201 ഹോബി സര്ക്യൂട്ടുകള്
The first thing you will want to know is: HOW DOES A TRANSISTOR WORK?
Diagram "A" shows an NPN transistor with the legs covering the symbol showing the name for each lead.
The
transistor is a "general purpose" type and and is the smallest and
cheapest type you can get. The number on the transistor will change
according to the country where the circuit was designed but the types we
refer to are all the SAME.
Diagram "B" shows two different
"general purpose" transistors and the different pinouts. You need to
refer to data sheets or test the transistor to find the correct
pinout.
Diagram "C" shows the equivalent of a transistor as a water
valve. As more current (water) enters the base, more water flows from
the collector to the emitter.
Diagram "D" shows the transistor
connected to the power rails. The collector connects to a resistor
called a LOAD and the emitter connects to the 0v rail or earth or
"ground."
Diagram "E" shows the transistor in SELF BIAS mode. This
is called a COMMON EMITTER stage and the resistance of the BASE BIAS
RESISTOR is selected so the voltage on the collector is half-rail
voltage. In this case it is 2.5v.
To keep the theory simple, here's how you do it. Use 22k as the load resistance.
Select the base bias resistor until the measured voltage on the collector 2.5v. The base bias will be about 2M2.
This is how the transistor reacts to the base bias resistor:
The
base bias resistor feeds a small current into the base and this
makes the transistor turn on and create a current-flow though the
collector-emitter leads.
This causes the same current to flow
through the load resistor and a voltage-drop is created across this
resistor. This lowers the voltage on the collector.
The lower
voltage causes a lower current to flow into the base and the
transistor stops turning on a slight amount. The transistor very quickly
settles down to allowing a certain current to flow through the
collector-emitter and produce a voltage at the collector that is just
sufficient to allow the right amount of current to enter the base.
Diagram
"F" shows the transistor being turned on via a finger. Press hard on
the two wires and the LED will illuminate brighter. As you press
harder, the resistance of your finger decreases. This allows more
current to flow into the base and the transistor turns on harder.
Diagram
"G" shows a second transistor to "amplify the effect of your finger"
and the LED illuminates about 100 times brighter.
Diagram "H" shows
the effect of putting a capacitor on the base lead. The capacitor
must be uncharged and when you apply pressure, the LED will flash
brightly then go off. This is because the capacitor gets charged when
you touch the wires. As soon as it is charged NO MORE CURRENT flows
though it. The first transistor stops receiving current and the
circuit does not keep the LED illuminated. To get the circuit to work
again, the capacitor must be discharged. This is a simple concept of
how a capacitor works. A large-value capacitor will keep the LED
illuminated for a longer period of time.
Diagram "I" shows the
effect of putting a capacitor on the output. It must be uncharged for
this effect to work. We know from Diagram G that the circuit will
stay on when the wires are touched but when a capacitor is placed in
the output, it gets charged when the circuit turns ON and only allows
the LED to flash. 1. This is a
simple explanation of how a transistor works. It amplifies the current
going into the base about 100 times and the higher current flowing
through the collector-emitter leads will illuminate a LED. 2. A capacitor allows current to flow through it until it gets charged. It must be discharged to see the effect again.
Read the full article HERE
RESISTOR COLOUR CODE 
See resistors from 0.22ohm to 22M in full colour at bottom of this page and another resistor table
TESTING AN unknown TRANSISTOR The
first thing you may want to do is test an unknown transistor for
COLLECTOR, BASE AND EMITTER. You also need to know if it is NPN or
PNP.
You need a cheap multimeter called an ANALOGUE METER - a multimeter with a scale and pointer (needle).
It
will measure resistance values (normally used to test resistors) -
(you can also test other components) and Voltage and Current. We use the
resistance settings. It may have ranges such as "x10" "x100"
"x1k" "x10"
Look at the resistance scale on the meter. It will be the top scale.
The scale starts at zero on the right and the high values are on the left. This is opposite to all the other scales. .
When
the two probes are touched together, the needle swings FULL SCALE
and reads "ZERO." Adjust the pot on the side of the meter to make the
pointer read exactly zero. How to read: "x10" "x100" "x1k" "x10"
Up-scale from the zero mark is "1"
When the needle swings to this position on the "x10" setting, the value is 10 ohms. When the needle swings to "1" on the "x100" setting, the value is 100 ohms.
When the needle swings to "1" on the "x1k" setting, the value is 1,000 ohms = 1k.
When the needle swings to "1" on the "x10k" setting, the value is 10,000 ohms = 10k.
Use this to work out all the other values on the scale.
Resistance
values get very close-together (and very inaccurate) at the high end
of the scale. [This is just a point to note and does not affect
testing a transistor.] Step 1 - FINDING THE BASE and determining NPN or PNPGet an unknown transistor and test it with a multimeter set to "x10"
Try
the 6 combinations and when you have the black probe on a pin and
the red probe touches the other pins and the meter swings nearly full
scale, you have an NPN transistor. The black probe is BASE
If the
red probe touches a pin and the black probe produces a swing on the
other two pins, you have a PNP transistor. The red probe is BASE
If the needle swings FULL SCALE or if it swings for more than 2 readings, the transistor is FAULTY.
Step 2 - FINDING THE COLLECTOR and EMITTERSet the meter to "x10k."
For
an NPN transistor, place the leads on the transistor and when you
press hard on the two leads shown in the diagram below, the needle
will swing almost full scale.
For a PNP transistor, set the meter to "x10k" place
the leads on the transistor and when you press hard on the two leads
shown in the diagram below, the needle will swing almost full scale.
 | WORLDS SIMPLEST CIRCUIT
This is the simplest circuit you can get. Any NPN transistor can be used.
Connect the LED, 220 ohm resistor and transistor as shown in the photo.
Touch the top point with two fingers of one hand and the lower point with
fingers of the other hand and squeeze.
The LED will turn on brighter when you squeeze harder. Your
body has resistance and when a voltage is present, current will
flow though your body (fingers). The transistor is amplifying the
current through your fingers about 200 times and this is enough to
illuminate the LED. |
 | FINDING THE NORTH POLE
The
diagrams show that a North Pole will be produced when the positive
of a battery is connected to wire wound in the direction shown. This
is Flemmings Right Hand Rule and applies to motors, solenoids and
coils and anything wound like the turns in the diagram. |
These
two circuits will flash a LED very bright and consume less than 2mA
average current. The second circuit allows you to use a high power
NPN transistor as the driver if a number of LEDs need to bee driven.
The second circuit is the basis for a simple motor speed control.
See note on 330k in Flashing Two LEDs below.
FLASHING TWO LEDS
These
two circuits will flash two LEDs very bright and consume less than
2mA average current. They require 6v supply. The 330k may need to be
470k to produce flashing on 6v as 330k turns on the first transistor
too much and the 10u does not turn the first transistor off a small
amount when it becomes fully charged and thus cycling is not
produced.
 | 1.5v LED FLASHER
This
will flash a LED, using a single 1.5v cell. It may even flash a
white LED even though this type of LED needs about 3.2v to 3.6v for
operation.
The circuit takes about 2mA but produces a very bright flash.
|
LED on 1.5v SUPPLY
A
red LED requires about 1.7v before it will start to illuminate -
below this voltage - NOTHING! This circuit takes about 12mA to
illuminate a red LED using a single cell, but the interesting feature
is the way the LED is illuminated.
The 1u electrolytic can be considered to be a 1v cell.
(If
you want to be technical: it charges to about 1.5v - 0.2v loss due
to collector-emitter = 1.3v and a lost of about 0.2v via
collector-emitter in diagram B.)
It is firstly charged by the 100R
resistor and the 3rd transistor (when it is fully turned ON via the
1k base resistor). This is shown in diagram "A." During
this time the second transistor is not turned on and that's why we
have omitted it from the diagram. When the second transistor is turned
ON, the 1v cell is pulled to the 0v rail and the negative of the cell
is actually 1v below the 0v rail as shown in diagram "B."
The
LED sees 1.5v from the battery and about 1v from the electrolytic
and this is sufficient to illuminate it. Follow the two voltages to see
how they add to 2.5v.
 | 3v WHITE LED FLASHER
This will flash a white LED, on 3v supply and produce a very bright flash. The
circuit produces a voltage higher than 5v if the LED is not in
circuit but the LED limits the voltage to its characteristic voltage
of 3.2v to 3.6v. The circuit takes about 2mA an is actually a
voltage-doubler (voltage incrementer) arrangement.
Note the
10k charges the 100u. It does not illuminate the LED because the 100u
is charging and the voltage across it is always less than 3v.
When the two transistors conduct, the collector of the BC557 rises to
rail voltage and pulls the 100u HIGH. The negative of the 100u
effectively sits just below the positive rail and the positive of the
electro is about 2v higher than this. All the energy in the electro
is pumped into the LED to produce a very bright flash. |
BRIGHT FLASH FROM FLAT BATTERY
This
circuit will flash a white LED, on a supply from 2v to 6v and
produce a very bright flash. The circuit takes about 2mA and old
cells can be used. The two 100u electros in parallel produce a better
flash when the supply is 6v.
 | DUAL 3v WHITE LED FLASHER
This circuit alternately flashes two white LEDs, on a 3v supply and produces a very bright flash. The
circuit produces a voltage higher than 5v if the LED is not in
circuit but the LED limits the voltage to its characteristic voltage
of 3.2v to 3.6v. The circuit takes about 2mA and is actually a
voltage-doubler (voltage incrementer) arrangement.
The 1k charges
the 100u and the diode drops 0.6v to prevent the LED from starting
to illuminate on 3v. When a transistor conducts, the collector pulls the
100u down towards the 0v rail and the negative of the electro is
actually about 2v below the 0v rail. The LED sees 3v + 2v and
illuminates very brightly when the voltage reaches about 3.4v. All
the energy in the electro is pumped into the LED to produce a very
bright flash. |
 | DUAL 1v5 WHITE LED FLASHER
This circuit alternately flashes two white LEDs, on a 1.5v supply and produces a very bright flash. The
circuit produces a voltage of about 25v when the LEDs are not
connected, but the LEDs reduce this as they have a characteristic
voltage-drop across them when they are illuminated. Do not use a
supply voltage higher than 1.5v. The circuit takes about 10mA.
The
transformer consists of 30 turns of very fine wire on a 1.6mm slug
6mm long, but any ferrite bead or slug can be used. The number of turns
is not critical.
The 1n is important and using any other value or connecting it to the positive line will increase the supply current.
Using LEDs other than white will alter the flash-rate considerably and both LEDs must be the same colour. |
to Index
 | DANCING FLOWER
This circuit was taken from a dancing flower.
A
motor at the base of the flower had a shaft up the stem and when the
microphone detected music, the bent shaft made the flower wiggle and
move.
The circuit will respond to a whistle, music or noise.
|
 | WHITE LINE FOLLOWER
This
circuit can be used for a toy car to follow a white line. The motor
is either a 3v type with gearing to steer the car or a rotary
actuator or a servo motor.
When equal light is detected by the photo
resistors the voltage on the base of the first transistor will be mid
rail and the circuit is adjusted via the 2k2 pot so the motor does
not receive any voltage. When one of the LDR's receives more (or
less) light, the motor is activated. And the same thing happens when
the other LDR receives less or more light. |
 | LED DETECTS LIGHT
All LEDs give off light of a particular colour but some LEDs are also able to detect light.
Obviously they are not as good as a device that has been specially
made to detect light; such as solar cell, photocell, photo resistor,
light dependent resistor, photo transistor, photo diode and other
photo sensitive devices.
A green LED will detect light and a
high-bright red LED will respond about 100 times better than a green
LED, but the LED in this position in the circuit is classified as
very high impedance and it requires a considerable amount of
amplification to turn the detection into a worthwhile current-source.
All other LEDs respond very poorly and are not worth trying.
The accompanying circuit amplifies the output of the LED and enables it to be used for a number of applications.
The
LED only responds when the light enters the end of the LED and this
makes it ideal for solar trackers and any time there is a large
difference between the dark and light conditions. It will not detect
the light in a room unless the lamp is very close. |
 | 12v RELAY ON 6V SUPPLY
This circuit allows a 12v relay to operate on a 6v or 9v supply. Most 12v relays need
about 12v to "pull-in" but will "hold" on about 6v. The 220u charges
via the 2k2 and bottom diode. When an input above 1.5v is applied to
the input of the circuit, both transistors are turned ON and the 5v
across the electrolytic causes the negative end of the electro to go
below the 0v rail by about 4.5v and this puts about 10v across the
relay.Alternatively you can rewind a 12v relay by removing about half the turns.
Join
up what is left to the terminals. Replace the turns you took off, by
connecting them in parallel with the original half, making sure the
turns go the same way around |
MAKE TIME FLY!
Connect this circuit to an old electronic clock mechanism and speed up the motor 100 times!
The
"motor" is a simple "stepper-motor" that performs a half-rotation
each time the electromagnet is energised. It normally takes 2 seconds
for one revolution. But our circuit is connected directly to the
winding and the frequency can be adjusted via the pot.
Take the
mechanism apart, remove the 32kHz crystal and cut one track to the
electromagnet. Connect the circuit below via wires and re-assemble the
clock.
As you adjust the pot, the "seconds hand" will move
clockwise or anticlockwise and you can watch the hours "fly by" or
make "time go backwards."
The multivibrator section needs
strong buffering to drive the 2,800 ohm inductive winding of the
motor and that's why push-pull outputs have been used. The
flip-flop circuit cannot drive the highly inductive load directly
(it upsets the waveform enormously).
From a 6v supply, the motor only gets about 4v due to the voltage drops across the transistors. Consumption is about 5mA.HOW THE MOTOR WORKS
The rotor is a magnet with the north pole shown with the red mark and the south pole opposite.
The
electromagnet actually produces poles. A strong North near the end
of the electromagnet, and a weak North at the bottom. A strong
South at the top left and weak South at bottom left. The rotor
rests with its poles being attracted to the 4 pole-pieces equally. Voltage
must be applied to the electromagnet around the correct way so
that repulsion occurs. Since the rotor is sitting equally between
the North poles, for example, it will see a strong pushing force
from the pole near the electromagnet and this is how the motor
direction is determined. A reversal of voltage will revolve the
rotor in the same direction as before. The design of the motor is
much more complex than you think!! | 
The crystal removed and a "cut track" to the coil. The 6 gears must be re-fitted for the hands to work.
A close-up of the clock motorAnother
clock motor is shown below. Note the pole faces spiral closer to
the rotor to make it revolve in one direction. What a clever
design!! |
  |
 | CONSTANT CURRENT SOURCE
This
circuit provides a constant current to the LED. The LED can be
replaced by any other component and the current through it will
depend on the value of R2. Suppose R2 is 560R. When 1mA flows through
R2, 0.56v will develop across this resistor and begin to turn on
the BC547. This will rob the base of BD 679 with turn-on voltage and
the transistor turns off slightly. If the supply voltage increases,
this will try to increase the current through the circuit. If the
current tries to increase, the voltage across R2 increases and the BD
679 turns off more and the additional voltage appears across the BD
679.
If R2 is 56R, the current through the circuit will be 10mA.
If R2 is 5R6, the current through the circuit will be 100mA -
although you cannot pass 100mA through a LED without damaging it. |
 | ON - OFF VIA MOMENTARY PUSH-BUTTONS
- see Also Push-ON Push-OFF (in 101-200 Circuits)
This circuit will supply current to the load RL.
The maximum current will depend on the second transistor. The
circuit is turned on via the "ON" push button and this action puts a
current through the load and thus a voltage develops across the load.
This voltage is passed to the PNP transistor and it turns ON. The
collector of the PNP keeps the power transistor ON.
To turn the
circuit OFF, the "OFF" button is pressed momentarily. The 1k between
base and emitter of the power transistor prevents the base floating
or receiving any slight current from the PNP transistor that would
keep the circuit latched ON.
The circuit was originally designed by a
Professor of Engineering at Penn State University. It had 4
mistakes. So much for testing a circuit!!!! It has been corrected in
the circuit on the left. |
 | SIREN
This
circuit produces a wailing or siren sound that gradually increases
and decreases in frequency as the 100u charges and discharges when
the push-button is pressed and released. In other words, the circuit
is not automatic. You need to press the button and release it to
produce the up/down sound. |
 | TICKING BOMB
This circuit produces a sound similar to a loud clicking clock. The frequency of the tick is adjusted by the 220k pot.
The
circuit starts by charging the 2u2 and when 0.65v is on the base of
the NPN transistor, it starts to turn on. This turns on the BC 557 and
the voltage on the collector rises. This pushes the small charge on
the 2u2 into the base of the BC547 to turn it on more.
This
continues when the negative end of the 2u2 is above 0.65v and now the
electro starts to charge in the opposite direction until both
transistors are fully turned on. The BC 547 receives less current into
the base and it starts to turn off. Both transistors turn off very
quickly and the cycle starts again. |
 | SIGNAL INJECTOR
This
circuit is rich in harmonics and is ideal for testing amplifier
circuits. To find a fault in an amplifier, connect the earth clip to
the 0v rail and move through each stage, starting at the speaker. An
increase in volume should be heard at each preceding stage. This
Injector will also go through the IF stages of radios and FM sound
sections in TV's. |
 | LIGHT ALARM - 1
This circuit operates when the Light Dependent Resistor receives light.
When no light falls on the LDR, its resistance is high and the transistor driving the speaker is not turned on.
When
light falls on the LDR its resistance decreases and the collector of
the second transistor falls. This turns off the first transistor
slightly via the second 100n and the first 100n puts an additional spike
into the base of the second transistor. This continues until the
second transistor is turned on as hard as it can go. The first 100n
is now nearly charged and it cannot keep the second transistor turned
on. The second transistor starts to turn off and both transistors
swap conditions to produce the second half of the cycle. |
 | LIGHT ALARM - 2
This
circuit is similar to Light Alarm -1 but produces a louder output
due to the speaker being connected directly to the circuit.
The
circuit is basically a high-gain amplifier that is turned on
initially by the LDR and then the 10n keeps the circuit turning on until
it can turn on no more.
The circuit then starts to turn off and eventually turns off completely. The current through the LDR starts the cycle again.
|
LIGHT ALARM - 3 (MOVEMENT DETECTOR)
This circuit is very sensitive and can be placed in a room to detect the movement of a person up to 2 metres from the unit.
The
circuit is basically a high-gain amplifier (made up of the first
three transistors) that is turned on by the LDR or photo Darlington
transistor. The third transistor charges the 100u via a diode and
this delivers turn-on voltage for the oscillator. The LDR has equal
sensitivity to the photo transistor in this circuit.
 | SOUND TRIGGERED LED
This circuit turns on a LED when the microphone detects a loud sound.
The
"charge-pump" section consists of the 100n, 10k, signal diode and
10u electrolytic. A signal on the collector of the first transistor is
passed to the 10u via the diode and this turns on the second
transistor, to illuminate the LED.
|
 | SIMPLE LOGIC PROBE
This circuit
consumes no current when the probe is not touching any circuitry. The
reason is the voltage across the green LED, the base-emitter
junction of the BC557, plus the voltage across the red LED and
base-emitter junction of the BC547 is approx: 2.1v + 0.6v + 1.7v +
0.6v = 5v and this is greater than the supply voltage.
When the
circuit detects a LOW, the BC557 is turned on and the green LED
illuminates. When a HIGH (above 2.3v) is detected, the red LED is
illuminated. |
LOGIC PROBE with PULSE
This
circuit has the advantage of providing a PULSE LED to show when a
logic level is HIGH and pulsing at the same time. It can be built for
less than $5.00 on a piece of matrix board or on a small strip of
copper clad board if you are using surface mount components. The
probe will detect a HIGH at 3v and thus the project can be used for
3v, 5v and CMOS circuits.
CONTINUITY TESTER
This circuit has the advantage of providing a
beep when a short-circuit is detected but does not detect the small
voltage drop across a diode. This is ideal when testing logic
circuits as it is quick and you can listen for the beep while
concentrating on the probe. Using a multimeter is much slower.
 | TRAIN THROTTLE
This
circuit is for model train enthusiasts. By adding this circuit to
your speed controller box, you will be able to simulate a train
starting slowly from rest.
Remove the wire-wound rheostat and
replace it with a 1k pot. This controls the base of the BC547 and the
2N3055 output is controlled by the BC547. The diodes protect the
transistors from reverse polarity from the input and spikes from the
rails.
|
 | GUITAR FUZZ
The
output of a guitar is connected to the input of the Fuzz circuit.
The output of this circuit is connected to the input of your
amplifier.
With the guitar at full volume, this circuit is
overdriven and distorts. The distorted signal is then clipped by the
diodes and your power amp amplifies the Fuzz effect. |
TH TESTER
This is a simple "staircase" circuit in which the LEDs come on as the resistance between the probes decreases.
When
the voltage on the base of the first transistor sees 0.6v + 0.6v +
0.6v = 1.8v, LED1 comes on. LEDs 1&2 will come on when the voltage
rises a further 0.6v. The amount of pressure needed on the probes to
produce a result, depends on the setting of the 200k pot.
 | FOG HORN
When
the push-button is pressed, the 100u will take time to charge and
this will provide the rising pitch and volume. When the push-button
is released, the level and pitch will die away. This is the
characteristic sound of a ship's fog horn. |
 | HEADS OR TAILS
When
the push-button is pressed, the circuit will oscillate at a high
rate and both LEDs will illuminate. When the push button is released,
one of the LEDs will remain illuminated. The 50k is designed to
equalise the slightly different values on each half of the circuit
and prevent a "bias." |
 | DYNAMIC MICROPHONE AMPLIFIER
This circuit takes the place of an electret microphone. It turns an ordinary mini speaker into a very sensitive microphone.
Any
NPN transistors such as BC 547 can be used. The circuit will work
from 3v to 9v. It is a common-base amplifier and accepts the low
impedance of the speaker to produce a gain of more than 100. |
SCR WITH TRANSISTORS
The SCR in circuit A produces a 'LATCH.' When the button is pressed, the LED remains illuminated.
The SCR can be replaced with two transistors as shown in circuit B.
To
turn off circuit A, the current through the SCR is reduced to zero by
the action of the OFF button. In circuit B the OFF button removes the
voltage on the base of the BC547. The OFF button could be placed
across the two transistors and the circuit will turn off.
HEE HAW SIREN
The
circuit consists of two multivibrators. The first multi-vibrator
operates at a low frequency and this provides the speed of the change
from Hee to Haw. It modifies the voltage to the tone multivibrator,
by firstly allowing full voltage to appear at the bottom of the 220R
and then a slightly lower voltage when the LED is illuminated.
 | MICROPHONE PRE-AMPLIFIER
This circuit consists of two directly coupled transistors operating as common-emitter amplifiers.
The ratio of the 10k resistor to the 100R sets the gain of the circuit at 100. |
 | HARTLEY OSCILLATOR
The
Hartley Oscillator is characterised by an LC circuit in its
collector. The base of the transistor is held steady and a small amount
of signal is taken from a tapping on the inductor and fed to the
emitter to keep the transistor in oscillation.
The transformer can be any speaker transformer with centre-tapped primary.
The frequency is adjusted by changing the 470p. |
 | COLPITTS OSCILLATOR
The
Colpitts Oscillator is characterised by tapping the mid-point of the
capacitive side of the oscillator section. The inductor can be the
primary side of a speaker transformer. The feedback comes via the
inductor. |
 | PHASESHIFT OSCILLATOR
The Phaseshift Oscillator is characterised by 3 high-pass filters, creating a 180° phase shift.
The
output is a sinewave. Take care not to load the output - this will
prevent reliable start-up and may stop the circuit from oscillating.
Reduced
the 3k3 load resistor if the load prevents the circuit oscillating.
See Phase Shift Oscillator in second section of 200 Transistor
Circuits for a better design. |
 | DOOR-KNOB ALARM
This
circuit can be used to detect when someone touches the handle of a
door. A loop of bare wire is connected to the point "touch plate" and
the project is hung on the door-knob. Anyone touching the metal
door-knob will kill the pulses going to the second transistor and it
will turn off. This will activate the "high-gain" amplifier/oscillator.
The circuit will also work as a "Touch Plate" as it does not rely on mains hum, as many other circuits do. |
SIMPLE MOTOR SPEED CONTROL
This
circuit is better than reducing the RPM of a motor via a resistor.
Firstly it is more efficient. And secondly it gives the motor a set of
pulses and this allows it to start at low RPM. It's a simple
Pulse-Width circuit or Pulse-Circuit.
to Index
to Index
MOTOR SPEED CONTROL - Circuit 3
 | ELECTRONIC DRUMS
The
circuit consists of two "twin-T" oscillators set to a point below
oscillation. Touching a Touch Pad will set the circuit into
oscillation.
Different effects are produced by touching the pads in different ways and a whole range of effects are available.
The two 25k pots are adjusted to a point just before oscillation.
A "drum roll" can be produced by shifting a finger rapidly across adjacent ground and drum pads.
|
 | LIGHT EXTENDER
This
circuit is a Courtesy Light Extender for cars. It extends the "ON"
time when a door is closed in a car, so the passenger can see where
he/she is sitting.
When the door switch is opened, the light
normally goes off immediately, but the circuit takes over and allows
current to flow because the 22u is not charged and the first BC 547
transistor is not turned ON. This turns on the second BC547 via the
100k and the BD679 is also turned on to illuminate the interior
light.
The 22u gradually charges via the 1M and the first BC547
turns on, robbing the second BC547 of "turn-on" voltage and it starts
to turn off the BD679.
The 1N4148 discharges the 22u when the door is opened. |
 | 20 WATT FLUORO INVERTER
This circuit will drive a 40 watt fluoro or two 20-watt tubes in series.
The transformer is wound on a ferrite rod 10mm dia and 8cm long.
The
wire diameters are not critical but our prototype used 0.61mm wire
for the primary and 0.28mm wire for the secondary and feedback winding.
Do not remove the tube when the circuit is operating as the spikes produced by the transformer will damage the transistor.
The
circuit will take approx 1.5amp on 12v, making it more efficient
than running the tubes from the mains. A normal fluoro takes 20 watts
for the tube and about 15 watts for the ballast. |
 | 6 to 12 WATT FLUORO INVERTER
This
circuit will drive a 40 watt fluoro or two 20-watt tubes in series
but with less brightness than the circuit above and it will take less
current.
2 x 20 watt tubes = 900mA to 1.2A and 1 x 20 watt tube 450mA to 900mA depending on pot setting.
The
transformer is wound on a ferrite rod 10mm dia and 8cm long. The
wire diameter is fairly critical and our prototype used 0.28mm wire
for all the windings.
Do not remove the tube when the circuit is
operating as the spikes produced by the transformer will damage the
transistor. The pot will adjust the brightness and vary the current
consumption. Adjust the pot and select the base-bias resistor to get
the same current as our prototype. Heat-sink must be greater than
40sq cm. Use heat-sink compound. |
PHASER GUN
This
is a very effective circuit. The sound is amazing. You have to build
it to appreciate the range of effects it produces. The 50k pot
provides the frequency of the sound while the switch provides fast or
slow speed.
IC RADIO
This circuit contains an IC but it looks like a 3-leaded transistor and that's why we have included it here.
The
IC is called a "Radio in a Chip" and it contains 10 transistors to
produce a TRF (tuned Radio Frequency) front end for our project.
The 3-transistor amplifier is taken from our SUPER EAR project with the electret microphone removed.
The two 1N 4148 diodes produce a constant voltage of 1.3v for the chip as it is designed for a maximum of 1.5v.
The "antenna coil" is 60t of 0.25mm wire wound on a 10mm ferrite rod. The tuning capacitor can be any value up to 450p.
5-TRANSISTOR RADIO
If you are not able to get the ZN414 IC, this circuit uses two transistors to take the place of the chip.
AUTOMATIC LIGHT
This
circuit automatically turns on a light when illumination is removed
from the LDR. It remains ON for the delay period set by the 2M2 pot.
The
important feature of this circuit is the building blocks it contains
- a delay circuit and Schmitt Trigger. These can be used when designing
other circuits.
5-LED CHASER
The LEDs in this circuit produce a chasing pattern similar the running LEDs display in video shops.
In
fact the effect is called: "Running Hole." All transistors will try
to come on at the same time when the power is applied, but some will
be faster due to their internal characteristics and some will get a
different turn-on current due to the exact value of the 22u
electrolytics. The last 22u will delay the voltage-rise to the base
of the first transistor and make the circuit start reliably. It is
very difficult to see where the hole starts and that's why you should
build the circuit and investigate it yourself. The circuit can be
extended to any number of odd stages.
MAKING 0-1Amp meter for the BENCH POWER SUPPLY
The
item in the photo is called a "Movement." A movement is a moving coil
with a pointer and no resistors connected to the leads.
Any Movement can be converted to an ammeter without any mathematics.
Simply
solder two 1R resistors (in parallel) across the terminals of any
movement and connect it in series with an ammeter on the output of the
Bench Power Supply. The second ammeter provides a reference so you can
calibrate the movement. Connect a globe and increase the voltage.
At 500mA, if the pointer is "up scale" (reading too high) add a trim-resistor. In our case it was 4R7.
The three shunt resistors can be clearly seen in the photo. Two 1R and the trim resistor is 4R7.
You
can get a movement from an old multimeter or they are available in
electronics shops as a separate item. The sensitivity does not matter.
It can be 20uA or 50uA FSD or any sensitivity.
MAKING A ZENER DIODESometimes
a zener diode of the required voltage is not available. Here are a
number of components that produce a characteristic voltage across
them. Since they all have different voltages, they can be placed in
series to produce the voltage you need. A reference voltage as low as
0.65v is available and you need at least 1 to 3mA through the device(s) to put them in a state of conduction (breakdown).
12v TRICKLE CHARGER
The
12v Trickle Charger circuit uses a TIP3055 power transistor to limit
the current to the battery by turning off when the battery voltage
reaches approx 14v or if the current rises above 2 amp. The signal to
turn off this transistor comes from two other transistors - the
BC557 and BC 547.
Firstly, the circuit turns on fully via the BD139
and TIP3055. The BC557 and BC 547 do not come into operation at the
moment. The current through the 0.47R creates a voltage across it to
charge the 22u and this puts a voltage between the base and emitter
of the BC547. The transistors turn on slightly and remove some of
the turn-on voltage to the BD139 and this turns off the TIP3055
slightly.
This is how the 2 amp max is created.
As the battery
voltage rises, the voltage divider made up of the 1k8 and 39k creates
a 0.65v between base and emitter of the BC557 and it starts to turn on
at approx 14v. This turns on the BC 547 and it robs the BD136 of
"turn-on" voltage and the TIP3055 is nearly fully turned off.
All battery chargers in Australia must be earthed. The negative of the output is taken to the earth pin.
1.5v to 10v INVERTER
This
very clever circuit will convert 1.5v to 10v to take the place of
those expensive 9v batteries and also provide a 5v supply for a
microcontroller project.
But the clever part is the voltage
regulating section. It reduces the current to less than 8mA when no
current is being drawn from the output. With a 470R load and 10v, the
output current is 20mA and the voltage drop is less than 10mV. The
pot will adjust the output voltage from 5.3v to 10v.
5v REGULATED SUPPLY FROM 3V
This
circuit will produce a 5v regulated output from 2 cells (3v). The
output current is limited to 50mA but will be ideal for many
microcontroller circuits.
The output voltage is set to 5v by the 3k9 and 560R resistors, making up a voltage divider network.
9v SUPPLY FROM 3V
You can replace a 9v battery with this circuit.
The output is about 10.4v on no load and 9.6v @30mA .
The advantage is the voltage stays over 9v for the life of the cells.
A normal 9v battery drops to 7v very quickly.
The
output voltage is set to 9-10v by the 6k8 and 390R resistors. The
470R gives the circuit an idling current of about 20mA and the spikes
are about 75mV.
By increasing the 470R, the quiescent current decreases but the voltage drops more when the current is 30mA.
 | 27MHz TRANSMITTER
The
transmitter is a very simple crystal oscillator. The heart of the
circuit is the tuned circuit consisting of the primary of the
transformer and a 10p capacitor. The frequency is adjusted by a
ferrite slug in the centre of the coil until it is exactly the same
as the crystal. The transistor is configured as a common emitter
amplifier. It has a 390R on the emitter for biasing purposes and
prevents a high current passing through the transistor as the
resistance of the transformer is very low.
The "pi" network matches the antenna to the output of the circuit. See full description in 27MHz Links article. |
27MHz RECEIVER
The
27MHz receiver is really a transmitter. It's a very weak transmitter
and delivers a low level signal to the surroundings via the antenna.
When another signal (from the transmitter) comes in contact with the
transmission from the receiver it creates an interference pattern
that reflects down the antenna and into the first stage of the
receiver.
The receiver is a super-regenerative design. It is
self-oscillating (or already oscillating) and makes it very sensitive
to nearby signals. See full description in 27MHz Links article.
 | 27MHz TRANSMITTER WITHOUT A CRYSTAL
A
27MHz transmitter without a crystal. When a circuit does not have a
crystal, the oscillator is said to be "voltage dependent" or "voltage
controlled" and when the supply voltage drops, the frequency
changes.
If the frequency drifts too much, the receiver will not
pick up the signal. For this reason, a simple circuit as shown is not
recommended. We have only included it as a concept to show how the
27MHz frequency is generated. It produces a tone and this is detected
by a receiver. See full description in 27MHz Links article. |
27MHz TRANSMITTER WITH SQUARE-WAVE OSCILLATORThe
circuit consists of two blocks. Block 1is a multivibrator and this
has an equal mark/space ratio to turn the RF stage on and off. Block 2
is an RF oscillator. The feedback to keep the stage operating is
provided by the 27p capacitor. The frequency-producing items are the
coil (made up of the full 7 turns) and the 47p air trimmer. These two
items are called a parallel tuned circuit. They are also called a
TANK CIRCUIT as they store energy just like a TANK of water and pass
it to the antenna. The frequency of the circuit is adjusted by the 47p
air trimmer. See full description in 27MHz Links article.
27MHz RECEIVER-2This
circuit matches with the 27MHz Transmitter with Square-wave
Oscillator. See full description on Talking Electronics website: 27MHz Links article.
The
receiver frequency is fixed. The transmitter is adjusted to suit the
receiver. The 3-27p trimmer is adjusted for maximum gain (10p trimmer
and 5p6 in our case) and this is a critical adjustment.
The
base-emitter junction of the first BC547 sets 0.7v (as it is heavily
turned on by the 10k) on the base of the oscillator Q1, and this is
fixed. Q1 is very lightly turned on (due to the emitter resistor), and
this makes it very sensitive when it is oscillating. Any 27MHz signal
from the surroundings will upset the oscillator and any tone in the
signal will be passed to the stages for amplification. The coil
is 13 turns. It can be replaced with 11 turns of 0.25mm wire on 3mm
dia slug 7mm long. Although the original Russian product worked very
well, our prototype did not have very good sensitivity. The circuit was
very difficult to set-up.
Note: When making the 27uH inductor and
checking its value on an inductance meter; if the meter does not
read low values accurately, put two inductors in series. Measure the
first inductor, say 100uH. The two inductors in series will be 127uH
as inductors combine just like resistors in series! The result is
the addition of the individual values.
WALKIE TALKIENearly
all the components in the 4-transistor circuit are used for both
transmitting and receiving. This makes it a very economical design.
The frequency-generating stage only needs the crystal to be removed
and it becomes a receiver. Next is a three transistor directly
coupled audio amplifier with very high gain. The first transistor is a
pre-amplifier and the next two are wired as a super-alpha pair,
commonly called a Darlington pair to drive the speaker transformer. See
full description in 27MHz Links article.
 | 27MHz TRANSMITTER - 2 CHANNEL
This
circuit does not use a crystal but has a clever feature of using the
two push buttons to turn the circuit on when it is required to
transmit.
The frequency of the multivibrator is determined by the
value of resistance on the base of each transistor. The multivibrator
is driven directly from the supply with the forward button and via a
150k for the reverse frequency.
The receiver requires a 1kHz tone for forward and 250Hz for reverse. See full description in 27MHz Links article. |
to Index
 | 27MHz TRANSMITTER - 4 CHANNEL
This circuit uses the same number of components as the 2-Channel circuit above but has 4 channels.
The frequency of the multivibrator is determined by the value of resistance on the base of each transistor.
A 4 channel receiver has been designed by talking Electronics using a PIC12F628 micro to detect the different frequencies. See P4 of:
2 Digit Up/Down Counter (see left index on Talking Electronics website).
2 Digit Up/Down Counter has the receiver section.A = 500Hz B = 550Hz C = 660Hz D = 1kHz |
 | 303MHz TRANSMITTER
The
transmitter circuit is made up of two building blocks - the 303MHz
RF oscillator and the 32kHz crystal controlled oscillator to generate a tone so the receiver does not false-trigger.
The
303MHz oscillator consists of a self-oscillating circuit made up of
the coil on the PC board and a 9p (9 puff) capacitor. See full description in Wireless Doorbell article. |
| Type: | | Gain: | Vbe | Vce | Current | Case |
| 2SC1815 | NPN | 100 | 1v | 50v | 150mA |  |
| 2SC3279 | NPN | 140 to
600
@0.5A | 0.75v | 10v | 2amp |
BC337
BC338 | NPN | 60
@300mA | 0.7v | 45v
25v | 800mA |  |
BC547
BC548
BC549 | NPN | 70
@100mA | 0.7v | 45v
30v
30v | 100mA | |
| BC557 | PNP | | | 45v | 100mA |
| BD139 | NPN | 70-100
@150mA | 0.5v | 80v | 1.5A |  |
| BD140 | PNP | 70-100
@150mA | 0.5v | 80v | 1.5A |
| 2SCxxx | | | | | |  |
| 8050 | NPN | | | 10v | 1.5A |
| 8550 | PNP | | | 10v | 1.5A |
| 9012 | PNP | | | | 500mA |
| 9013 | NPN | | 1v | 20v | 500mA |
| 9014 | NPN | | | | 100mA |
| 9015 | PNP | | | | 100mA |
| 9018 | NPN | 700MHz | 15v | 50mA |
The 1/10th watt resistors used in this circuit, compared with 0.25watt resistors.
5 TRANSISTOR WALKIE TALKIE - 1This
walkie talkie circuit does not have a crystal or speaker
transformer, with the board measuring just 3cm x 4cm and using 1/10th
watt resistors, it is one of the smallest units on the market, for
just $9.50 to $12.00. The wires in the photo go to the battery,
speaker, call-switch and antenna. The most difficult component in the
circuit to duplicate is the oscillator coil. See the photo for the
size and shape. The coil dia is 5mm and uses 0.25mm wire. The actual
full-turn or half turn on the coil is also important. Almost all 5
transistor walkie talkies use this circuit or slight variations. See
the article: 27MHz Transmitters for theory on how these transmitters work - it is fascinating.
5 TRANSISTOR WALKIE TALKIE - 2Here is another walkie talkie circuit, using slightly different values for some of the components. See the article: 27MHz Transmitters for theory on how these transmitters work.
WALKIE TALKIE with LM386Here
is a more up-to-date version of the walkie talkie, using an LM
386 amplifier IC to take the place of 4 transistors.
 | HEARING AID 1.5v SUPPLY
This
simple circuit will detect very faint sounds and deliver them to an 8
ohm earpiece. The circuit is designed for 1.5v operation. |
 | HEARING AID with PUSH PULL OUTPUT
This circuit will detect very faint sounds and deliver them to an 8 ohm earpiece. It is designed for 3v operation. |
HEARING AID with CONSTANT VOLUME
This is a very handy circuit as it provides constant volume. It is designed for 3v operation.
SOLAR ENGINE
This
circuit is called Type-1 SE. Low current from a solar cell is stored
in a large capacitor and when a preset voltage-level is reached, the
energy from the capacitor is released to a motor.
For full details on how the circuit works and how to modify it, see:
SUN EATER-I
An
improved design over Solar Engine circuit above. It has a clever
2-transistor self-latching arrangement to keep the circuit ON until
the voltage drops to 1.5v. The circuit turns on at 2.8v. This gives
the motor more energy from the electrolytic at each "pulse." For full
details on how the circuit works and how to modify it, see:
SOLAR PHOTOVOREThe green LEDs cause the Solar Engine on the opposite side to fire and the Solar Photovore turns toward the light source. The motors are two pager
"vibe" motors with the weights removed. The 100k pot on the "head"
balances the two Solar Engines. If you cannot get the circuit to work
with green LEDs, use photo-transistors. For full details on how the circuit works and how to modify it, see:
SIGNAL BY-PASSThis
circuit allows a class-A amplifier to drive a low impedance speaker
and has a low quiescent current. The 220R in series with the speaker
limits the "wasted" current to about 20mA max as the transistor is
generally biased at mid-voltage. However the transistor will be
almost directly driving the speaker when a signal is being processed
and the only limitation is the ability of the 220R to discharge the
100u during each cycle.
The circuit is called a signal by-pass as the signal by-passes the 220R and drives the speaker directly (via the 100u).
SOUND-TO-LIGHTThe LED illuminates when the piezo diaphragm detects sound.
Some
piezo diaphragms are very sensitive and produce 100mV when whistling
at 50cm. Others produce 1mV. You must test them with a CRO.
The sensitivity of the diaphragm will determine the sensitivity of the circuit.
The
transmitter is built on a small length of PC board, cut into
lands with a file. The photo clearly shows how all the components
are mounted and how the board is fitted into a toothbrush holder.
The flashing LED shows the unit is ON and serves to control the
beep-beep-beep of the circuit. The flashing LED is not an ordinary
LED.
You cannot use an ordinary LED. It must be a FLASHING LED as
this type of LED has a built-in resistor and a chip to make the
LED flash.
The circuit does not make the LED flash, the LED makes
the circuit beep-beep-beep due to the on-off from the chip inside
the LED.
One constructor used an ordinary LED - and BANG! That's
why we are the first in the world to create a symbol for a
flashing LED. The extra bar represents the chip inside the LED. |
This is the professional unit | 
TRANSMITTER CIRCUIT |
RECEIVER CIRCUIT | The
receiver circuit is a high-gain amplifier and produces constant
background noise so the slightest magnetic field can be detected.
The 10mH choke can be any value but the largest number of turns on the core is best.
The mini speaker can be a 16R earpiece but these are not as loud as a mini speaker.
Quiescent current is 50mA so the on-off switch can be a push-button. |
CABLE TRACER
Why
pay $100 for a cable tracer when you can build one for less than
$10.00! This type of tracer is used by telephone technicians,
electricians and anyone laying, replacing or wiring anything, using
long cables, such as intercoms, television or security.
Our cable
tracer consists of two units. One unit has a multivibrator with an
output of 4v p-p at approx 5kHz. This is called the transmitter. The
other unit is a very sensitive amplifier with capacitive input for
detecting the tone from the transmitter and a magnetic pickup for
detecting magnetic lines of force from power cables carrying 240v. This
is called the receiver. The circuit also has an inductive loop,
made up of a length of wire, to pick up stray signals from power
cables, so if one detector does not detect the signal, the other will.
Our circuit is nothing like that in the professional unit shown above.
 | LED TORCH with 1.5v SUPPLY
This
simple circuit will illuminate a super-bright white LED to full
brightness with 28mA from a 1.5v cell. The LED is 20,000mcd (20cd @
15° viewing angle) and has an output of approx 1lumen.
The
transformer is wound on a small ferrite slug 2.6mm dia and 6mm long.
It is made from F29 ferrite material as the circuit operates at a high
frequency (100kHz to 500kHz).
The efficiency of the circuit
revolves around the fact that a LED will produce a very high output
when delivered pulses, but the overall current will be less than a
steady DC current.
BC 337 has a collector-emitter voltage of 45v.
(BC338 has 25v collector-emitter voltage rating.) The voltage across
the transistor is no more than 4v as the LED absorbs the spikes. Do
not remove the LED as the spikes from the transformer will damage the
transistor.
The circuit will drive 1 or 2 while LEDs in series.
|
 | WHITE LED FLASHER
This circuit will flash a super-bright white LED from a 1.5v cell.
The transformer is wound on a small ferrite slug 2.6mm dia and 6mm long as shown in a project above.
The
circuit uses the zener characteristic of the reverse-base-emitter
junction of a BC 547 to pass current and flash the LED.
|
 | LED TORCH with ADJUSTABLE BRIGHTNESS
This
circuit will drive up to 3 high-bright white LEDs from a 3v supply.
The circuit has a pot to adjust the brightness to provide optimum
brightness for the current you wish to draw from the battery.
The transformer is wound on a ferrite slug 2.6mm dia and 6mm long as shown in the LED Torch with 1.5v Supply project.
This
circuit is a "Boost Converter" meaning the supply is less than the
voltage of the LEDs. If the supply is greater than the voltage across
the LEDs, they will be damaged.
|
 | BUCK CONVERTER for HIGH-POWER LED 170mA
This circuit is slightly simpler than above but it does not have the feature of being able to adjust the drive-current.
The inductor is the same as the photo above but has a feedback winding of 15 turns.
Connect the circuit via a 220R resistor and if the LED does not illuminate, reverse the feedback winding.
The driver transistor will need a small heatsink. |
 | BUCK CONVERTER for HIGH-POWER LED 210mA
This circuit will drive 1 high-power white LED from a 12v supply and is capable of delivering 210mA.
The driver transistor is BD 139 and the details of the inductor are shown above.
The voltage across the LED is approx 3.3v - 3.5v
The driver transistor will need a small heatsink.
The 2R2 can be increased if a lower drive-current is required. |
Designed 12-8-2011 | BUCK CONVERTER for HIGH-POWER LED 250mA - 1watt LED
This circuit will drive 1watt white LED from a 12v supply and is capable of delivering 300mA.
The driver transistor is BD 327 and the inductor is 70 turns of 0.25mm wire wound on the core of a 10mH inductor.
The voltage across the LED is approx 3.3v - 3.5v
The 1R is used to measure the mV across it. 300mV equals 300mA LED current.
The diode MUST be high speed. Non-highspeed diode increases current 50mA!
This
circuit is the best design as it does not put peaks of current
though the LED. Reduce 390R slightly to increase max. current. |
AUTOMATIC GARDEN LIGHT
This
circuit automatically turns on and illuminates the LEDs when the solar
panel does not detect any light. It switches off when the solar panel
produces more than 1v and charges the battery when the panel
produces more than 1.5v + 0.6v = 2.1v
27MHz DOOR PHONE
This
circuit turns a walkie talkie into a handy wireless door phone. It
saves wiring and the receiver can be taken with you upstairs or outside,
without loosing a call from a visitor.
A 5-Transistor walkie talkie can be used (see circuit above) and the modifications made to the transmitter and receiver are shown below:
THE TRANSMITTER
Only
three sections of the transmit/ receive switch are used in the
walkie talkie circuit and our modification uses the fourth section.
Cut the tracks to the lands of the unused section so it can be
used for our circuit.
There are a number of different printed
circuit boards on the market, all using the same circuit and some
will be physically different to that shown in the photo. But one of
the sections of the switch will be unused.
Build the 2-transistor
delay circuit and connect it to the walkie talkie board as shown.
When the "push-to-talk" switch is pressed, the PC board will be
activated as the delay circuit effectively connects the negative lead
of the battery to the negative rail of the board for about 30
seconds.
The 100u gradually discharges via the 1M after the
"press-to-talk" switch is released and the two transistors turn
off and the current drops to less than 1 micro-amp - that's why the
power switch can be left on. .
The transmitter walkie talkie is
placed at the front door and the power switch is turned on. To call,
push the "push-to-talk" switch and the "CALL" button at the same time
for about 5 seconds. The circuit will activate and when the
"push-to-talk" switch is released, the circuit will produce
background noise for about 30 seconds and you will hear when call is
answered.
The "push-to-talk" switch is then used to talk to the
other end and this will activate the circuit for a further 30
seconds. If the walkie talkie does not have a "CALL" switch, 3
components can be added to provide feedback, as shown in the circuit
below, to produce a tone.
THE RECEIVER
The receiver
circuit needs modification and a 2-transistor circuit is added. This
circuit detects the tone and activates the 3-transistor
direct-coupled amplifier so that the speaker produces a tone.
The
receiver circuit is switched on and the 2-transistor circuit we
connect to the PC board effectively turns on the 3-transistor amplifier
so that the quiescent current drops from 10mA to about 2-3mA. It also
mutes the speaker as the amplifier is not activated. The circuit
remains on all the time so it will be able to detect a "CALL." When a
tone is picked up by the first two transistors in the walkie talkie,
it is passed to the first transistor in our "add-on" section and
this transistor produces a signal with sufficient amplitude to remove
the charge on the 1u electrolytic. This switches off the second
transistor and this allows the 3-transistor amplifier to pass the
tone to the speaker. The operator then slides a switch called
"OPERATE" to ON (down) and this turns on the 3-transistor amplifier.
Pressing the "push-to-talk" switch (labelled T/R) allows a
conversation with the person at the door. Slide the "OPERATE" switch up
when finished. 
The receiver walkie talkie with the 2-transistor "add-on"
SCHMITT TRIGGER
A
Schmitt Trigger is any circuit that has a fast change-over from one
state to the other. In our case we have used 2 transistors to produce
this effect and the third is an emitter-follower buffer.
The
circuit will drive a LED or relay and the purpose is to turn the LED
ON quickly at a particular level of illumination and OFF at a higher
level. The gap between ON and OFF is called the HYSTERESIS GAP. SCHMITT TRIGGER-2The following circuit is a Schmitt Trigger made with NPN and PNP transistors:
PHONE TAPE - 1
This simple circuit will allow you to tape-record a conversation from a phone line.
It must be placed between the plug on the wall and the phone.
The
easiest way is to cut an extension lead. Wind 300-500 turns of 0.095mm
wire on a plastic straw and place the reed switch inside. Start with
300 turns and see if the reed switch activates, Keep adding turns
until the switch is reliable.
Fit two 100n capacitors to the ends
of the winding for the audio. Plug the Audio into "Mic" on tape
recorder. Plug the remote into "remote" on the tape recorder and push
"record." The tape recorder will turn on when the phone is lifted
and record the conversation.
PHONE TAPE - 2
The
circuit is turned off when the phone line is 45v as the voltage
divider made up of the 470k, 1M and 100k puts 3.5v on the base of the
first BC557 transistor. If you are not able to cut the lead to the
phone, the circuit above will record a conversation from an
extension lead. The remote plug must be wired around the correct way
for the motor to operate.
PHONE ALERT
Two
circuits are available to show when a phone is being used. The first
circuit must be placed between the socket on the wall and the phone -
such as cutting into the lead and insert the bridge and diode.
But
if you cannot cut the lead to the phone, you will have to add an
extension cord and place the second circuit at the end of the line. You
can also connect a phone at the end if needed.
THE LISTENER
This
circuit consists of a 4-transistor amplifier and a 3-transistor
"switch" that detects when the phone line is in use, and turns on the
amplifier. The voltage divider at the front end produces about 11v
on the base of the first BC557 and this keeps the transistor off.
Switch the unit off when removed from the phone line.
PHONE TRANSMITTER - 1 see also Phone Bug (101-200 circuits)
The
circuit will transmit a phone conversation to an FM radio on the
88-108MHz band. It uses energy from the phone line to transmit about
100metres. It uses the phone wire as the antenna and is activated when
the phone is picked up. The components are mounted on a small PC board
and the lower photo clearly shows the track-work.
PHONE TRANSMITTER - 2 see also Phone Bug (101-200 circuits)
The
circuit will transmit a phone conversation to an FM radio on the
88-108MHz band. It uses energy from the phone line to transmit about
200metres. It uses the phone wire as the antenna and is activated when
the phone is picked up.
PHONE TRANSMITTER - 3 see also Phone Bug (101-200 circuits)
This
circuit has poor features but you can try it and see how it
performs. It uses a PNP transistor and requires a separate antenna. It
also has a supply of less than 1.9v, via the red LED. It would be
better to put 2 LEDs in series to get a higher voltage. It is
activated when the phone is picked up. 
PHONE TRANSMITTER - 4 see also Phone Bug (101-200 circuits)
The
circuit was originally designed by me and presented in Poptronics
magazine. It will transmit a phone conversation to an FM radio on the
88-108MHz band. It uses energy from the phone line to transmit about
200metres and uses the phone wire as the antenna. It is activated when
the phone is picked up. The 22p air trimmer is shown as well as the 3
coils. Q2 is a buffer transistor between the oscillator and phone
line and will provide a higher output than the previous circuits.
SWITCH DEBOUNCER and PULSE PRODUCERThus is one of the simplest and cleverest circuits ever produced (by Ron: http://www.zen22142.zen.co.uk/ronj/tg1.html
It
produces a complete pulse every time the button is pressed. When the
button is pressed, the output goes low for 3uS and produces a pulse
to activate the clock-line of a chip. Our circuit produced 100%
reliability and the cap takes 0.1sec to charge.
All the resistor colours:
See 101-200 Circuits for resistors in parallel and series and
capacitors in parallel and series. You can make ANY VALUE by simply
connecting resistors in parallel or series. And the same with capacitors.
