Projects

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Circuits

An electronic circuit is composed of individual electronic components, such as resistors, transistors, capacitors, inductors and diodes, connected by conductive wires or traces through which electric current can flow. The combination of components and wires allows various simple and complex operations to be performed: signals can be amplified, computations can be performed, and data can be moved from one place to another.[1] Circuits can be constructed of discrete components connected by individual pieces of wire, but today it is much more common to create interconnections by photolithographic techniques on a laminated substrate (a printed circuit board or PCB) and solder the components to these interconnections to create a finished circuit. In an integrated circuit or IC, the components and interconnections are formed on the same substrate, typically a semiconductor such as silicon or (less commonly) gallium arsenide.[2]
Breadboards, perfboards, and stripboards are common for testing new designs. They allow the designer to make quick changes to the circuit during development.
An electronic circuit can usually be categorized as an analog circuit, a digital circuit, or a mixed-signal circuit (a combination of analog circuits and digital circuits).

Programmable Electronic Dice

Here’s a simple programmable electronic dice with numeric display. This dice can be programmed using a 4-way DIP switch to display any random number between ‘1’ and ‘2,’ ‘1’ and ‘3,’ ….. or ‘1’ and ‘9.’
To obtain the desired dice range, inner switches A, B, C and D of DIP switch are to be set as per the table. For example, if you want the electronic dice to count from 1 to 8, close switches A and D and keep B and C open. On pressing switch S1, the display varies fast between ‘1’ and ‘8.’ When you release S1, the display stops shuffling and the last (latest) number remains on it.
IC1 is a dual 4-input Schmitt trigger NAND gate 74LS13. Gate N1 is used as an oscillator built using resistor R2 and capacitor C1 to produce approximately 70kHz clock frequency, which is fed to IC2. Gate N2 loads data at the inputs of IC2.
IC2 is a presettable binary counter (74LS191) with parallel loading facility. Whenever its pin 11 goes low, the data present at its inputs D through A (which is ‘0001’) appears at its outputs QD through QA when all the inner switches of DIP switch are open and DIS1 shows the minimum count as ‘1’ (and not ‘0’).
With inner switches of DIP switch in positions shown in the table, the count output can go from ‘0001’ to the maximum count shown under ‘Dice Range’ in the table when switch S1 is depressed. On releasing switch S1, the last count within the dice range gets displayed.
The outputs of IC2 are displayed on common-anode, 7-segment display LTS542 (DIS1). BCD-to-7-segment decoder IC 7447 (IC3) is used to drive the display. Resistor R8 limits the current through DIS1.

PC-Based Candle Ignitor

Here’s a PC-based lighting system that lets you light up a candle using matchsticks by just pressing the ‘Enter’ key on the PC’s keyboard. It is especially useful when celebrating such occasions as birthdays and anniversaries.
The number of matchsticks required to light up the candle is placed on the candle (alongside its wick) as shown in the figure. The heating coil for igniting the matchsticks is kept near them.
The interface circuitry between the PC and the heating coil for the candle-matchsticks arrangement comprises an inverter, monostable and relay driver. Transistor BC548 (T1) acts as the inverter, IC 555 (IC1) is configured as the monostable circuit and transistor SL100 (T2) is the relay driver.
When you press ‘Enter’ key on the keyboard, the inverted output at the collector of transistor T1 goes low to trigger IC1 through its pin 2. Output pin 3 of the monostable goes high and transistor T2 conducts for around 50 seconds.
The conduction of transistor T2 energises relay RL1, which, in turn, connects the heating coil to 230V AC through the normally opened (N/O) contact. In place of the heating coil, you can also use an electric cigarrette lighter. The heating coil becomes red hot when connected across the 230V AC and ignites the matchsticks. The flames of the matchsticks light up the candle.
The program, written in ‘C’ language, is simple and easy to understand. The parallel-port D-type female connector normally available on the back of the PC is used for outputting the data to the interfacing circuitry. The address 378H of parallel-port LPT1 is used in the program. The parallel-port pin 2 corresponding to data bit D0 sends the control signal to energise the relay, which, in turn, connects the load to AC mains.
This circuit uses only one output of the PC’s parallel port to light up the candle, but it can be extended to light up up to eight diyas/candles in thiruvillaku (as called in South India) by using eight outputs with a slight change in the program and adding seven similar circuits.

Handy Tester

For beginners, here’s a low-cost multitester that can be used to test the condition of almost all the electronic components from resistors to ICs. It uses only a few components but can also detect polarity, continuity, logic states and activity of multivibrators.
The circuit is extremely simple and exploits the biasing property of bipolar transistors. Transistors T1 and T2 act as transistor switches driving the red and green halves of bicolour LED1 independently to give results of the test.
When power is applied by pressing switch S1, transistor T1 stops conducting due to the lack of forward bias. At the same time, transistor T2 takes base bias voltage from the battery through resistor R1 and conducts. This allows the red half of bicolour LED1 to illuminate.
When the base of transistor T1 gets positive voltage through resistor R3, it conducts to light up the green half of bicolour LED1. When transistor T1 conducts, the base of transistor T2 is grounded and it cuts off to turn off the red half of bicolour LED1. The functioning of the circuit thus depends on the signal obtained at the base of transistor T1. The table gives the testing procedures for various components with the expected indications/results.

Timer with Musical Alarm

This low-cost timer can be used for introducing a delay of one minute to two hours. After the timing period is over, a musical song is heard.
The circuit is built around popular CMOS oscillator/divider CD4060 (IC1). It works off a 9V PP3 battery and its standby current drain is very low.
Fig. 1: Pin configuration of melody generator ICUM66
By adjusting preset VR1, the time delay can be adjusted. After time delay is over, output pin 3 of IC1 goes high and npn transistor T1 conducts to provide positive power supply to melody generator IC UM66 (IC2) at its pin 2. Zener diode ZD1 reduces this power supply to 3.3V required for operation of IC2.
The output of IC2 is fed to the loudspeaker (LS1) via driver transistor T1. Preset VR2 is used to control the volume of the loudspeaker.
Fig. 2: The circuit of timer with musical alarm
The timer gets activated when power is supplied by pressing switch S1. To switch off the alarm, you need to switch off the power supply.

Noise Meter

Normally, sound intensity up to 30 dB is pleasant. Above 80 dB, it becomes annoying. And if it goes beyond 100 dB, it may affect your psychomotor performance, detracting your attention and causing stress. Noise pollution may also affect your hearing ability.
Noise intensity level in households is around 47 dB. But hi-fi music systems and TV sets operated at high volumes add to this sound, posing a health hazard.
Here’s a simple circuit that senses and displays the noise intensity level in your room. It also gives a warning beep when noise crosses the safe level of 30 dB.
The circuit comprises a sound intensity sensor and a display unit. The regulator circuit built around regulator IC 7809 (IC1) provides regulated 9V power supply to the circuit.
The sound intensity sensor is built around a condenser microphone, op-amp IC CA3130 (IC2) and associated components. Op-amp IC2 is configured as a high-gain inverting amplifier. The voltage supply to IC2 at its non-inverting pin 3 is divided to half by resistors R3 and R4, which is also used as the reference voltage. Resistor R1 determines the sensitivity of the condenser microphone.
The microphone picks up sound vibrations and converts them into the corresponding electric pulses, which are fed to the inverting input of IC2 (pin 2) via capacitor C4 and resistor R2. Capacitor C4 block sany DC entering the op-amp, since it may affect the functioning of the op-amp. The output of IC2 is connected to the inverting input through resistor R5 (10 mega-ohms) for negative feedback. Since the input impedance of IC2 is very high, even a small current can activate the op-amp.
The output of IC2 is given to preset VR1 via capacitor C5, which is used to control the volume. Capacitor C5 blocks DC, allowing only AC to pass through preset VR1. The AC signals from the wiper of VR1 are fed to a diode pump comprising diodes Dl and D2. The diode pump rectifies the AC and maintains it at the output level of IC2. Capacitor C6 acts as a reservoir capacitor for DC and resistor R6 provides the path for its discharge.
The display circuit is built around monolithic IC LM3914 (IC3), which senses the analogue voltage and drives ten LEDs to provide a logarithmic analogue display. Current through the LEDs is regulated by the internal resistors of IC3, eliminating the need for external resistors. The built-in low-bias input buffer of IC3 accepts signals down to ground potential and drives ten individual comparators inside IC3. The outputs of IC3 go low in a descending order from 18 to 10 as the input voltage increases.
Each LED connected to the output of IC3 represents the sound level of 3 dB, so when all the ten LEDs glow, it means the sound intensity is 30 dB.
Pin 9 of IC3 is connected to 9V to get the dot-mode display. In the dot-mode display, there is a small amount of overlap between segments. This assures that at no time will all LEDs be ‘off.’
When output pin 10 of IC3 goes low, pnp transistor T1 gets base bias (normally cut-off due to resistor R7) to sound the piezobuzzer (PZ1) connected to its collector.
The circuit can be constructed on any general-purpose PCB. Condenser microphone should be connected using a shield wire and enclosed in a tube to increase its sensitivity. For audiovisual indications, use a small DC piezobuzzer and transparent LEDs. Adjust preset VR1 until only the first LED (LED1) lights up. Keep the circuit near the audio equipment or TV set to monitor the audio level.

Automatic Water Pump Controller

Here’s a circuit that automatically controls the water pump motor. The motor gets automatically switched on when water in the overhead tank (OHT) falls below the lower limit. Similarly, it gets switched off when the tank is filled up. Built around only one NAND gate IC (CD4011), the circuit is simple, compact and economical. It works off a 12V DC power supply and consumes very little power.
The circuit can be divided into two parts: controller circuit and indicator circuit.
Fig. 1 shows the controller circuit. Let us consider two reference probes ‘A’ and ‘B’ inside the tank, where ‘A’ is the lower-limit probe and ‘B’ is the upper-limit probe. The 12V DC power supply is given to probe C, which is the limit for minimum water always stored in the tank.
Fig. 1: Controller circuit
The lower limit ‘A’ is connected to the base of transistor T1 (BC547), the collector of which is connected to the 12V power supply and the emitter is connected to relay RL1. Relay RL1 is connected to pin 13 of NAND gate N3.
Similarly, the upper-limit probe ‘B’ is connected to the base of transistor T2 (BC547), the collector of which is connected to the 12V power supply and the emitter is connected to pins 1 and 2 of NAND gate N1 and ground via resistor R3. The output pin 4 of NAND gate N2 is connected to pin 12 of NAND gate N3. The output of N3 is connected to input pin 6 of N2 and the base of transistor T3 via resistor R4. Relay RL2 connected to the emitter of transistor T3 is used to drive the motor.
If the tank is filled below probe A, transistors T1 and T2 do not conduct and the output of N3 goes high. This high output energises relay RL2 to drive the motor and it starts pumping water into the tank.
When the tank is filled above probe A but below probe B, water inside the tank provides base voltage to drive transistor T1 and relay RL1 energises to make pin 13 of gate N3 high. However, water inside the tank does not provide base voltage to transistor T2, so it does not conduct and the logic built around NAND gates N1 and N2 outputs low to pin 12 of gate N3. The net effect is that the output of N3 remains high and the motor continues pumping water into the tank.
When the tank is filled up to probe B level, water inside the tank still provides base voltage to transistor T1 and relay RL1 energises to make pin 13 of gate N3 high. At the same time, water inside the tank also provides base voltage to drive transistor T2 and the logic built around NAND gates N1 and N2 outputs high to pin 12 of gate N3. The net effect is that the output at pin 11 of N3 goes low and the motor stops pumping water into the tank.
When water level falls below probe B but above probe A, water inside the tank still provides base voltage to transistor T1 and relay RL1 remains energised to make pin 13 of gate N3 high. However, transistor T2 doesn’t conduct and the logic built around NAND gates N1 and N2 outputs high to pin 12 of N3. As a result, the output of N3 remains low and motor remains stopped.
When water level falls below probe A, both transistors T1 and T2 do not conduct. NAND gate N3 gives a high output to drive relay RL2 and the motor restarts pumping water into the tank.
Fig. 2: Indicator/monitoring circuit
Fig. 2 shows the indicator/monitoring circuit. It consists of five LEDs, which glow to indicate the level of water in the overhead tank. Since 12V power supply is given to water at the base of the tank, transistors T3 through T7 get base voltage and conduct to light up the LEDs (LED5 down through LED1).
When water in the tank reaches the minimum at level C, transistor T7 conducts and LED1 glows. When water level rises to one-fourth of the tank, transistor T6 conducts and LED1 and LED2 glow. When water level rises to half of the tank, transistor T5 conducts and LED1, LED2 and LED3 glow. When water level rises to three-fourth of the tank, transistor T4 conducts and LED1 through LED4 glow. When the tank is full, transistor T3 conducts and all the five LEDs glow. So, from glowing of LEDs, one can know water level in the tank (see the table). The LEDs can be mounted anywhere for easy monitoring.
Note. The user can adjust the level to which water must be filled in the tank by adjusting the heights of probes A and B. The stand and adjusting screws should be insulated to avoid shorting.

Soldering Iron Temperature Controller

Here is a simple circuit to control the temperature of a soldering iron. It is especially useful if the soldering iron is to be kept on for long since you can control the heat dissipation from the iron. When a soldering iron is switched on, the iron takes time to reach the solder’s melting point. Simply connect this circuit to the soldering iron as shown in the figure and the iron reaches the solder’s melting point quickly.
Triac BT136 is fired at different phase angles to get temperatures varying from zero to maximum. A diac is used to control the triac firing in both directions. Potentiometer VR1 is used for setting the temperature of the soldering iron.
The circuit can be housed in a box with the potentiometer fixed on the side such that its knob can be used from outside the box to adjust the soldering iron’s temperature.

Multipurpose White-LED Light

Standard fluorescent lamps and their smaller versions called compact fluorescent lamps (CFLs) radiate light in all directions (360°) and tend to increase the room temperature. In emergency lights using these lamps, the battery lasts only a few hours due to the power loss during conversion of DC into AC. These limitations can be overcome by using ultra-bright white LEDs.
Here is a torch-cum-table lamp using white LEDs that can also be modified to act as an emergency-cum-bedroom light. Its main features are long and continuous operation, very low power consumption, selectable light angle, very long life and negligible heat radiation.
Fig. 1 shows the circuit of white LEDs-based torch-cum-table lamp. The circuit is very simple and uses a battery charger unit built around IC LM317 (IC1) and a combination of white LEDs. Resistor R3 (4.7-ohm, 2W) limits the current through the battery. The radiation angles selected for white LEDs are 60° and 20°. Three columns of LED clusters (A, B and C) are made on separate transparent acrylic sheets, with each sheet having a total of twelve LEDs affixed to it.
Fig. 1: Cluster LED searchlight/table lamp
The left (A) and right (C) columns use 20° LEDs, while the middle column (B) uses 60° LEDs. All the twelve LEDs of each column are connected in series to separate 15-ohm current-equalisation resistors (R8 through R19) as shown in Fig. 2, and to current-limiter resistors R7 (10-ohm, 1W) and R6 (5-ohm, 1W) as shown in Fig. 1. The entire unit is powered by a 6V, 4Ah maintenance-free rechargeable battery.
The continuous lighting life is around 7 hours in torchlight mode and around 14 hours in table lamp mode, depending on the battery capacity and quality. For the torch mode, only the left and right LED columns are used. These LEDs beam light up to 6 metres. In table lamp (spread light) mode, only the middle column of LEDs is used.
Fig. 2: Arrangement of LEDs for column A, B or C
You can select between the table lamp and torch modes by using rotary switch S1, which is a single-pole, 3-way switch. When the pole of switch S1 is set at position 1, the C column of 60° LEDs lights up and the system acts as a table lamp. When the pole of switch S1 is set at position 3, columns A and C light up and the system acts as a torch. When the pole of switch S1 is at position 2, both the table lamp and the torch modes remain off.
When mains is switched on, LED2 glows. To charge the battery, flip switch S2 to ‘on’ position. To check the status of the battery, flip switch S3 to ‘on’ position. This will give an indication of battery charge. If low-battery indicator LED1 turns off, the battery needs to be charged.
Fig. 3 shows the circuit of emergency lamp with brightness control, which is derived from Fig. 1 with slight modification in the combination of LEDs. Built around four multichip (MC) LEDs, it is very compact and simple, and can work in two modes, namely, bedroom lamp and emergency lamp.
In bedroom lamp mode, only one blue LED glows. This LED is mounted at the top in upside down position to avoid direct viewing of the blue light. The arrangement gives a pleasant, well-spread light.
In emergency lamp mode, 8mm, 80° bright-white multichip LEDs give 80° spread light, which is sufficient for indoor uses. Circular PCBs for multichip LEDs have four internal junctions each. Solder LED17 through LED20 in the first PCB, LED21 through LED24 in the second PCB, LED25 through LED28 in the third PCB and LED29 through LED32 in the fourth PCB, with a spacing of 3 to 4 cm between two adjacent LEDs. Finally, house all the four circular PCBs in a compact cabinet along with the reflector such that light can spread out in the room.
Fig. 3: Emergency lamp with brightness control
Each multichip LED gives a power of 32 candles. Therefore use of four 8mm multichip LEDs will give a total power of 128 candles.
In emergency lamp mode (selected through rotary switch S5), all the four multichip LEDs (including LED17 through LED32) glow. The DC power source is a 6V, 4Ah chargeable battery, with charging circuit built around popular IC LM317 (IC2). Resistor R21 (2.2-ohm, 1W) acts as the current limiter for the battery.
You can control the candle power (brightness) of LEDs as per your requirements. Transistor SL100 (T1) and its associated components form the candle controller (brightness controller). The base biasing voltage of the transistor is stabilised by resistor R24 and diodes N3 and N4 (1N4001). This constant voltage is given to the base of the transistor through a potentiometer VR1 (4.7k lin.). By adjusting the potentiometer, you can control the intensity of the multichip LEDs. No heat-sink is required for the transistor.

Mains Failure/Resumption Alarm

This mains indicator sounds an alarm whenever AC mains fails or resumes. It is very useful in industrial installations, cinema halls, hospitals, etc.
The mains detector circuit is built around capacitors C1 and C2, resistor R1, and diodes D1 and D2. It provides sufficient voltage for the glowing of internal LED of optocoupler MCT2E (IC1).
Initially SPDT switch S1 is at position 1. When mains fails, pin 5 of gate N2 goes high and the oscillator built around gates N2 and N3 of IC2 produces low-frequency oscillations at pin 10, which are further given to pin 4 of IC 555 (IC3). The oscillation frequency can be varied from 0.662 Hz to 1.855 kHz using preset VR1.
IC 555 (IC3) is wired as an audio tone generator. The tone of this audio oscillator can be varied from 472 Hz to 1.555 kHz using preset VR2. The low-frequency input activates IC3 to generate audio tones and loudspeaker LS1 connected to its output pin 3 sounds an alarm indicating mains failure.
To turn off the alarm, slide the pole of switch S1 to position 2. Now the circuit is ready for sensing the mains resumption.
When mains resumes, pin 5 of gate N2 goes high and the oscillator built around gates N2 and N3 of IC2 produces low-frequency oscillations at pin 10, which are given to reset pin 4 of IC3. As a result, loudspeaker LS1 again sounds to indicate that mains has resumed. To turn off the alarm, slide the pole of switch S1 back to position 1. Now the circuit is again ready for sensing the mains failure.
The circuit works off a 9V battery. It can be housed in a box and installed where you want to monitor the status of mains.

Twilight Lamp Blinker

During sunset or sunrise, the ambient light is not adequate to lead you through the open doorway or make your way around obstructions. To avoid any mishap, here is a twilight lamp blinker that you can place near obstructions.
Fig. 1 shows the circuit of the twilight lamp blinker. For powering the circuit, the mains input (230V AC) is down-converted by resistors R1 and R2, capacitor C1 and diodes D1 and D2 into a DC voltage that is low enough to safely charge the back-up battery pack. Resistor R2 across capacitor C1 functions as a bleeder resistor. Zener diode ZD2 protects against over-voltage.
Fig. 1: Circuit diagram of twilight lamp blinker
Miniature Ni-Cd battery packs for cordless telephones are easily available at reasonable rates. Use such a battery pack with 4.8V, 500mAh rating for efficient and long-time back-up. The pole of switch S1 should be in position 2 if you use a battery. If you are not interested in the back-up facility, flip switch S1 to position 1.
The rest of the circuit includes a light-detector resistor (LDR1), IC CD4093 (IC1) and a preset (VR1) for brightness control. LDR1 is used as a sensor that has a low resistance during daytime and a high resistance at night.
Fig. 2: Proposed enclosure
When light falls on the LDR, its low resistance provides low level at the inputs of NAND gate N1. The high input from N1 makes the output of N2 low and the relaxation oscillator (built around NAND gates N3 and N4 of IC1, capacitor C3 and resistor R3) does not oscillate. As a result, transistor T1 does not conduct and LED1 does not blink.
On the other hand, in darkness, the high resistance of LDR1 provides a high level at the input pins of NAND gate N1. The low output from N1 makes the output of N2 high and the relaxation oscillator oscillates. As a result, transistor T1 conducts and LED1 blinks.
Transistor T1 is the LED driver. Resistor R4 limits the current flowing through LED1 and hence its brightness. You may connect one or two additional LEDs in series with LED1 to get more light. The low brightness of LED1 will extend the battery back-up time.
Since the circuit is directly connected to the high-voltage AC supply, enclose it in a plastic case (shown in Fig. 2) to avoid any fatal electric shock. On the front side of the cabinet, leave a hole for LDR1 so that light can easily fall on it. Fix preset VR1 on the other side. You can place the gadget anywhere you want, provided ambient light falls directly on the LDR.

Sound-Operated Intruder Alarm

When this burglar alarm detects any sound, such as that created by opening of a door or inserting a key into the lock, it starts flashing a light as well as sounding an intermittent audio alarm to alert you of an intruder. Both the light and the alarm are automatically turned off by the next sound pulse.
230V AC mains is stepped down bytransformer X1, rectified by diode D1 andfiltered by capacitor C1 to give 12V DC. The voltage at the non-inverting input (pin 3) of op-amp CA3140 (IC1) is treated as the reference voltage and it can be set using preset VR1. The voltage at the inverting input (pin 2) is the same as that across the condenser microphone. The condenser microphone should be carefully set for a high sensitivity of the sound. A high reference value means a subtle sound is enough to change the output of IC1 at pin 6. Fix the reference voltage such that the output remains unchanged during any false triggering.
In the absence of any sound, the voltage at input pin 2 of IC1 is almost equal to the full DC voltage and therefore the output of IC1 remains low. Since IC CD4027 is wired in toggle mode, its output pin 15 is also low. This makes reset pin 4 of IC3 low to reset the astable multivibrator built around timer 555 (IC3). As a result, transistor T1 is cut-off and relay RL1 remains de-energised. In de-energised state, both the N/O contacts of relay RL1, i.e. RL1(a) and RL1(b), remain open. RL1(a) contacts keep the lamp turned off, whereas RL(b) contact disconnects the output of the astable multivibrator built around IC 555 (IC4) to disable the speaker.
In the case of any noise, a current flows through the microphone and the voltage at pin 2 reduces to make the output of op-amp IC1 high. IC2 gets triggered by the pulse available at its pin 13 and its output at pin 15 goes high to enable astable multivibrator IC3. The output of IC3 goes high for three seconds and then goes low for 1.5 seconds. This repeats until pin 15 of IC2 remains high. The high output of IC3 energises the relay via driver transistor T1, while the low output de-energies the relay.
When relay RL1 is energised, relay contact RL1(a) passes on the AC power to bulb B1 and it lights up. At the same time, relay contact RL1(b) allows the output of astable multivibrator IC4 to the speaker and an audio tone is generated. The frequency of this audio tone is approximately 480 Hz. Both the flashing of the bulb and the audio tone continue as long as the output of flip-flop IC2 remains high.
Now if the circuit detects any further sound, the output of flip-flop IC2 goes low. This makes reset pin 4 of astable multivibrator IC3 low and IC3 stops oscillating. The low output of IC3 de-energises the relay to turn the bulb and the tone off.

Electronic Street Light Switch

Here’s a simple and low-cost street light switch. This switch automatically turns on the light at sunset and turns it off at sunrise. The automatic function saves electricity besides man-power.
Broadly, the circuit can be divided into power supply and switching sections.
Pressing switch S1 connects mains to power the circuit. Mains is stepped down to 9.1V DC by resistor R1, diode D1 and zener diode ZD1. The output across ZD1 is filtered by capacitors C1 and C2. The output voltage can be increased up to 18V or decreased to 5V by changing the value of zener diode ZD1.
The switching circuit is built around light-dependent resistor LDR1, transistors T1 through T3 and timer IC1. The resistance of LDR1 remains low in daytime and high at night. Timer IC1 is designed to work as an inverter, so a low input at its pin 2 provides a high output at pin 3, and vice versa. The inverter is used to activate triac 1 and turn street bulb B1 on.
During daytime, light falls on LDR1 and transistors T1 and T2 remain cut-off to make pins 4 and 8 of IC1 low. Since transistor T3 is also cut-off, IC1 is not triggered. As a result, output pin 3 of IC1 (connected to the gate of triac 1 via resistor R5 and red LED1) remains low and the street bulb does not glow.
At night, no light falls on LDR1 and transistors T1 and T2 conduct to make pins 4 and 8 of IC1 high. Due to the conduction of transistor T3, trigger pin 2 of IC1 remains low. The high output of IC2 at its pin 3 turns the street bulb ‘on.’
Assemble the circuit, except LDR1, on any general-purpose PCB. Use long wires for LDR1 connections so that it can be mounted at a place where sufficient light falls on it.

Chanting Player

Chanting combines singing and music with mantras. The sweetness of chanting stills the mind, dissolves worries and opens the heart. Chanting forms an integral part of the practice schedule at siddha yoga retreats, centres and ashrams. Here are a few electronic chanting players for some popular mantras and artis.
At the heart of these circuits is a preprogrammed read-only memory (ROM) chip bonded on a hylam board. (The ROM chip is a complementary metal-oxide semiconductor (CMOS), large-scale integrated (LSI) chip.) Known as chip-on-board (COB), these boards are available in different sizes, under a blob of epoxy, with chips programmed with single or multiple mantras/artis such as gayatri mantra, ganapati mantra, krishna mantra, om namah shivaye, shri ram jai ram and satnaam wahe guru.
Fig. 1: The circuit for 3-in-1 mantra player including the power supply
The COBs are available in 7-, 8-, 9- and 16-pin pad configurations. Pin connections of these COBs are shown in Fig. 6, Figs 1, 2 and 4, Figs 3 and 5, and Fig. 7, respectively. Some manufacturers make these COBs with different pad configurations, so their specifications should be strictly followed.
Besides a preprogrammed data ROM, the COBs contain an inbuilt oscillator, counter, shift register, adaptive differential pulse-code modulation (ADPCM) synthesiser and digital-to-analogue converter (DAC).
The timing pulses generated by the oscillator regulate the pace of the mantra and other activity inside the chip. Its frequency is decided by an external resistor (Rosc) connected between its two input pins. The controller controls all the activities inside the chip. It sends appropriate signals to the counter and the shift register to read the data in the ROM. The output of the ROM is fed back to the controller, which directs it to the ADPCM synthesiser. The synthesiser’s output is sent to the DAC, which converts it into audio. The audio output from the DAC is reproduced by the loudspeaker. The potentiometer connected to the input of the loudspeaker acts as a volume controller.
Fig. 2: The COB circuit for 2-in-1 mantraplayer
Fig. 3: The COB circuit for 6-in-1 mantraplayer
The COB works off 3V DC and is capable of driving the loudspeaker directly.
Fig. 1 can be divided into power supply and COB sections. The same power supply section is to be used for the COB circuits shown in Figs 2 through 5 as well. The 3V power supply for the COB is derived by using a 3V-0-3V center-tapped transformer (X1). The secondary output of the transformer is applied to a full-wave rectifier comprising diodes D1 and D2. The output of the full-wave rectifier is filtered by capacitor C1 to provide 3V DC to the COB.
For 3-in-1 mantra player, connect A and B terminals of the power supply section to the corresponding points of the COB section as shown in Fig. 1. Then connect 230V AC mains to the primary of transformer X1. Now the circuit is ready to play.
Fig. 4: The COB circuit for 5-in-1 mantraplayer
Fig. 5: The COB circuit for another 2-in-1mantra player
The desired mantra can be selected by applying positive supply to trigger pin 3 of IC1 by pressing push-to-on switch S1 momentarily. When you press switch S1 for the first time, “wahe guru” is played. When you press switch S1 second time, “satnam wahe guru” is played. When you press switch S1 third time, “satnam karta purush” is played. Using preset VR1, the volume of the sound can be controlled.
For 2-in-1 mantra player, connect the power supply section of Fig. 1 to the COB section shown in Fig. 2. The desired mantra can be selected by applying positive supply to trigger pin 3 of IC2 by pressing push-to-on switch S2 momentarily. When you press switch S2 for the first time, “jai ganesh jai ganesh deva” is played. When you press switch S2 second time, “aarti kijje hanuman lala ki” is played.
For 6-in-1 mantra player, connect the power supply section of Fig. 1 to the COB section shown in Fig. 3. The desired mantra can be selected by applying positive supply to trigger pin 4 of IC3 by pressing push-to-on switch S3 momentarily. When you press switch 3 for the first time, the circuit starts playing “om bhurbhua swaha” When you press switch S3 second time, “om namah shivaye” is played. When you press switch S3 third time, “jai ganesh, jai ganesh deva” is played. When you press switch S3 fourth time, “govind bolo hari gopal bolo” is played. When you press switch S3 fifth time, “shriman narayan narayan” is played. When you press switch S3 sixth time, “om krishna yadhamah” is played.
Fig. 6: The circuit (including power supply) for playing a single mantra with amplified sound
For 5-in-1 mantra player, connect the power supply section of Fig. 1 to the COB section shown in Fig. 4. When you press switch S4 for the first time, “om bhurbhua swaha” is played. On consequent pressing of switch S4, “om namo shivaye,” “jai ganesh, jai ganesh deva” “jai siya ram” and “govind bolo hari gopal bolo” are played in that order.
For another 2-in-1 mantra player, connect the power supply section of Fig. 1 to the COB section shown in Fig. 5. When you press switch S5 for the first time, “om bhurbhua swaha” is played. When switch S5 is pressed second time, “om namah shivaye” is played.
The circuit for playing a single mantra with loud sound is shown in Fig. 6. The circuit comprises power supply, COB (shown within dotted lines) and low-power audio amplifier sections. Low-power audio amplifier IC LM386 (IC6) is used here to get louder sound.
The power supply section uses a 6V-0-6V centre-tapped transformer (X2) instead of the 3V-0-3V centre-tapped transformer. The secondary output of the transformer is rectified by a full-wave rectifier comprising diodes D3 and D4, and filtered by capacitor C4 to provide 6V DC to the power amplifier (IC6). Zener diode ZD1 in series with resistor R6 reduces the supply voltage to 3V for the COB section.
Fig. 7: The COB circuit for 2-mantra player
Connect all the three sections together by connecting their identical terminals. Then connect 230V AC mains to the primary of transformer X2. Now the circuit is ready to work. Simply press switch S6 to provide the power supply to IC6 and IC7 and “om namah shivaye” start playing loudly. Using preset VR6, you can control the volume of the sound.
For a 2-mantra player with loud sound, disconnect the COB circuit shown within dotted lines in Fig. 6 and replace it with the COB circuit shown in Fig. 7. The desired mantra can be selected by applying positive supply to trigger pin 15 or 16 of IC8 by changing the position of switch S7. Note that switch S6 should be kept pressed.
When switch S7 is in position 1, “shri krishanah sharnam namah” is played. The mantra repeats continuously. To stop it, either release switch S6 or shift switch S7 to position 2. If you choose to shift switch S7, “shri krishana” stops playing but “hari krishana, hari krishana” starts playing. The mantra repeats continuously. To stop it, either release switch S6 or shift switch S7 to position 1.
For ease of construction, assemble a small printed circuit board (PCB) for the amplifier and power supply circuits. Various types of plastic enclosures for electronic chanting players are available in the market. Use a suitable enclosure for this player. Take care while handling and soldering the COBs as the CMOS chips can get damaged due to static charge.

Versatile LED Display

This circuit uses an erasable programmable read-only memory (EPROM) to display various light patterns on LEDs. Since bicolour LEDs (comprising green and red LEDs) have been used, display is possible in three colours (green, red and amber).
The circuit is powered by 5V DC. IC 555 (IC1) is wired as an astable multivibrator, whose oscillation frequency can be varied using preset VR1. The output of IC1 clocks 12-stage binary counter IC CD4040 (IC2), which, in turn, provides address data to EPROM IC 2716 (IC3). IC3 contains the code (see Table I) for the display.
The high logic at any data pin causes the corresponding LED to glow. When the data at address location 00H is addressed, the red LED of LED1 glows. The data byte 44H at address location 09H causes both the green and red LEDs of LED2 to glow (refer the table).
The binary outputs of IC2 comprising Q0, Q1, Q2, Q5, Q6, Q7, Q8 and Q9 have been connected to address pins A0 through A7 of EPROM IC3 (2716). Q3 and Q4 outputs of IC2 have not been used. This causes each display pattern to be repeated eight times before the next pattern is displayed. You can adjust the number of times a display pattern repeats by changing the output lines of IC2 connected to the EPROM’s address pins A0 through A7.
The circuit uses a total of four bicolour LEDs. However, more LEDs in pairs of four can be added in the dotted lines (see the figure). Suppose you want to connect four more bicolour LEDs (LED5 through LED8, not shown in the figure). For this, you’ll have to connect them in parallel to LED1 through LED4, respectively. The speed of the display can be changed by varying preset VR1, which changes the clock frequency.
You can also create other display patterns by coding the EPROM accordingly. Note that the code should be burnt into the EPROM (by using a programmer kit) before it is inserted into the circuit.

Long-range Burglar Alarm Using Laser Torch

Laser torch-based burglar alarms normally work in darkness only. But this long-range photoelectric alarm can work reliably in daytime also to warn you against intruders in your big compounds, etc. The alarm comprises laser transmitter and receiver units, which are to be mounted on the opposite pillars of the entry gate. Whenever anyone enters to interrupt the transmitted laser beam falling on the receiver, the buzzer in the receiver circuit sounds an alarm.
The range of this burglar alarm is around 30 metres, which means you can place the transmitter and the receiver up to 30 metres apart. Since the laser torch can transmit light up to a distance of 500 metres, this range can be increased by orienting the phototransistor sensor properly. To avoid false triggering by sunlight, mount the phototransistor sensor such that it doesn’t directly face sunlight.
Fig. 1: Circuit of laser torch based transmitter
The transmitter circuit is powered by 3V DC. The astable multivibrator built around timer 7555 (IC1) produces 5.25kHz frequency. CMOS version of timer 7555 is used for low-voltage operation. The body of the laser torch is connected to the emitter of npn transistor T1 and the spring-loaded lead protruding from inside the torch is connected to the ground.
The receiver circuit is powered by 12V DC. It uses photoDarlington 2N5777 (T2) to sense the laser beam transmitted from the laser torch. The output beam signals from photoDarlington are given to the two-stage amplifier followed by switching circuit, etc. As long as the laser beam falls on photoDarlington T2, relay RL1 remains un-energised and the buzzer does not sound. Also, LED1 doesn’t glow.
Fig. 2: Receiver circuit When anyone interrupts the laser beam falling on photoDarlington T2, npn transistor T6 stops conducting and npn transistor T7 is driven into conduction. As a result, LED1 glows and relay RL1 energises to sound the buzzer for a few seconds (determined by the values of resistor R15 and capacitor C10). At the same time, the large indication load (230V AC alarm for louder sounds or any other device for momentary indication) also gets activated as it is connected to 230V AC mains via normally opened (N/O) contact of relay RL1.

Infrared Object Counter

This infrared object counter can be installed at the entry gate to count the total number of people entering any venue. For example, it can be used at the railway stations or bus stands to count the people arriving per day or week.
The counter uses an infrared transmitter-receiver pair and a simple, low-cost calculator. It works even in the presence of normal light. The maximum detection range is about 10 metres. That means the transmitter and the receiver are to be installed (at the opposite pillars of the gate) not more than 10 metres apart. No focusing lens is required. If an 8-digit calculator is used the counter can count up to 99,999,999 easily, and if a 10-digit calculator is used the counter can count up to 9,999,999,999.
Fig. 1: Transmitter circuit
Powered by a 9V battery, the transmitter circuit (see Fig. 1) comprises IC 555 (IC1), which is wired as an astable multivibrator with a centre frequency of about 38 kHz, and two infrared light-emitting diodes (LEDs). The receiver circuit (see Fig. 2) is powered by a 5V regulated power supply built around transformer X1, bridge rectifier comprising diodes D1 through D4 and regulator IC2. It uses an infrared receiver (IR) module (RX1), optocoupler (IC3) and a simple calculator.
When switch S1 is in ‘on’ position, the transmitter circuit activates to produce a square wave at its output pin 3. The two infrared LEDs (IR LED1 and IR LED2) connected at its output transmit modulated IR beams at the same frequency (38 kHz). The oscillator frequency can be adjusted using preset VR1.
In the receiver circuit, IR receiver module TSOP1738, which is commonly used in colour televisions for sensing the IR signals transmitted from the TV remote, is used as the sensor.
The IR beams transmitted by IR LED1 and LED2 fall on infrared receiver module IR RX1 of the receiver circuit to produce a low output at its pin 2. This keeps transistor T1 in non-conduction mode.
Now when anyone enters through the gate to interrupt the IR beam, the IR receiver module produces a high output pulse at its pin 3. As a result, transistor T1 conducts to activate IC3 and its internal transistor shorts key ‘=’ of the calculator to advance the count by one.
Fig. 2: Receiver-cum-counter circuit
Both the transmitter and the receiver can be assembled on any general-purpose PCB. Place the transmitter and the receiver around one metre apart.
For calibration, press switches S1 and S2 followed by ‘on’ key of the calculator. Now press ‘1’ and ‘+’ keys sequentially to get ‘1’ on the screen of the calculator. Then, place a piece of cardboard between the transmitter and the receiver to interrupt the IR rays two times. If the calculator counts ‘2,’ the counter is working properly for that range. Repeat this procedure for higher ranges as well. If there is any problem, adjust VR1.
For installation, switch off the transmitter, receiver and calculator, and mount the transmitter and the receiver on the opposite pillars of the main entry gate such that they are properly orientated towards each other. Mount the calculator where you can read it easily. Connect pins 4 and 5 of IC3 across ‘=’ key connections on the PCB of the calculator.
Now switch on the transmitter and the receiver by pressing switches S1 and S2, respectively. Thereafter, switch on the calculator and press ‘1’ followed by ‘+’ key of the calculator to initialise it. Now your counter is ready to count.
The calculator reads ‘1’ after one interruption, ‘2’ after second interruption and so on.

Automatic Soldering Iron Switch

Quite often, we forget to turn off the soldering iron. This results in not only a smoking oxidised iron but also waste of electricity. To solve this problem, here’s a circuit that automatically switches off the soldering iron after a predetermined time. The circuit draws no power when it is inactive. The circuit can also be used for controlling the electric iron, kitchen timer or other appliances.
At the heart of the circuit is a monostable multivibrator built around timer IC 555. When the circuit is in sleep mode, to switch on the soldering iron, you should push switch S1 momentarily. The multivibrator gets triggered and its output pin 3 goes high for around 18 minutes to keep relay RL1 energised via transistor T1. At the same time, capacitor C3 charges and AC supply is provided to switch on the soldering iron via normally opened (N/O) contacts of relay RL1.
The soldering iron remains ‘on’ for the time period predetermined by resistor R1 and capacitor C2. Here, this time is set for 18 minutes. Flashing of LED1 indicates the heating progress of the soldering iron. When the predetermined time is over, relay RL1 de-energises to turn off the soldering iron and the buzzer sounds until capacitor C3 gets discharged.
For switching on the circuit, use either a bell push switch or a similar switch with appropriate current carrying capacity.

Pen cell Charge Indicator

Small-size AA cells and button cells used in electronic devices providing a terminal voltage of 1.5V are normally rated at 500 mAh. As the cells discharge, their internal impedance increases to form a potential divider along with the load and the battery terminal voltage reduces. This, in turn, reduces the performance of the gadget and we are forced to replace the battery with a new one. But the same battery can be used again in some other application that requires less current.
Here’s a simple tester for quick checking of discharged pencells and button cells before throwing them away. The tester detects the holding charge of the battery and the terminal voltage to indicate whether the battery is suitable for a particular gadget or not.
A 9V battery can power the circuit with sufficient voltage and current. When you close switch S1, it provides stable 6V DC to the circuit.
The circuit uses op-amp CA3140 (IC1) as a voltage comparator. It can sense even a slight voltage variation between its inverting and non-inverting inputs. The non-inverting input (pin 3) of IC1 is supplied with a voltage obtained from the battery under test, while its inverting input pin 2 is provided with a reference voltage of 1.4V derived by resistor R4 and series combination of diodes D1 and D2. Resistors R1 and R2 provide a loading of 10 mA and 100 mA, respectively, for checking the charge capacity.
When a new battery is connected to the test terminals, the non-inverting input of IC1 gets 1.5V, which exceeds the voltage of the inverting input and the output of IC1 goes high. This high output provides forward bias to transistor T1 through resistor R4 and it conducts to light up the green half of the bicolour LED (LED1). Simultaneously, the base of transistor T2 is pulled down and it turns off and the red half of bicolour LED1 remains off.
When a partially discharged battery (with a terminal voltage of less than 1.4 V) is connected to the test terminals, the output of IC1 goes low to switch off transistor T1. This allows transistor T2 to forward bias by taking bias voltage through resistor R5 and the red LED within bicolour LED1 glows.
Slide switch S2 is used to check whether the battery is holding sufficient current to drive a load of 10 mA or 100 mA. If the discharged battery holds more than 100mA current, the green LED within bicolour LED1 glows, indicating that the battery can be used again in a low-drain circuit.
The circuit can be easily constructed on a perforated board using readily available components. Enclose it in a small case with probes or battery holder for testing.

PC-Based Timer

Timers are very useful both for industrial applications and household appliances. Here is a PC-based timer that can be used for controlling the appliances for up to 18 hours. For control, the timer uses a simple program and interface circuit. It is very cost-effective and efficient for those who have a PC at workplace or home. The tolerance is ±1 second. The circuit for interfacing the PC’s parallel port with the load is very simple. It uses only one IC MCT2E, which isolates the PC and the relay driver circuits. The IC prevents the PC from any short circuit that may occur in the relay driver circuit or appliance. The glowing of LED1 indicates that the appliance is turned on. Transistor BC548 is used as the relay driver. The program code is written in ‘C’ language and compiled using ‘Turbo C’ compiler. When the program is run, it prompts the user to input the time duration in seconds or minutes to control the appliance. After entering the required timing, press any key from the keyboard. Suppose you input the total duration as ‘x’ minutes, of which ‘on’ and ‘off’ durations are ‘y’ and ‘z’ minutes, respectively. The program will repeat the on-off cycle for x/(y+z) number of times. After completion of the total time, to repeat the cycle, you will have to reset the time in the program to activate the circuit. The program uses two bytes for storing integer type data. So when input is given in terms of seconds or minutes, it can hold 216–1=65,535 seconds or 18 hours at the maximum. The sleep() function in the program is used to hold the appliance in ‘on’ or ‘off’ condition for the ‘on’ and ‘off’ periods as entered by the user against prompts. The sound() function is used to give a beep during ‘on’ condition of the appliance. EFY note. The source code and executable file of this program have been included in this month’s EFY-CD.

Miser Flash

A flashing LED at the doorstep of your garage or home will trick the thieves into believing that a sophisticated security gadget is installed. The circuit is nothing but a low-current drain flasher. It uses a single CMOS timer that is configured as a free running oscillator using a few additional components. As the LED flashes very briefly, the average current through the LED is around 150 µA with a high peak value, which is sufficient for normal viewing. This makes it a real miser. The 9V battery source is connected via ‘on’/‘off’ switch S1 to the circuit. When switch S1 is closed, the IC receives power from capacitor C1, which is constantly charged through resistor R1. As capacitor C1 delivers power to IC1, it saves the battery from drain. Most LEDs consume a current of 20 mA, which in many instances is higher than the power consumed by the rest of the circuit. This is undesirable if the device is battery-powered. In this circuit, the energy consumed by the LED is a small fraction of the normal value. Capacitor C2 charges through resistor R2 and diode D1. When the voltage across C2 reaches two-third of the supply voltage, threshold pin 7 of IC1 switches on as a current sink. The capacitor discharges through LED1 into pin 7 rapidly. Diode 1N4148 (D1) provides the one-way charging path for capacitor C2 via resistor R2. LED1 illuminates briefly for a while with the accumulated charges in C2. Again, the charging cycle repeats. This way, LED continues flashing. A 9V PP3 battery can perfectly handle this job.

Digital Frequency Comparator

Here’s a digital frequency comparator for oscillators that indicates the result through a 7-segment display and a light-emitting diode (LED). When the frequency count of an oscillator is below ‘8,’ the corresponding LED remains turned off. As soon as the count reaches ‘8,’ the LED turns on and the 7-segment display shows ‘8.’
This demo circuit uses two NE555 timers configured as astable free-running oscillators, whose frequencies are to be compared.
The circuit of the digital frequency comparator portion comprises two 74LS90 decade counter ICs (IC2 and IC6), two 74LS47 7-segment display driver ICs (IC3 and IC7), 74LS74 set/reset flip-flop (IC4), 74LS00 NAND gate (IC8) and two 7-segment displays (DIS1 and DIS2). The astable free-running oscillators built around the timers are the frequency sources for the corresponding counters.
When power supply to the circuit is switched on, timing capacitor C1 starts charging through resistor R1 and potmeter VR1. When the capacitor voltage reaches 2/3Vcc, the internal comparator of IC1 triggers the flip-flop and the capacitor starts discharging towards ground though VR1. When the capacitor voltage reaches 1/3Vcc, the lower comparator of IC1 is triggered and the capacitor starts charging again. The charge-discharge cycle repeats. That means, the capacitor charges and discharges periodically between two-third and one-third of the power supply (Vcc). The output of NE555 is high during charging and low during discharging of capacitor C1.
The other oscillator (IC5) works similarly. The oscillator frequency can be varied by the potentiometer (VR1 or VR2). Output pins (pin 3) of the oscillators (IC1 and IC5) are connected to the respective decade counters (IC2 and IC6) through the DPDT switch.
IC2 and IC6 count the initial eight cycles. IC 74LS90 is a 4-bit ripple decade counter. It consists of a divide-by-two section and a divide-by-five section counter. Each section has a separate clock input. The input of the divide-by-five section (CP1) is externally connected to the P output (pin 12) of the divide-by-two section (CP0). When the divide-by-two section receives clock pulse, it becomes a divide-by-ten counter.
Decade counter 74LS90 is reset by a high pulse at its pins 2 and 3. Initially, pins 2 and 3 are pulled down by resistor R2. The P through S outputs of IC2 are connected to the A through D inputs of IC3. Pin 11 (S) of IC2 is also connected to pin 3 of IC4(A) for providing the clock pulse. The count is displayed on the 7-segment display.
The 7-segment decoder/driver (74LS47) accepts four binary-coded decimals (8421), generates their complements internally and decodes the data with seven AND/OR gates having the open-collector output to drive the display segments directly. Each segment-driver output is capable of sinking 40mA current in the ‘on’ state. Pins 3, 4 and 5 of the display driver are connected to Vcc to disable the ripple-blanking input (RBI), blanking input (BI)/ripple-blanking output (RBO) and lamp test (LT).
IC3 provides segment data to the 7-segment display through current-limiting resistors R3 through R9 (each 220 ohms).
IC 74LS74 (IC4) controls the reset pin (RST) of NE555. It is a dual D-type flip-flop with direct clear and set inputs and complementary outputs. The input data is transferred to the outputs on the positive edge of the clock pulse. Since the Q output is connected to the data input D, the flip-flops work in toggle mode.
Initially, reset pins 1 and 13 of the flip-flops are pulled high via resistor R10. When the reset pin of any flip-flop receives a low pulse from NAND gate N2 of IC8, the flip-flop is reset and its Q output goes high. On receiving a clock pulse, the Q and Q outputs of the flip-flop go high and low, respectively, and the LED turns on. The low output of IC4 resets the oscillators. The reset signal is derived with the help of NAND gates N3 and N4.
When switch S2 is pressed, both the oscillators and the respective counters start working. As soon as any of the counters counts ‘8,’ the corresponding display shows ‘8’ and LED glows. This means that oscillator has a higher frequency. Now both the counters stop counting because the flip-flop output goes low to reset both the astable oscillators.
In case the frequencies of both the astable oscillators are same, both the displays show ‘8’ and LED1 and LED2 glow at the same time.

Wireless Stepper Motor Controllers

Here is a low-cost and simple wireless stepper motor controller using infrared signals. Using this circuit you can control the stepper motor from a distance of up to four metres.
The circuit comprises transmitter and receiver sections. The communication between the transmitter and receiver sections is achieved through infrared signals.
Fig. 1: Infrared transmitter
In the transmitter section, timer NE555 ICs (IC1 and IC2) are configured as astable multivibrators with frequencies of around 1 Hz and 38 kHz, respectively. The output of IC1 is given to reset pin 4 of IC2, so the 38kHz carrier signal is modulated by 1Hz modulating signal. The modulated signal from pin 3 of IC2 is transmitted by the infrared LED. Resistor R5 limits the current through the IR LED.
The transmitted signal is sensed by IR receiver module TSOP1738 (IC6) of the receiver section and its output at pin 3 is used as clocks for dual flip-flop 74LS74 Ics (IC3 and IC4), which are configured as a ring counter.
Fig. 2: Infrared receiver and stepper motor driver circuit
When the power is switched on, the first flip-flop is set and its Q1 output goes high, while the other three flip-flops are reset and their outputs go low. On receiving the first clock pulse, the high output of the first flip-flop gets shifted to the second flip-flop. Thus on reception of every clock pulse, the high output keeps shifting in a ring fashion.
The outputs of flip-flops are amplified by the Darlington transistor array inside ULN2003 (IC5) and connected to the stepper motor windings marked ‘A’ through ‘D.’ The common point of the windings is connected to +12V DC supply.
To stop the motor, the flip-flops can be reset manually by pressing reset switch S1. On releasing the reset switch, the stepper motor again starts moving. If any interruption occurs between the transmitter and the receiver, the motor stops.

Manual Eprom Programmer

The programmer devices required for programming the electrically programmable read only memories (EPROMs) are generally expensive. Here is a low-cost circuit to program binary data into 2716 and 2732 EPROMs.
The circuit uses timer NE555 (IC2) wired as a monostable. When push-to-on switch S1 is pressed, IC2 generates a 50ms pulse, which is given to the program pin 18 of the ZIF SOCKET through switch S2. EPROM is inserted into the ZIF SOCKET for programming. LED1 glows to indicate the application of the programming pulse to the EPROM. Before applying the programming pulse to the EPROM, select the programming voltage (25V, 21V or 12.5V, as specified by the manufacturer) applied to pin 21 of the ZIF SOCKET by using jumper J1. The programming voltage required for an EPROM is sometimes written on its body. The address and data for the EPROM (ZIF SOCKET) are set by using DIP switches SW1 and SW2, respectively, whose pins are initially pulled high via 10-kilo-ohm resistors.
The AC mains is stepped down by transformer X1 to deliver 30V, 250 mA from the secondary. The secondary output is rectified by diode D1 and filtered by capacitor C1. The programming voltages of 25V, 21V and 12.5V are generated with the help of zener diodes ZD1, ZD2 and ZD3. IC1 is used to provide +5V regulated supply to the circuit.
To begin with, first read the programming voltage written on the EPROM. Now insert the EPROM chip into the 24-pin ZIF socket and slide switch S2 as per EPROM. Then connect the power supply to provide regulated 5V DC to the circuit. Select the programming voltage using jumper J1 and set the programming address and data value using switches SW1 and SW2, respectively. After providing the required programming voltage, press switch S1 to program the data at the desired address. Repeat this procedure for the next address and corresponding data.

DIGITAL AUDIO/ VIDEO INPUT SELECTOR

Need to connect more than one audio-video (AV) source to your colour television? Don’t worry, here’s an AV input expander for your TV. It is inexpensive and easy to construct.
The working of the circuit is simple and straightforward. Whenever 12V DC is applied to the circuit, power-on LED1 glows. Now reset the decade counter by momentarily pressing switch S2 to make Q0 output of IC1 high. LED2 glows to indicate that the circuit is ready to work.Switch S1 is used for selecting a particular audio-video (AV) signal. To select the first AV signal, press switch S1 once. To select the second AV signal, press switch S1 twice. In the same way, you can select the other two signals.
Momentarily pressing of switch S1 once results in clocking of the decade counter and relay driver transistor T1 conducts to energise relay RL1. Now normally opened (N/O) contacts of two-changeover relay RL1 connect the television set’s inputs to the first AV signal (marked as Video-In 1 and Audio-in 1). LED3 glows to indicate this.
When you press switch S1 twice, the Q2 output of IC1 goes high. Consequently, 2C/O relay RL2 (not shown in the circuit) energises and television inputs are connected to the second AV signal (not shown in the figure). LED4 (not shown in figure) glows to indicate this.
Similarly, pressing switch S1 thrice makes the Q3 output of IC1 high. Consequently, 2C/O relay RL3 (not shown in the figure) energises and the television inputs are connected to the third AV signal source. LED5 (not shown in the figure) glows to indicate this. Again, pressing switch S1 four times makes the Q4 output of IC1 high. Consequently, 2C/O relay RL4 energises and the TV inputs are connected to the fourth AV signal source (marked as Video-in 4 and Audio-in 4). LED6 glows to indicate this.
Further pressing of switch S1 resets the decade counter and LED2 glows again. Thereafter, the cycle repeats. The circuit is wired for four-input selection, therefore the Q5 output of IC1 is connected to reset pin 15 of IC1.
Enclose the assembled PCB along with the relays in a cabinet with the input/output sockets and indicators mounted on the body of the cabinet.

Automatic Bathroom Light with Back-up Lamp

sometimes we forget to switch off the bathroom light and it remains on unnoticed for long periods. This circuit solves the problem of electricity wastage by switching off the lamp automatically after 30 minutes once it is switched on. The back-up LED lamp provided in the circuit turns on for three minutes when mains fails. This is helpful especially when you are taking a shower at night.
The circuit is built around binary counter CD4060 (IC2), which has a built-in oscillator and 14 cascaded bistable multivibrators. The oscillator generates clock pulses based on the values of resistors R3 and R4 and capacitor C3.
For the given values, Q11 output of IC2 goes high after 30 minutes of power-on. Resistor R2 resets the IC for proper operation. The output of IC2 is fed to the gate of the SCR via resistor R6 and LED2, which function as a voltage dropper as well as output status indicator.
When the SCR gets gate drive, it fires to energise relay RL1. The latching function of the SCR keeps the relay energised until the power to the circuit is switched off using switch S1. When the relay energises, its normally closed (N/C) contacts break and light turns off. LED1 indicates that the oscillator is working.
The back-up white-LED lamp comprising LED3 and LED4 gives ample light in the event of mains failure. It is powered by a 9V rechargeable battery, which is charged at around 200mA current via diode D6 and resistor R7 when the circuit is switched on.
The back-up lamp circuit is built around timer NE555 (IC3) designed as a monostable. The output of IC3 goes high for three minutes based on the values of preset VR1 and capacitor C9. When the circuit is switched on, IC3 gets power supply via diode D6 and its trigger pin 2 remains high due to resistor R8. As a result, its output remains low as long as mains is present.
When power fails, pin 2 of IC3 get striggered via capacitor C8 and the monostable output goes high to switch on the white LEDs (LED3 and LED4). Resistor R9 limits the current through the LEDs to a safe level. Diode D7 is forward biased to give full voltage to the monostable when power fails.
The power supply for the circuit is derived from a 15V AC, 250mA transformer. The secondary output is rectified by a full-wave rectifier comprising diodes D1 through D4. Capacitor C1 smoothes the resulting DC. Regulator IC 7812 (IC1) and capacitors C4 and C5 provide stabilised 12V for the circuit.
Assemble the circuit on a Vero board and enclose it in a watertight plastic case. Connect the bathroom lamp (either 25-watt bulb or 11-watt CFL tube) to the circuit via N/C contacts of the relay, so that it turns on when switch S1 is pressed. For easy access, fix switch S1 along with the neon indicator outside the bathroom.

Simple Low-Power Inverter

Here is a simple low-power inverter that converts 12V DC into 230-250V AC. It can be used to power very light loads like window chargers and night lamps, or simply give shock to keep the intruders away. The circuit is built around just two ICs, namely, IC CD4047 and IC ULN2004.
IC CD4047 (IC1) is a monostable/astable multivibrator. It is wired in astable mode and produces symmetrical pulses of 50 to 400 Hz, which are given to IC2 via resistors R1 and R2.
IC ULN2004 (IC2) is a popular 7-channel Darlington array IC. Here, the three Darlington stages are paralleled to amplify the frequencies received from IC1. The output of IC2 is fed to transformer X1 via resistors R3 and R4.
Transformer X1 (9V-0-9V, 500mA secondary) is an ordinary step-down transformer that is used here for the reverse function, i.e., step up. That means it produces a high voltage. Resistors R3 and R4 are used to limit the output current from the ULN to safe values. The 230-250V AC output is available across the high-impedance winding of the transformer’s primary windings.

Power-on Reminder with LED Lamp

Many a times equipment at workstations remain switched on unnoticed. In this situation, these may get damaged due to overheating. Here is an add-on device for the workbench power supply that reminds you of the power-on status of the connected devices every hour or so by sounding a buzzer for around 20 seconds. It also has a white LED that provides good enough light to locate objects when mains fails. Fig. 1: Circuit of power-on reminder with LED lamp Fig. 1 shows the circuit of power-on reminder with LED lamp. Here, IC NE555 (IC1) is wired as an astable multivibrator, whose time period is set to around six minutes using resistors R1 and R2, preset VR1 and capacitor C1 for sounding the buzzer every hour. The output of IC1 is fed to the clock input of IC CD4017 (IC2). Capacitor C3 and resistor R3 provide power-on-reset pulse to IC2. When power to the circuit is switched on, pin 3 of IC2 goes high. After around one hour, its output pin 11 (Q9) goes high and the buzzer sounds. This cycle repeats until the two npn transistors. The LDR offers a very high resistance in darkness, i.e., when no light falls on it. Therefore when power fails, transistor T1 gets reverse biased to drive transistor T2 and the white LED (LED2) glows. The lamp circuit is powered by a 9V rechargeable battery, which is charged via resistor R5 when mains is present. Thus in darkness, the LED remains power to the circuit is switched off. Fig. 2: Power supply circuit The automatic lamp is built around a light-dependent resistor (LDR) and ‘on.’ Fig. 2 shows the power supply circuit. The AC mains is stepped down by transformer X1 to deliver a secondary output of 15V AC at 500 mA. The transformer output is rectified by a bridge rectifier comprising diodes D1 through D4, filtered by capacitor C5 and regulated by IC 7812 (IC3) to provide regulated 12V to the circuit. Capacitor C6 bypasses any ripple in the regulated output.

Mains Interruption Counter with Indicator

This circuit counts mains supply interruptions (up to 9) and shows the number on a 7-segment display. It is highly useful for automobile battery chargers. Based on the number of mains interruptions, the user can extend the charging time for lead-acid batteries.
Fig. 1: Circuit of mains interruption counter with indicator
Fig. 1 shows the circuit of the interruption counter with indicator. A 9V (PP3 or 6F22) battery powers the entire circuit. Fig. 2 shows the block diagram of the mains interruption counter circuit along with the battery charger and lead-acid battery as used in automobile battery charger shops.
When 9V is applied to the circuit, IC2 is reset by the power-on-reset signal provided by capacitor C3 and resistor R5 and the 7-segment display (DIS1) shows ‘0.’ The 230V AC mains is fed to mains-voltage detection optocoupler IC MCT2E (IC1) via capacitor C1 and resistors R1 and R2 followed by bridge rectifier BR1, smoothing capacitor C2 and current-limiting resistor R2. Illumination of the LED inside optocoupler IC1 activates its internal phototransistor and clock input pin 1 of IC2 is pulled down to low level.
Fig. 2: Block diagram of the arrangement used in automobile battery charger shops
IC CD4033 (IC2) is a decade counter/7-segment decoder. Its pin 3 is held high so that the display initially shows ‘0.’ Clock pulses are applied to clock input pin 1 and clock-enable pin 2 is held low to enable the counter.
Seven-segment, common-cathode display DIS1 (LTS543) indicates the mains interruption count. Capacitor C2 provides a small turn-on delay for the display.
When mains fails for the first time, clock input pin 1 of IC2 again goes high and display DIS1 shows ‘1.’ When mains resumes, pin 1 of IC2 goes low and DIS1 continues to show ‘1.’ When mains fails for the second time, clock input pin 1 of IC2 goes high and display DIS1 shows ‘2.’ When mains resumes, pin 1 of IC2 again goes low and DIS1 continues to show ‘2.’ This way, the counter keeps incrementing by ‘1’ on every mains interruption. Note that this circuit can count up to nine mains interruptions only.

Accurate Foot-Switch

Certain industrial controls require accurate switching operations. For example, in case of a foot-switch for precise drilling work, even a small error in switching may cause considerable loss. This low-cost but accurate foot-operated switch can prevent that.
IC NE555 is wired in one-shot mode. Its output pin 3 goes high only when both switches S1 and S2 are pressed simultaneously. You can release any one of the switches without changing the output state. When you release both the switches, the output goes low.
Fig. 1: Circuit of the foot-switch
The switches are placed under a foot paddle as shown in Fig. 2. LED1 is used as a warning indicator. If either S1 or S2 gets pressed erroneously, LED1 blinks to warn the operator. The operator can then withdraw his foot in case of a mistake or depress the other switch also to trigger the circuit. LED1 is to be mounted on the operator’s desk.
The circuit operation is simple. Resistors R2, R3 and R4 form a voltage divider. IC NE555 has two comparators, a flip-flop and power output section built into it. Pressing either S1 or S2 puts the input voltage between the upper comparator (2/3Vcc) and the lower comparator (1/3Vcc). Thus, it has no effect on the state of the internal flip-flop of IC NE555. Pressing the two switches simultaneously sets the flip-flop and the output of NE555 goes high. Transistor T2 energises relay RL1 for driving the load.
Fig. 2: Foot paddle switch
Releasing any of the switches brings the comparator voltage back to the initial level inside NE555 and it has no effect on the state of the flip-flop. Releasing both the switches brings the input level with respect to ground below the low trigger level, and thus it resets the output.
Use of the voltage divider results in stable operation over the entire permissible supply voltage range. The RC circuit at pin 4 provides power-on reset.
When only S1 is pressed, R3 (1 kilo-ohm) is less than R5 (1.5 kilo-ohms) and IC1 is not triggered. However, transistor T1 (BC548) gets forward biased and LED1 glows. When both S1 and S2 are pressed, the effective resistance between +Vcc and pin 2 of IC1 is about 500 ohms, which is less than R5 (1.5 kilo-ohms), and IC NE555 gets triggered.

TV Pattern Generator

This single-IC TV pattern generator is useful for fault finding in TV sets. You can correct the alignment of the timing circuits of the TV set with the help of this circuit. The vertical stripes (bars) produced by the pattern generator on the TV screen help you align the vertical scanning synchronisation circuit of the receiver.
To test the TV set, you need to connect the video and audio outputs of the circuit to the respective inputs of the TV set one by one. If the video section of the TV set is working the circuit generates vertical white lines on the TV screen, and if the audio section is working you hear sound from the TV’s speakers. You can also adjust the width of vertical lines.
The circuit uses hex Schmitt inverter IC CD40106 (IC1). NOT gate N1 generates horizontally synchronised (Hsync) pulses for the PAL video signal. Presets VR1 and VR2 are used to control the ‘on’ and ‘off’ time durations of the oscillator, respectively. For PAL, you need to adjust VR2 for ‘off’ duration of 4.7 µs, while VR1 needs to be adjusted for ‘on’ duration of around 60 µs.
If vertical lines appear on the TV screen on connecting the video output of the circuit to the video input of the TV, the video section of the TV set is working. You can control the starting position of the lines using potmeter VR3, the end position of the lines using potmeter VR4, and the line width and the number of lines using potmeter VR5.
If you don’t have an oscilloscope, set presets VR1 and VR2 to 150k and 22k, respectively, to get the required ‘on’ and ‘off’ periods for the oscillator and see the vertical line pattern on the TV.
The audio frequency oscillator is built around NOT gate N6. Its oscillation frequency is decided by resistor R6 and capacitor C5. Connect the audio output of the circuit to the audio input of the TV. If you hear sound from the TV’s speakers, the audio section of the TV set is working.

Sound-Operated Switch for Lamps

This inexpensive, fully transistorised switch is very sensitive to sound signals and turns on a lamp when you clap within 1.5 metres of the switch. One of its interesting applications is in discotheques, where lights could be turned on or off in sync with the music beats or clapping.
The condenser microphone senses the sound and converts it into electrical variations. The electrical signals are amplified by the two-stage direct-coupled (DC) amplifier formed by transistors T1 and T2 and fed to the switching circuit. The switching circuit comprises transistors T3, T4 and T5, which conduct only when the circuit senses sound signals. Transistor T5 supplies sufficient gate voltage to the triac to drive the 230V lamp.
The regulated 12V DC power supply for the circuit is derived from AC mains by using resistor R14, diode D1 and zener diode ZD1. The circuit can be assembled on any general-purpose PCB.

Mock Alarm with Call Bell

Here is a fully automatic mock alarm to ward off any intruder to your house. The alarm becomes active at sunset and remains ‘on’ till morning. The flashing light-emitting diodes (LEDs) and beeps from the unit simulate the functioning of a sophisticated alarm system. Besides, the circuit turns on and off a lamp regularly at an interval of 30 minutes throughout the night. It also has a call bell facility.
The circuit is built around CMOS IC CD4060B (IC1), which has an internal oscillator and a 14-stage binary divider to provide a long delay without using a high-value resistor and capacitor.
Press switch S2 to provide 9V power supply to the circuit. During daytime, light-dependent resistor LDR1 offers little resistance and transistor T1 conducts. This drives transistor T2 into the cut-off mode, as its base is pulled to ground via transistor T1. Reset pin 12 of IC1 remains high as long as transistor T2 is cut off. This keeps the oscillator of IC1 (comprising resistors R5 and R6 and capacitor C1) disabled and its outputs remain low. The sensitivity of LDR1 can be adjusted using preset VR1.
When the sunlight decreases in the evening, the resistance of LDR1 increases to cut off transistor T1. This drives transistor T2 into conduction mode and its collector voltage goes low. At the same time, reset pin 12 of IC1 goes low to enable the oscillator of IC1 and the oscillator starts oscillating. The O3 output (pin 7) of IC1 goes high every five seconds to light up the LEDs (LED1 and LED2) and activate the buzzer. Resistor R8 limits the tone produced from the buzzer.
At the same time, O13 output of IC1 (pin 3) goes high every 30 minutes to forward bias transistor T3 to energise relay RL1 and lamp L1 connected to the normally opened (N/O) contacts of relay RL1 glows. This cycle repeats till morning. The call bell is built around IC 4822 (IC2). Its inbuilt musical tone generator generates different tones at each trigger. The frequency of the tone can be controlled through external components R11 and C2. The output at pin 11 of IC2 is amplified by transistor T4.
When push-to-on switch S1 is pressed once, trigger pin 4 of IC2 gets a positive trigger from the positive rail (reduced by zener diode to 3.3V) via resistor R10 and IC2 starts producing a melody. Resistor R10 limits the current to the trigger pin of IC2 and resistor R12 prevents any false triggering. Zener diode ZD1 provides the 3.3V required for IC 4822.
The circuit works off 9V regulated power supply. Assemble the circuit on any general-purpose PCB and enclose it in a waterproof plastic box with holes for mounting LEDs on the rear and the LDR on the top of the box. Place the LDR such that sunlight falls on it directly. Mount the unit on the pillar of the entrance gate. To avoid unnecessary illumination of the LDR, install lamp L1 away from the unit in the porch of the house. Keep the speaker inside the room.

Rechargeable Torch Based on White LED

Rechargeable torches don’t come without problems. You need to replace the bulbs and charge the batteries frequently. The average incandescent light-emitting diode (LED) based torch, for instance, consumes around 2 watts. Here’s a rechargeable white LED-based torch that consumes just 300 mW and has 60 per cent longer service life than an average incandescent torch.
Fig. 1: Circuit diagram of rechargeable torch Fig. 1 shows the circuit of the rechargeable white LED-based torch. The reactive impedance of capacitors C1 through C3 (rated for 250V AC) limits the current to the charger circuit. The resistor across the capacitors provides a discharge path for the capacitors after the battery is charged. The red LED1 indicates that the circuit is active for charging.
The torch uses three NiMH rechargeable button cells, each of 1.2V, 225 mAH. A normal recharge will take at least 12 hours. Each full recharge will give a continuous operational time of approximately 2.5 hours. Recharge the battery to full capacity immediately after use to ensure its reliability and durability. The charging current is around 25 mA.
Fig. 2: Suggested enclosure for the torch
A voltage booster circuit is required for powering the white LEDs (LED2 through LED4). An inverter circuit is used to achieve voltage boosting. Winding details of the inverter transformer using an insulated ferrite toroidal core is given in the schematic. The number of 35 SWG wire turns in the primary and secondary coils (NP and NS) are 30 and 3, respectively. If the inverter does not oscillate, swap the polarity of either (but not both) the primary or the secondary winding. A reference voltage from resistor R5 provides a reflected biasing to the transistor, and keeps the output constant and regulated.
The suggested enclosure for the torch is shown in Fig. 2.

Multidoor Opening Alarm with Indicator

This door-opening alarm alerts you of intruders. You can use it for up to three doors.
You simply need to fit a small unit including reed switch on each doorframe and fix a magnet on the moving door such that the magnet aligns with the reed switch when the door is closed. A separate unit incorporating the power supply, three LEDs and a buzzer is to be kept by your side inside your room. A three-core ribbon cable from this unit goes to each door unit. One core goes to the positive terminal, second to the negative terminal and third to the output of the unit.
When the door is closed, the reed switch terminals are shorted and the alarm does not sound.
When door 1 is opened by someone, transistors in the corresponding door unit conduct and the buzzer sounds. LED1 glows to indicate opening of door 1. Due to diode latching action, the alarm will sound continuously even after the door is closed. It can be stopped only by pressing the reset switch of the door unit.
Regulated 9V to 12V DC for operating the circuit is derived from AC mains and fed to the three units mounted on the doors. Battery-backup is also provided. When all the three doors are simultaneously opened, all the three LEDs will glow.
This arrangement can be extended for more doors by increasing the number of door units connected to the audio-visual indication unit.

BRAKE FAILURE INDICATOR

Do you want to get an early warning of brake failure while driving? Here is a brake failure indicator circuit that constantly monitors the condition of the brake and gives an audio-visual indication. When the brake is applied, the green LED blinks and the piezobuzzer beeps for around one second if the brake system is intact. If the brake fails, the red LED glows and the buzzer stops beeping.
The circuit will work only in vehicles with negative grounding. It also gives an indication of brake switch failure.
In hydraulic brake systems of vehicles, a brake switch is mounted on the brake cylinder to operate the rear brake lamps. The brake switch is fluid-operated and doesn’t function if the fluid pressure drops due to leakage. The fluid leakage cannot be detected easily unless there is a severe pressure drop in the brake pedal. This circuit senses the chance of a brake failure by monitoring the brake switch and reminds you of the condition of the brake every time the brake is applied.
The circuit uses an op-amp IC CA3140 (IC2) as voltage comparator and timer NE555 (IC3) in monostable configuration for alarm. Voltage comparator IC2 senses the voltage level across the brake switch. Its non-inverting input (pin 3) gets half the supply voltage through potential divider resistors R3 and R4 of 10 kilo-ohms each. The inverting input (pin 2) of IC2 is connected to the brake switch through diode D1, IC 7812 (IC1) and resistor R2. It receives a higher voltage when the brake is applied.
Normally, when the brake is not applied, the output of IC2 remains high and the red LED (LED1) glows. The output of IC2 is fed to trigger pin 2 of the monostable through coupling capacitor C2. Resistor R1 is used for the input stability of IC2. IC1 and C1 provide a ripple-free regulated supply to the inverting input of IC2.
IC3 is wired as a monostable to give pulse output of one second. Timing elements R7 and C4 make the output high for one second to activate the buzzer and LED2. Usually, the trigger pin of IC3 is high due to R6 and the buzzer and LED2 remain ‘off.’
When the brake pedal is pressed, pin 2 of IC2 gets a higher voltage from the brake switch and its output goes low to switch off the red LED. The low output of IC2 gives a short negative pulse to the monostable through C2 to trigger it. This activates the buzzer and LED2 to indicate that the brake system is working. When there is pressure drop in the brake system due to leakage, LED1 remains ‘on’ and the buzzer does not sound when the brake is applied.
The circuit can be assembled on any general-purpose PCB or perforated board. Connect point A to that terminal of the brake switch which goes to the brake lamps. The circuit can be powered from the vehicle’s battery.
The circuit requires well-regulated power supply to avoid unwanted triggering while the battery is charging from the dynamo. IC4, C6 and C7 provide regulated 12V to the circuit. The power supply should be taken from the ignition switch and the circuit ground should be clamped to the vehicle’s body. A bicolour LED can be used in place of LED1 and LED2 if desired.

Battery Charger with Automatic Switch-off

This smart charger automatically switches off when your rechargeable batteries reach the full charge.
The circuit comprises a bistable multivibrator wired around timer IC 555. The bistable output is fed to an ammeter (via diode D1) and potmeter VR1 before it goes to three Ni-Cd batteries that are to be charged.
Normally, the full charge potential of an Ni-Cd cell is 1.2V. Trigger the bistable by pressing switch S1 and adjust potmeter VR1 for 60mA current through the ammeter.
Now remove the ammeter and connect a jumper wire between its points ‘a’ and ‘b.’ Connect the positive output terminal of the batteries to the emitter of pnp transistor T1. The base of transistor T1 is held at 2.9V by adjusting potmeter VR2. The output of transistor T1 is inverted twice by npn transistors T2 and T3.
Thus when the batteries are fully charged to 3×1.2V=3.6V, a voltage higher than this makes transistor T1 to conduct. Transistor T2 also conducts and transistor T3 goes off. The threshold level of timer 555 reaches 6V, which is more than 2/3×VCC = 2/ 3×6=4V, to turn off the timer.
During charging, the threshold level of the timer is held low. The green LED (LED1) glows during charging of the batteries and goes off at the attainment of full charge.
Note that this circuit can be used only for 1.2V, 600mAH Ni-Cd rechargeable batteries that require 60 mA of current for 15 hours to charge fully.

16-Way Clap-Operated Switch

Control your home appliances without getting out of your bed. You just have to clap in the vicinity of the microphone used in this circuit, which you can keep by the bedside. You can switch on/off up to four different electrical equipment (TV, fan, light, etc) in 16 different ways.
This circuit is built around timer IC 555 (IC1), CMOS IC 74LS93 (IC2) and five BC547 npn transistors (T1, T2, T3, T4 and T5). Transistor T1 is used as the preamplifier and the rest are used for driving the relays.
A small condenser microphone is connected at the base of transistor T1, which is biased from resistor R1 (10 kilo-ohms). The clapping sound is converted into electrical energy by the microphone and amplified by transistor T1. The transistor output is fed to the monostable circuit wired around IC 555. Output pin 3 of the timer is connected to the clock input of divide-by-16 IC 74LS93.
The outputs of IC2 are fed to npn transistors T2, T3, T4 and T5 via 100ohm resistors to drive relays RL1, RL2, RL3 and RL4 connected to appliances 1 though 4, respectively. Freewheeling diodes D1 through D4 connected across the relays protect the transistors from the back electromagnetic field (e.m.f.) produced by the relays.
The output states of IC 74LS93 (Q0 through Q3) for different numbers of claps are shown in the table.
The circuit is powered from regulated 5V DC. For testing the circuit, disconnect the resistors from the outputs of IC2 and connect four LEDs in series with 220-ohm resistors between the outputs and ground. Now switch on the power supply and clap near the microphone. You can see the four LEDs glowing in the manner shown in the table. A reset push switch is provided to switch off all the ‘on’ devices.
Now you can connect the desired appliances to the relays and control them with your claps.

White LED-Based Emergency Lamp and Turning Indicator

White LEDs are replacing the conventional incandescent and fluorescent bulbs due to their high power efficiency and low operating voltage. These can be utilised optimally for emergency lamp and vehicle turning indication. The circuits for the purpose are given here.
Fig. 1: White LED based emergency lamp
Fig. 1 shows the circuit of a white-LED based emergency lamp. You can also use arrays of white LEDs as daytime running lamps in automobiles.
In the emergency lamp, seven 1.2V AA-size Ni-Cd cells giving 8.4V have been used as the power source. The brightness is controlled by duty-cycle variation of an astable multivibrator working at 1 kHz. The astable multivibrator is built around IC1. Its output is connected to LED-driver transistor T1.
Up to six branches of white LEDs can be connected in parallel, with each branch containing two LEDs in series (only three branches are used here). Depending on the application, different combinations of battery voltages and the number of LEDs in series can be made such as to keep the resistive losses low.
Fig. 2: Battery charger
The charger circuit for a Ni-Cd battery is shown in Fig. 2. When the battery voltage is less than 9.8V, charging takes place since the voltage at the emitter of transistor T2 (VE) is 9.8V. The value of resistor R8 is chosen such that the battery charges at a rate of 70 mA per hour. The full charge voltage of the battery is 9.8V. When the battery reaches full voltage, the current reduces to approach the tickle charge value of few milliamperes.
Assemble both the circuits shown in Figs 1 and 2 on a general-purpose PCB. LEDs can also be mounted on the reflector of a lamp. After assembling, connect points A and GND of the emergency lamp circuit to the respective points of the battery charger circuit. Now your emergency lamp is ready to work.
To use the emergency lamp, switch on the circuit using switch S1. All the LEDs (LED1 through LED6) will glow to provide sufficient light.
Fig. 3: Turning indicator
Turning indicator shown in Fig. 3 is another application of the LEDs. It can be used for two-wheelers and draws limited power from the dynamo/battery. At low revolutions, headlight dims because of the increase in load. The white LED-based turning indicator circuit draws a fraction of the power drawn by conventional bulbs, and may last longer than the vehicle itself.
The circuit comprises two identical sections for left and right turn indications. The right turn indicator circuit is built around transistors T3 through T5 and white/yellow LEDs (LED8 through LED13). Similarly, the left turn indicator circuit is built around transistors T4, T6 and T7 and white/yellow LEDs (LED15 through LED20). Transistor T4 and the piezobuzzer are common for both-side indicators.
When you slide switch S2 towards right, blinking LED7, right-front LEDs (LED8 through LED10) and rear LEDs (LED11 through LED13) start blinking. Similarly, when you slide switch S2 towards left, blinking LED14, left-front LEDs (LED15 through LED17) and rear LEDs (LED18 through LED20) start blinking.
Transistor T3 acts as the buffer, while transistor T4 drives the buzzer. Transistors T5 and T7 drive the LEDs.
The LED array can be built using white LEDs or yellow LEDs depending on the colour of the indicator’s cover. In case you use yellow LEDs, keep in mind that the forward drop voltage is around 1.8V for a single yellow LED and therefore the value of the resistance should be changed in accordance with the increase in the number of LEDs in series.
Three white LEDs produce the light intensity of six yellow LEDs.

Inexpensive car Protection Unit

For car protection, custom-made units are available but they are costly.
Here’s a circuit to protect car stereo, etc from pilferage that costs less and requires no adjustments in the car but a good car cover.
Place the circuit at your bedside and bring the two wires from the unit to the car (parked outside your home) and connect one wire-end to the cover and the other to the ground, with both wire-ends shorted by some weight such as a brick. So outwardly the mechanism is not visible.
Fig. 1: Circuit of car protection unit with alarm
If someone tries to remove the cover, the alarm of the circuit starts sounding to alert you. The alarm can be switched off by resetting it using switch S1.
The car protection circuit comprises two timer ICs: one for the alarm circuit (see IC2 in Fig.1) and the other to indicate that the battery has taken over as the power source (see IC3 in Fig. 2). Normally, the protector operates off AC mains and the battery takes over only when mains fail. As the battery current is not high, the battery will last long.
As long as the two wires remain shorted, transistor T1 remains cut off. When shorting is removed, transistor T1 gets forward biased and its collector voltage drops to trigger IC2 and the piezobuzzer starts sounding.
Fig. 2: Battery takeover indicator
If mains fails, the battery-takeover indicator (shown in Fig. 2 and connected to points A, B and C in Fig. 1) immediately gets triggered at pin 2 of IC3. Its high output activates the battery-operation alarm for a couple of seconds. IC1 draws power from the battery to activate the protection unit.
After setting up the unit properly and shorting both the wires, press test switch S2. If there is no fault in the circuit, the alarm will sound. Now release test switch S2 and momentarily press reset switch S1 to switch off the alarm.

IC 555 Timer Tester

This simple and easy-to-use gadget not only tests the IC 555 timer in all its basic configurations but also tests the functionality of each pin of the timer. Once a timer is declared fit by this gadget, it will function satisfactorily in whatever mode or configuration you may try it.
The two basic configurations in which a timer IC 555 can be used are the astable and the monostable modes of operation.
When the DPDT switch (S2) is in position 1-1, the timer under test automatically gets wired as a monostable multivibrator. In this case, the monoshot can be triggered by the microswitch (S1). The debouncing circuit constituted by the two NAND gates of IC1 (N1 and N2) produces a clean rectangular pulse when the microswitch is pressed. Resistor R3, capacitor C1 and diode D1 ensure that the trigger terminal of timer IC 555 (pin 2 is the trigger terminal) gets the desired positive-to-ground trigger pulse. This differentiator circuit also ensures that the width of the trigger pulse is less than the expected monoshot output pulse.
The monoshot output pulse width is a function of the series combination of resistor R8 and potentiometer VR2, and capacitor C4. When DPDT switch S2 is in position 2-2, the timer gets configured for the astable mode of operation. The output is a pulse train with the high time period determined by the series combination of resistors R8, potentiometer VR2, resistor R9 and capacitor C4, whereas the low time period is determined by resistor R9 and capacitor C4.
The reset terminal of timer IC (pin 4) should be tied to Vcc normally. More precisely, the voltage at pin 4 should be greater than 0.8V. A voltage less than that resets the output. Whether you have connected the timer in the monoshot or astable mode of operation, the output goes low the moment you bring the reset terminal below 0.8V.
The control terminal (pin 5) can be used to change the high time (‘on’ time) of the output pulse train in the astable mode and the output pulse width in the monoshot mode by applying an external voltage. This external voltage basically changes the reference voltage levels of the comparators inside the IC. The levels are set by three identical resistors of usually 5 kilo-ohms inside the IC connected from Vcc to ground, at 2/3Vcc for pin 5 and 1/3Vcc for pin 2. These levels can be changed by connecting an external resistor between pin 5 and ground. Resistor R10 and potentiometer VR3 have been connected for this purpose.
The pulse width in the monoshotmode is given by:
1.1×total charging resistance×charging capacitance
This expression is valid when there is no external resistor connected at pin 5. The pulse width can be reduced by connecting an external resistor.
The high and low time periods in the astable mode are:
High time period = 0.69×chargingresistance×charging capacitance
Low time period = 0.69×dischargeresistance×capacitance
Again the expressions are true with no external resistor at pin 5. The high time period can be made to decrease by connecting an external resistor between pin 5 and ground.
The circuit can thus be used to check:
1. The timer IC in astable configuration.
2. The timer IC in monostable configuration.
3. The capability of the reset terminal to override all functions and rest the output to low.
4. The function of the control terminal to change the ‘on’ or the ‘high’ time of the output waveform in astable mode of operation and the output pulse width in monostable mode of operation.
The circuit operates off a 9V battery, which makes the gadget portable. You can construct it easily on any general-purpose PCB along with the 8-pin socket.
To test an IC 555:
1. Insert it into the socket.
2. Set switch S2 in position 1-1.
3. Switch on the power supply by flipping switch S3 to ‘on’ position. Power-indicator LED (LED3) glows to indicate that the circuit is ready to test the IC timer.
4. If the IC is okay, LED1 glows because the IC is wired as a monoshot and in the absence of any trigger, its output is low.
5. Apply the trigger pulse by momentarily pressing switch S1. LED1 stops glowing and, in turn, LED2 glows. This confirms that the output of the monoshot has gone high. After the predetermined time period, LED2 goes off and LED1 again glows. Vary preset VR2 and trigger the monoshot again through switch S1. You will find that LED2 glows this time for a longer or a smaller time period depending upon whether you increased or decreased VR2 resistance.
6. For checking the reset function of the timer, trigger the monoshot again, and before the expected time is over, quickly decrease the potmeter VR1 resistance so as to bring the voltage at pin 4 below 0.8V. You will observe the output going low (indicated by glowing LED1 and extinguished LED2).
7. For checking the control function of the timer IC, set potmeter VR1 again in the maximum resistance position. Also set preset VR3 in the minimum resistance position. Trigger the monoshot using switch S1. You’ll observe its output going high for a time period that is much less than that determined from the series combination of R8 and VR2, and capacitor C4. In fact, for any fixed setting of this series combination, the output pulse width can be observed to vary for different values of potmeter VR3 resistance—by triggering the monoshot several times, once for each setting of VR3.
8. Now set the DPDT switch in position 2-2. LED1 and LED2 glow alternatively with the timing determined by the resistances in the charge and discharge paths. This means the timer IC is okay and wired in astable mode.
9. The functions of reset and control pins can be checked in astable configuration too in the same way as discussed above for the monoshot configuration.

Dog Caller

Dog trainers use a whistle to call dogs. But why blow that irritating, loud whistle when the dog can hear a sound inaudible to the humans? We the humans can hear up to 20 kHz, but dogs can hear ultrasound (sound ranging between 20 and 30 kHz) also.
Here’s a circuit that generates 21 to 22 kHz (frequencies just above the audible range), so it can be used to call your pets by generating ultrasonic sound.
IC 555 is used as an oscillator. By adjusting the preset, ultrasonic sound of 21-22kHz frequency can be generated. Whistle effectiveness depends on the speaker used. Use of a low-wattage tweeter is recommended. (Don’t use an ultrasonic transducer, because it is designed for 40 kHz only.)
The circuit works off 9V. For portability, use a 9V PP3 battery and house the unit inside a pocket radio cabinet.

Timer for Geyser

This timer circuit for geyser sounds an alarm after the set timing of 22 minutes when the water is heated up.
The circuit comprises a timer IC 555 wired as an astable multivibrator with adjustable time period of 15 seconds. The astable output after inversion by an inverter drives decade counters IC3 and IC4 (each IC 7490) connected in cascade. The decade counters output is connected to decoders IC5 and IC6 (each IC 7442), respectively. The decoder outputs (Q8 outputs of IC5 and IC6) are fed to inverters and the inverter outputs, in turn, are fed to an AND gate. The AND output is connected to the reset pin of the astable multivibrator built around another timer IC 555 to sound the alarm. Now you can turn off the geyser.
A green LED (LED1) has been used as the power supply indicator.
Switch on the timer and the geyser at the same time. When the alarm sounds, it means that the water in the geyser has heated up and can be used.
You can assemble the timer circuit on a general-purpose PCB and install it near your bathroom so that both the timer circuit and the geyser can be switched on simultaneously.
After the siren sounds, if required, we can increase the time by another 22 minutes for geyser by resetting the circuit by pushing reset switches S1 and S2 momentarily.
If you want to change the preset time of the geyser, the same can be easily done by combining appropriate outputs of IC5 and IC6 using DIP switches (S3 and S4) while keeping in mind IC5 outputs (Q0 through Q9) are spaced 15 seconds apart and IC6 outputs are spaced 150 seconds (2.5 minutes) apart.
Caution. Please note that the timer circuit has no connection with the geyser circuit. The geyser works off 220V AC, while the timer works off 5V DC.

Multicell Charger

Using this charger, you can safely charge up to two pieces of Ni-Cd cells or Ni-MH cells. The circuit is compact, inexpensive and easy-to-use.
The 230V AC mains is down-converted to 12V AC (at 500 mA) by step-down transformer X1, converted into pulsating DC voltage by diodes D1 and D2, and fed to the battery charger terminals via current-limiting resistor R1 and silicon-controlled rectifier SCR1.
SCR1 is at the heart of the charger. Normally, it conducts due to the gate biasing voltage available through resistor R2 and diode D3, and the battery is in charging mode, which is indicated by LED1. Resistor R2 limits the charging current to a safe value. Charging current of this circuit is about 250 mA.
When the battery reaches full charge, SCR2 conducts to pull down the gate of SCR1. This state is indicated by LED2. Now remove the cells from the charger. Normally, Ni-Cd cell with a rating of 500 mAH will take around 2.5 hours to reach full charge, while the charging time for Ni-MH cell with a rating of 1500 mAH will be around 7 hours. Charging time may vary depending on the settings of the charger and input supply line conditions.
After construction, a minor adjustment is required for ensuring proper performance: Power on the circuit without cells and adjust VR1 such that LED2 lights up. Now measure voltage across the charger output terminals, which should be around 5V DC. Now insert the two cells into the holder and connect it to the charger output terminals for charging. LED1 instantly lights up to indicate the charging process. If LED1 glows dimly, readjust VR1 for proper glowing of LED1. Now the circuit is ready for use.
Use of a small heat-sink is recommended for SCR1.

Light Dimmer that Doubles as Voltmeter

Measure AC mains voltage without using a multimeter. All you need to do is to slightly modify the light dimmer fitted at the base of a table lamp for use as a voltmeter. When the dimmer is turned anticlockwise to a point where the filament glow is just visible, that point can be used as the reference point for measuring the voltage.
Fig. 1: Light dimmer
First, remove the old knob and fix a circular white paper around the shaft. Now put back a skirted knob with a cursor as close to the paper as possible and mark two extremities of the pot on the paper as CW and ACW (see Fig. 2).
Fig. 2: AC volts scale marking
Switch on the lamp via a variac and feed 50 volts. Rotate the potmeter knob anticlockwise until the filament glow is just visible and mark that point against the cursor as 50V. Keep on increasing the voltage to 100, 150, 180, 200 and 220 using the variac and calibrating the scale for all the voltages. Now a voltage scale is created. The only snag is that the voltage is increasing in anti-clock-wise direction, which should not be a problem. The scale will not however be linear unlike the one shown in the sketch. Accuracy will depend on the calibration standard used and the tolerance is of the order of 1 percent ±5 volts. The diameter of the knob of potmeter and fineness of cursor can be of help in getting better accuracy and tolerance.
Fig. 3: Pin configuration of BT134
An ordinary fan regulator can be used with a lamp of 40, 60 or 100 watts and calibrated accordingly. The minimum measurable voltage is naturally limited to the one required for ‘just visible’ condition. With R1 open circuited the maximum scale voltage will be around 220 volts.

220V Live Wire Scanner

This simple circuit lets you scan a 220V live wire. The clock input of the IC is connected to a wire, which acts as the sensor. Here, we have used 10cm length of 22SWG wire as the sensor.
When you hold the sensor (metallic conductor or copper wire) close to the live wire, electric field from mains activates the circuit. As the input impedance of the CMOS IC is high, the electric field induced in the sensor is sufficient to clock it. The output obtained at pin 11 of CD4017 drives the LED. Flashing of the LED (LED2) indicates the presence of mains, while LED1 indicates that the scanner is active.
The circuit can be used to find stray leakage from electrical appliances like fans, mixers, refrigerators, etc. It can be easily assembled on any general-purpose board or the discrete components can be directly soldered on the IC.
A 9V PP3 battery powers the circuit. If you use a mains adaptor, make sure that it is well regulated and isolated; otherwise, even the stray electric field from mains transformer will clock the circuit.
Caution. Use insulated wire as sensor to avoid risk of exposure to live AC mains.

Smart Switch

To switch on the mains voltage, either a mechanical switch or a relay offers a simple solution. However, the relay and its associated components occupy a lot of space and cannot be accommodated in a standard switch box. The ‘smart switch’ circuit, shown here, offers a better alternative. It is nothing but an ‘on’/‘off’ controller and uses an electronic circuit that behaves like a normal switch. A flat pushbutton control provides an aesthetic look to your switch panel.
Fig. 1: Circuit of the smart switch
The switching circuit comprises an optocoupler circuit that receives input from a bistable switch formed by a couple of Schmidt trigger gates that control a triac. The load can be switched on/off by simply pushing the pushbutton switch for a brief period. Every time the switch receives a push, the optocoupler toggles the triac. A special zero-crossing detector in the optocoupler supresses radio interference, unlike the arbitrary phase switching.
Fig. 2: Pin configuration of triac BT136
Since mains is not isolated, use a good-quality pushbutton switch with proper insulation to avoid lethal shock. Make sure that the triac can handle the current you are going to draw through it. If required, several pushbuttons can be wired in parallel to allow toggling of the triac from different locations.

Power Failure and Resumption Alarm

This circuit gives audio-visual indication of the failure and resumption of mains power. The circuit is built around dual timer IC LM556. When mains is present the bicolour LED glows in green colour, and when mains fails it turns red.
The AC mains is stepped down by transformer X1 to deliver the secondary output of 12V at 250 mA. The transformer output is rectified by a full-wave bridge rectifier comprising diodes D1 through D4, filtered by capacitor C1 and regulated by IC 7809 (IC1) to give regulated 9V DC to operate the circuit.
9V battery and pnp transistor T1 have been used here as the power source for red light indication of the absence of power. Transistor T1 can be made to conduct or cut-off easily by varying preset VR1.
Initially, when mains is present, pnp transistor T1 is in cut-off state and therefore bicolour LED1 glows in green colour.
When power fails, pnp transistor T1 starts conducting and bicolour LED1 glows in red colour. Due to non-availability of Vcc voltage at pin 14 of IC2, its output pin 9 remains low and transistor T3 does not conduct. However, capacitor C7 (4700µF) holds adequate charge and hence transistor T4 conducts and piezobuzzer PZ1 sounds continuously for around eleven seconds until capacitor C7 discharges completely.
When power resumes, bicolour LED1 glows in green colour and the buzzer beeps for around 14 seconds.
Dual timer IC LM556 (IC2) sections have been used here in monostable and astable modes, respectively.
In the monostable section, location of the external timing capacitor determines whether a positive or negative output pulse is generated. Diode D7 ensures that even a momentary power loss will cause a pulse to be generated when the power resumes. With capacitor C3 connected to ground, a positive output pulse is generated according to the following relationship:
T = 1.1×R5×C3. This positive output is present at pin 5 of IC2. Since IC2 is a dual-timer IC, its first output is directly fed to reset pin 10 of second section. Therefore the second timer of IC2 starts oscillating. Its frequency of oscillations (F0) is determined by resistors R6 and R11 and capacitor C6 as follows: F0=1.4/(R6+2R11)×C6.
IC LM556 outputs frequencies in the form of pulses at its pin 9. These pulses are coupled to npn transistor T3, which conducts and cuts off depending on the output at pin 9 of IC2. Red LED2 is connected to pin 9 via current-limiting resistor R7 (270-ohm) to indicate power resumption.
The collector output of transistor T3 is directly fed to the base of pnp transistor T4, due to which base biasing of T4 varies and the buzzer beeps for around 14 seconds.

Doorbell-Cum-Visitor Indicator

This doorbell circuit can also give identification of the visitor to your home in your absence. When you’re home, you can use it simply as a normal doorbell.
The circuit (see Fig. 1) comprises a monostable built around timer IC 555 (IC1), relay driver transistor BC548 (T1), inverter section built around IC 7404 (IC2), latching section built around IC 555 (IC3) and LED display driver transistor BC548 (T2).
Fig. 1: Circuit diagram of doorbell-cum-visitor indicator
The monostable output drives the relay through transistor amplifier T1. The normally-opened (N/O) contact of the relay connects an electric bell with mains supply as shown in Fig. 1. The output of the monostable also goes to inverter N1, which, in turn, enables the latching circuit built around IC3 in conjunction with DPDT slide switch S2. The output of IC3 at its pin 3 is fed to LED1 (visitor-out indicator) and LED2 through LED16 via LED driver transistor T2.
The switch panel shown in Fig. 2 is to be mounted on the entry gate or door, while the LED display panel shown in Fig. 3 is to be kept inside the house near the owner’s desk.
Fig. 2: Switch panel to be mounted on the gate
Fig. 3: Suggested LED display to be kept nearthe owner’s desk
Before you leave your house, slide switch S2 (Out) to ‘on’ position and press reset switch S3 once. When somebody visits your home and presses doorbell switch S1, it will trigger the monostable (IC1) and also energise the relay to ring the bell. The monostable output through inverter N1 will enable the latching circuit and LED1 will glow continuously to indicate that you’re out of the house.
The message “Please indicate the first letter of your name (single alphabet in English only) by flipping switches S4 through S18 to ‘on’ position,” as required, is written just below LED1 indicator as shown in Fig. 2.
Suppose the visitor’s name is Tina Chopra. As the initial alphabet of her first name is ‘T,’ she has to flip the topmost-row and middle-column switches towards ‘on’ position. The position of the switches for this example is shown in Fig. 2.
When you return home, just flip switch S19 to ‘on’ position to check the visitor’s initial. The topmost-row and middle-column LEDs will glow to indicate the alphabet ‘T,’ so you know that one of your friends having name starting with ‘T’ had visited your place in your absence. The status of the LEDs for this example is shown in Fig. 3 by the red LEDs.
Now you can slide switch S2 to ‘off’ position or press reset button S3 once. When the indicator is not in use, slide DPDT switch S2 to ‘off’ position to bypass the latching and LED display sections by cutting off the power supply and thus prevent unnecessary continuous drain of power. Now the rest of the circuit will work as a doorbell only.
The rows and columns may be increased to accommodate a 5×7 display matrix. The circuit can be used for a single visitor only.

Zener Value Evaluator

Using this simple circuit and a known-value zener diode, you can find the breakdown voltage value of any zener diode. The circuit is divided into two sections: zener evaluator and display unit. Regulated 12V and 5V are required to power the zener evaluator section, while the display section works off only 5V. Connect +5V, point A and ground of the zener evaluator section to the respective terminals of the display section.
The zener evaluator circuit (Fig. 1) comprises a linear ramp generator built around timer NE555 and an astable multivibrator built around another NE555. The resistor of the monostable is replaced with a constant-current source formed by transistor T1. Capacitor C2 is charged linearly by the constant-current source formed by transistor T1.
The time period T of the linear ramp generated by IC1 at its pin 6 across capacitor C2 is given by:
On substituting the values shown in Fig. 1, you get:
T= 0.15 second (approx.)
This value is equal to Ton of the monostable without connecting the zener at the control voltage terminal pin 5.
Now connect the zener to the control voltage terminal and trigger the monostable (IC1) by momentarily pressing switch S1. The output pulse width of IC1 is fed to the astable multivibrator (IC2). The time period of the astable multivibrator is around 7 milliseconds (ms) and it oscillates as long as the ramp output of IC1 is high.
The display unit comprising decade counter ICs 74LS90, decoder/driver ICs 74LS47 and 7-segment common-anode displays LTS542 is shown in Fig. 2. Decade counters IC3 and IC4 count the frequency applied on clock pin 14 of IC3 from pin 3 of IC2.
IC 74LS90 is a 4-bit ripple decade counter. When the output of IC3 is ‘10’ (1001), it provides clock at pin 14 of IC4 (via AND gate N1) for further counting. For resetting IC3 and IC4, simply press reset switch S3 momentarily.
The outputs of decade counters IC3 and IC4 are connected to 7-segment decoders/drivers IC6 and IC5, respectively, which, in turn, are connected to common-anode displays DIS1 and DIS2 for displaying the frequency of astable multivibrator IC2 used to evaluate the unknown value of the zener diode.
Let’s say the counter counts up to N1 for the known zener diode breakdown voltage value X1 and up to N2 for the unknown zener diode value X2. Now, you can calculate the value of the unknown zener diode from the following relationship:
Suppose you have a zener diode rated at 6.8V (X1). Insert it at the position marked ‘ZUT’ of Fig. 1 and press trigger switch S1 momentarily. The counter counts up to, say, ‘29’ (N1), which is shown on the display. Now remove the 6.8V zener and insert the zener of unknown value (X2). The display now shows, say, 15 counts (N2).
From Eq. (2), the value of the unknown zener (X2) can be calculated as 3.5V.

Liquid-Level Alarm

In water-level controllers for tanks, a DC current is passed through the metallic probes fitted in the water tank to sense the water level. This causes electrolysis and corrosion of probes, inhibiting the conduction of current and degrading its performance. As a consequence, probes have to be replaced regularly to maintain proper current flow.
The liquid-level alarm given here overcomes this problem. A 1kHz AC signal is passed through the probes, so there will be no electrolysis and therefore the probes last longer.
Fig. 1: Block diagram of liquid level alarm
The block diagram for the liquid-level alarm is shown in Fig. 1. The signal generator sends the generated signal to the first metallic probe. The second metallic probe is connected to the sensing circuit followed by the alarm circuit.
The complete circuit for the liquid-level alarm is shown in Fig. 2. The astable multibrator built around IC 555 (IC1) generates 1kHz square wave signal, which is fed to one of the probes via a DC blocking capacitor.
Fig. 2: Liquid level alarm
Fig. 3: Pin configuration of UM66
When the water tank is empty, pnp transistor T1 does not get negative base bias. But as water fills up in the tank, it receives 1kHz signal from IC1 via the probes immersed in water and conducts during the negative half cycle of 1kHz signals. Due to the presence of capacitor C7 (2.2µF), npn transistor T2 continues to get base bias and conducts to provide 3.3V DC to melody generator IC UM66 (IC2). Pin configuration of IC UM66 is shown in Fig. 3. Preset VR1 acts as the output loudness controller. It can be varied to set the alarm sound from the speaker at the desired level.
The circuit works off 12V unregulated power and can be used to detect any conductive liquid.

Electronic Fuse

An absolute necessity of every electronics lab is a workbench power supply. The power supply should be regulated and protected against short circuit.
Most power-supply protection circuits use a low-value, high-wattage resistor connected in series with the load for current sensing. The voltage drop across the sensor resistor is weighed to activate the protection circuit. The given circuit is based on a polyfuse application, which is a resettable fuse by itself.
Fig. 1: Electronic fuse
Initially, when the circuit is powered, silicon-controlled rectifier SCR1 is ‘off.’ Relay RL1 energises through the polyfuse and the load is connected through the normally opened (N/O) contact of the relay. When the current drawn by the load increases above a certain level (which depends on the number of turns in the winding on reed relay, see Fig. 2 and the accompanying table), the contacts of reed relay RL2 close to trigger SCR1. As a result, relay RL1 de-energises and the load gets disconnected. The polyfuse remains in high-resistance state until SCR1 is turned off.
Fig. 2: Reed relay with coil winding
The circuit can be reset either by switching off the power supply or by pushing reset switch S1. LED2 indicates that the power supply is working normally. LED1 indicates that the power supply unit is under the protection mode and the buzzer sounds to warn the user.
The turns of reed relay winding are based on the current drawn through the load, so refer to the table for winding details for your load current requirements. At EFY, testing was done for approximately 1.85A AC load current at 230V AC mains and accordingly 16 turns of 22SWG copper-enamelled wire were wound on the reed relay.

Bicycle Guard

This antitheft device for bicycles is inexpensive and can be constructed easily using a few components.
At the heart of the circuit is a wheel rotation detector, realised using a DC micro motor. For the purpose, you can use the micromotor (spindle motor) of a discarded local CD deck mechanism. With a little skill and patience, you can easily attach a small metallic pulley covered with a rubber washer to the motor spindle. Thereafter, fix the unit in the back wheel of the cycle, like the existing dynamo assembly.
Power supply switch S1 should be kept ‘on’ when you are using this bicycle guard. When it is flipped towards ‘on’ position, the circuit gets power from the miniature 12V battery. Now LED1 lights up and resistor R4 limits the LED current. Next, the monostable built around IC1, which is CMOS version of timer LM555, is powered through a low-current, fixed-voltage regulator IC2 (78L05).
Initially, when the bicycle is standing still, the monostable output at pin 3 of IC1 is low and the circuit is in idle state. In the event of a theft attempt, forward or reverse rotation of the DC motor induces a small voltage at its DC input terminals and the internal LED of 4-pin DIP AC input isolator optocoupler IC3 (PS2505-1 or PC814) glows. As a result, the internal transistor of IC3 conducts and pin 2 of IC1 is pulled low by the optocoupler and the monostable built around IC1 is triggered.
The output at pin 3 of IC1 now drives piezobuzzer-driver transistor T1 via resistor R3 and the buzzer starts sounding to alert you. In this circuit, the buzzer remains ‘on’ for around two minutes. You can change this time by changing the values of resistor R2 and capacitor C1.
Zener diodes ZD1 and ZD2 (each 5.1V) act as a protector for optocoupler IC3. The costly GP12V/27A battery is used here due to its compact size and reliability. 12V active buzzers with high-pitched tone output may be used with this circuit. These are readily available in the market.
Note. The specific optocoupler is used here deliberately, instead of a bridge rectifier, to increase the circuit’s detection sensitivity. Never replace the same with a DC optocoupler.

Water-Tank Overflow Indicator

Water is a vital but scarce natural resource. To prevent water wastage, this water-tank overflow indicator comes in handy. It gives audio as well visual alarm whenever the water tank overflows.
Fig. 1: Circuit of the water-tank overflow audio-visual indicator
Fig. 1 shows the water-tank overflow indicator circuit and Fig. 2 shows the power supply circuit.
In the power supply unit, mains AC is stepped down by transformer X1 to deliver secondary output of 9V-0-9V AC at 300 mA. The transformer output is rectified by a full-wave bridge rectifier comprising diodes D1 through D4 and filtered by capacitors C1 and C2 to provide +9V at ‘+B’ point and –9V at ‘–B’ point. Connect ‘+B,’ ‘–B’ and ‘GND’ terminals of the power supply unit to the respective terminals of the water-tank overflow indicator circuit.
Fig. 2: Power supply circuit
The circuit is built around op-amp LM741 (IC1), which is used as a comparator. The pin configuration of melody generator IC1(UM66) is shown in Fig. 3.
When water in the tank is below the metal plate sensors, inverting pin 2 of IC1 is at a higher potential than non-inverting pin 3. Output pin 6 of the op-amp is low and there is no music from programmable melody generator IC UM66 (IC2). Transistor BC547 (T1) remains cut-off and therefore LED1 doesn’t glow and the loudspeaker remains silent.
Fig. 3: Pinconfigurationof UM66
When water in the tank touches the metal plate sensors, it extends ground to pin 2 of IC1. Now pin 3 of IC1 is at a higher potential than pin 2. The high output of the op-amp generates 3.1V across zener diode ZD1. Melody generator IC2produces a melody, which drives the transistor to light up LED1 and sound an alarm from the loudspeaker. Rectifier diode D5 is used to prevent negative polarity to the cathode of the zener diode

Transistor Tester

You can test both npn and pnp transistors using this circuit. The circuit indicates whether the transistor is good, open or shorted through two light-emitting diodes (LEDs).
The circuit comprises two NE555 timer ICs: one (IC1) is wired in the astable mode and the other (IC2) in the monostable mode. The time period of the astable multivibrator is around 0.5 second. Its output goes to the base of the npn/pnp transistor under test via DPDT switch S2.
Switch S2 selects the npn/pnp transistor you are going to test, which means that at a time only an npn or a pnp transistor can be tested. The collector of npn or pnp transistor goes to reset pin 4 of the monostable (IC2). Switch S3 is used to trigger the monostable. The time period of the monostable multivibrator is around two seconds.
To test a transistor, insert it at the appropriate place shown within dotted lines and slide switch S2 towards the transistor type (npn or pnp) being tested. From glowing of LED1 and LED2 on triggering of the monostable via switch S3, you can infer whether the transistor is good, short or open-circuited, as shown in the table.

Simple Smoke Detector

This simple smoke detector is highly sensitive but inexpensive. It uses a Darlington-pair amplifier employing two npn transistors and an infrared photo-interrupter module as the sensor. The circuit gives audio-visual alarm whenever thick smoke is present in the environment.
Fig. 1: Top and bottom views of the photo-interrupter module (H21A1)
The photo-interrupter module (H21A1) consists of a gallium-arsenide infrared LED coupled to a silicon phototransistor in a plastic housing. The slot (gap) between the infrared diode and the transistor (see Fig. 1) allows interruption of the signal with smoke, switching the module output from ‘on’ to ‘off’ state.
The circuit of the smoke detector is shown in Fig. 2.
Fig. 2: Schematic of the smoke detector
When the smoke enters the gap, the IR rays falling on the phototransistor are obstructed. As a result, the phototransistor stops conducting and the Darlington-pair transistors conduct to activate the buzzer and light up LED1.
When the smoke in the gap is cleared, light from the IR LED falls on the phototransistor and it starts conducting. As a result, Darlington-pair transistors stop conducting and the buzzer and LED1 turn off.
For maximum sensitivity, adjust presets VR1 and VR2. VR1 is used to control the sensitivity of the photo-interrupter module, while VR2 is used to control the sensitivity of Darlington-pair transistors.

Remote Emergency Alarm for Unmanned Lifts

In unmanned lifts or elevators, sudden power failure cannot be detected from the remote operating room, and this can prove dangerous for the lift users. Here is a simple circuit that sounds an alarm in the remote lift/elevator control room in the event of power failure. The circuit operates off a 6V DC battery.
If the power shutdown continues, the buzzer would keep sounding and drain the battery. To prevent the battery from draining, the buzzer is made to stop sounding after a predetermined period. As soon as power resumes, the alarm circuit disconnects from the battery and the battery starts charging from mains.
For charging the battery, 230V AC mains is stepped down and rectified by diodes D1 and D2. The battery is charged through diode D5 and the normally-closed (N/C) contacts of relay RL1.
During this time, transistor T1 conducts because diode D4 is conducting, and that drives transistor T2 into cut-off region. So relay RL1 de-energises and the battery continues charging up.
When power fails, transistor T1 cuts off to drive transistor T2 into conduction and relay RL1 energises. Now the battery disconnects from the charging position and connects to the normally-open (N/O) contacts of relay RL1. Timers IC2 and IC3 get power supply (6V) from the battery through N/O contacts of relay RL1.
Timer IC3 generates pulses with a fixed interruption for reset pin (pin 4) of IC2, which, in turn, generates 1kH zaudio pulse for audio amplifier IC5. The loudspeaker sounds an alarm with interruption determined by IC3.
To stop the buzzer after a preprogrammed time, preprogrammable IC CD4541 (IC1) is used. After the preset time period (defined by (R1+ VR1).C2), output pin 8 of IC1 goes high to drive transistor T1. Consequently, transistor T2 cuts off and relay RL1 de-energises. As a result, IC2 and IC3 disconnect from the battery supply and the loudspeaker stops sounding.
IC1 starts counting only when its pin 6 goes low, i.e., when power fails. This is achieved by connecting pin 6 of IC1 to the power supply through diode D4.
In the presence of mains, IC1 does not oscillate. When mains fails, pin 6 (master reset pin) of IC1 goes low and it starts counting time. Here pins 12 and 13 of IC1 are set high so that the maximum time count can be obtained. Here time count is around 50 seconds. That means the loudspeaker sounds an alarm for 50 seconds with interruption.
IC5 is a simple low-power audio amplifier that produces a high output power with low harmonic and crossover distortion. It is used here for amplifying the alarm sound produced by IC2.
Assemble the circuit on a simple PCB and install it in the control room for the lift/elevator such that mains input is common for the lift/elevator as well as the circuit.

Audio-Controlled Running Light

This mains-operated audio-controlled running light can be used in discotheques. The lamps glow in running sequence as per the sound of music. Of the ten AC lamps, only one lamp permanently glows if there is no sound. When music is played, light starts ‘running’ through the lamps.
Fig. 1: Circuit for audio-controlled running light
Fig. 1 shows the circuit for the audio-controlled running light, while Fig. 2 shows the pin configuration of triac BT136.
The condenser mic converts audio signals into electric signals. Transistor T1 amplifies the microphone signals, which provide clock pulses to decade counter IC CD4017 (IC1). Preset VR1 is used to vary the signal level. The Q9 output provides reset signal to pin 15 of IC1. Divided-by-10 signals are fed to clock pin 14 of IC2, which is another decade counter. The outputs of IC2 drive transistors to provide triggering pulses to triacs. Triacs, in turn, drive the AC lamps. You can now see the lights running with the sound of the music.
Fig. 2: Pinconfigurationof triac BT136
Frequency-divider ICs CD4017 have been used here to reduce the audio to a noticeably low frequency.
AC mains is rectified by diode D11, regulated by zener diode ZD1 and filtered by capacitor C4 to power the circuit.

Power Supply Reversal Correcter-Cum-Preventer

When power-supply polarities of an electronic device are accidentally interchanged, the device runs the risk of damage. The danger can be avoided by adding this tiny circuit to the power supply section of the device. The circuit will instantly correct the interchanged poles of the power supply and warn of the error by raising an alarm accompanied with a visual indication. The control element is nothing but a relay (RL1). Its supply is made unidirectional by a series-connected diode (D1). Thus in normal state the relay is idle. This arrangement is connected in reverse across the power-supply output. The output of the add-on circuit, which is taken through the normally closed contacts of the relay, via fuse F1, supplies power to the load. This way, unwanted loading of the power supply by the relay is avoided. The normally closed (N/C) contacts of relay RL1 are connected to the input terminals of the power supply in the required polarity, while the normally open (N/O) contacts are connected to the reverse-polarity terminals. LED1 and LED2 are bicolour LEDs having only two pins each as against the usual three pins. You can also make the 2-lead bicolour LED by soldering the leads of a green LED and a red LED together in reverse polarity. When the input polarity is correct the LEDs glow green, and to indicate error these glow red. LED1 indicates the status of the input DC, while LED2 shows the status of the output. LED2 is primarily used to warn of stuck-up contacts of RL1. Another simple protection circuit added at the output of this circuit comprises diode D2 and fuse F1. Diode D2 is connected across the power supply in reverse polarity, so that if the relay takes a little longer to energise during any error, supply reversal forward biases diode D2 and virtually shorts the DC output. Thus the fuse is blown instantly to remove power from the load and the consequent damage to the circuit is averted. If the supply to the add-on circuit gets reversed, diode D1 forward biases to activate relay RL1. The contacts of relay RL1 change over and so the output is interchanged with respect to the input polarity, resulting in the correct output polarity. At the same time, piezobuzzer PZ1 also gets forward biased through diode D1 to start sounding the alarm indicating supply reversal. This indication by LED1 and PZ1 goes on until the error is corrected. There is no harm to the electronic device connected to the power supply even if one neglects to correct the interchanged polarities, since they are attended to automatically and corrected. This circuit is far more efficient than the commonly used diode-bridge protection arrangement because the bridge diodes affect the power supply levels, especially if it is a low-voltage storage battery. The circuit can be assembled on the relay itself since all other components like the LEDs and the fuse are mounted on the front panel in their respective holders. The supply input and load terminals can be screw type for ease of connections if it is to be used as a standalone unit.

Panic Plate

WUseful for the elderly and ailing persons, this touch-sensitive circuit sounds a panic alarm to catch the attention of others for immediate help. The touch plate fixed on the wall near the bedside gives an easy access to the person on bedrest so that he may call for assistance without much difficulty. Yellow LED3 on the panel indicates the call and the red LED indicates an immediate attention.
The circuit is based on the electric charge in the human body generated from within or obtained from the surrounding electric field. When the sensor plate is touched, the electric charge present in the body biases transistor T1 and it conducts. This triggers IC 7555 (IC1) and its output goes high for a while. IC 7555 is the CMOS version of the common timer IC 555.
Fig. 1: Circuit for panic plate
The high-going output pulse from IC1 triggers IC HEF4017 (IC2), which is a Johnson decade counter IC with ten decoded outputs. The counters within the IC advance one by one at the positive clock pulse on its clock pin 14, provided clock-inhibit pin 13 is low. Pin 15 resets the IC when it receives a positive signal.
When IC2 receives the clock pulse from IC1 through diode D1 and resistor R2, its Q1 output goes high to power LED3 and melody generator IC3. The tone from IC3 is amplified by T2 and heard from the speaker. Forward-biased diode D2 and resistor R6 provide current to IC3.
When IC2 receives another clock pulse, its Q2 output goes high and the call tone stops and red LED lights to indicate the need for immediate attention. The green LED indicates the standby mode and glows at power-on. Capacitor C2 prevents accidental triggering of the alarm.
The Q3 output (pin 7) of IC2 is shorted with reset pin 15, so IC2 resets when it receives the third clock pulse.
In short, the first touch will make a call, the second touch will stop the call and light up the red LED, and the third touch will reset the IC and light up the green LED after getting the assistance. So if the red LED remains lit, it indicates that nobody has attended the person even after a call was made. IC4 and C3 provide regulated 5V DC to the circuit.
Fig. 2: Pin configurationsof UM66 and 5V regulator
The circuit can be easily assembled on a general-purpose PCB. Use 7555 and HEF4017 ICs for higher reliability, and 5mm glass LEDs for indication. The touch plate can be made from a 15×15cm aluminium or tin sheet. It should be connected to the circuit using a 1m plastic-jacketed wire. Fix the speaker at a place where the sound can be heard easily by the family members.
The circuit can be powered by a 9V PP3 battery or mains operated 9V adaptor. Fig. 2 shows pin configurations of UM66 and 6V regulators.