Sunday, September 4, 2011

Home Security System

Home Security System

(Click on the image to enlarge it)

Interesting circuit of a home security alarm is described here.This circuit is wired
around three popular and less expensive CMOS ICs CD4000,CD 4011 and
CD4047.The circuit works off a 9volt battery supply.For intruder detection ,six
normally closed micro switches are used here. However one can use wire loops
instead of these switches to enhance the performance.When all switches/wire loops
are intact,the green LED(D1) in the circuit lights up.Similarly, the optional ouput
signal(available from pin 10 of IC2) for remote monitoring purpose is in logic low(L)

IC1 is a dual 3 input NOR gate and inverter.All input pins of this IC1 are grounded
by the sensor switches / wire loops.In standby state ,all six inputs(3x2) of IC1
remain low to give high state outputs at pins 6 and 10.This high outputs are further
given to the inputs of the first gate(pins 1 and 2) of IC2 CD4011(Quad 2 input
NAND gate pack).As a result,its output ,at pin 3 remains low and the audio
frequency oscillator built around IC3 is disabled.

When any switch is open or any loop is broken,corresponding output of the NOR
gate IC1 goes low while the output of NAND gate IC2 goes high.Now the astable
IC3 is enabled by the High level signal at its pin 5 and the speaker sounds to alert
the user.D1 instantly goes out and the condition of remote monitoring output is
reversed ie it changes from logic low(L) to logic high(H).

The astable / monostable CD 4047 (IC3),here wired as an astable, has a much lower
power output than the 555 but it needs only one external resistor and capacitor.
Ouput tone of the alarm signal can be changed by changing the values of C4
and(or)R8(Output frequency f = 0.23/R8xC4 hertz).For driving large
speaker(s),replace transistor T1 and T2 with suitable high power darlingtons.
by TK Hareendran

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