13-05-2011, 03:38 PM
Here is a circuit to thwart theattempt of burglary in yourhome. When an intruder triesto open the door of your house, itsounds a loud alarm and switches onthe porch light. The alarm disablesonly when the door is closed again orS2 is switched off. The circuit isfailproof and reliable and, unlike IRor555 timer-based burglar alarms,overcomes the problem of false triggering.The circuit is based on the triggeringaction of the low-powermonostable/astable multivibrator ICCD4047 (IC1). It is wired in the dual J-K master/slave flip-flop havingindependent J, K, set, reset and clockinputs. The flip-flops are edge-sensitiveto the clock input and Q and Qoutputs change states on the positivegoingtransition of the clock pulses.IC2 is wired such that its Q outputturns high when reset pin 4 receives ahigh pulse (as indicated by LED2).When set pin 7 receives a high pulse monostable mode to set and reset ICCD4027 (IC2). The output pulse widthof IC1 depends on the externalR3-C2 network connected to its pins1, 2 and 3.Monostable operation is achievedwhen IC1 is triggered by low-to-hightransition of the positive trigger input(pin 8). It can be retriggered by anotherlow-to-high transition of pin 8.When the master reset input (pin 9) ishigh, Q output (pin 10) goes low andQ output (pin 11) goes high. Here themonostable configuration of IC1 hasR3-C2 network with values of C2 andR3 as 0.0047 μF and 470 kilo-ohms, respectively.Trigger pin 8 is connectedto the emitter of T1. Q output goeslow and Q outputgoes high.This lights upLED3 anddrives transistorT2 (BC548). Theoutput fromtransistor T2 isused to activate Normally, with reed switch S1closed, transistor T1 is non-conductingand its emitter current is zero. Sothe monostable remains in the standbymode with low output at pin 10. WhenT1 conducts upon opening reed switchS1, a positive pulse from the emitterof T1 triggers the monostable and ashort positive pulse is available to Qoutput of the monostable. It resetswhen T1 is made non-conducting byclosing reed switch S1. The negativetrigger pin 6 and astable input pin 5of IC1 are tied to the ground alongwith ground pin 7.The short-duration (one-second)low-to-high output from IC1 is usedto set and reset IC2. It is a low-power triac1 (BT136) and alarm tone generatorIC3. The triac is used to switch onporch lamp.
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