Two alarm circuits are presentedhere. One produces bird-chirpingsound and the other Britishpolice siren tone.Fig. 1 shows the circuit of the birdchirping-sound alarm unit along withthe circuit of the control unit. Fig. 2shows the circuit of only the Britishpolice siren tone generator, which hasto be integrated with the control circuitportion of Fig. 1 at points A and B tocomplete the circuit diagram of automatedalarm.The control unit is built around ICsCD4047 and CD4027 (as shown on theleft side of the dotted line in Fig. 1). Asmentioned earlier, it is common to boththe alarm circuits. IC CD4047 (IC1)is wired in positive-edge-triggeringmonostable multivibrator mode to setand reset IC CD4027 (IC2). The outputpulse width of IC1 depends on thevalues of capacitor C2 and resistor R3connected to its pins 1, 2 and 3. Normally, when the door is closed,reed switch S1 is closed, transistor T1conducts and the monostable multivibrator(IC1) remains in standby modewith ‘low’ output at pin 10.When the door is opened, reedswitch S1 gets disconnected, T1 stopsconducting and low-to-high pulse at pin 8 of IC1 triggers the monostable and ashort-duration positive pulse of about10 seconds is available as Q output atpin 10. At the same time, complementaryoutput Q goes low at pin 11. The outputfrom IC1 is used to set and reset IC2.IC2 is a low-power, dual J-K master/slave flip-flop having independentJ, K, set, reset and clock inputs.The flip-flops change states on thepositive-going transition of the clockpulses. IC2 is wired such that its Qoutput turns ‘high’ when reset pin 4receives a high pulse. When set pin 7receives a high pulse, Q output goeslow and Q output goes high. Thislights up LED2 and drives transistorT2 (BC548), which enables the alarmcircuit.The output at point A is used toenable the alarm tone generator circuit(on the right side of the dotted line)consisting of two 555 timer ICs markedas IC3 and IC4. The R-C network determinesthe frequency of the soundproduced. The triangular waveformof the astable multivibrator is takenout from the junction of pins 2 and6 of IC3. This waveform is fed as thecontrol voltage at pin 5 of IC4 throughresistor R18. The output received frompin 3 of IC4 is fed to the base of transistorT3 to drive an 8-ohm loudspeaker(LS1), which generates the bird-chirping sound.For the chirping-sound alarm generator,assemble the circuit shown inFig. 1 on a separate general-purposePCB and enclose in a small box. And ifyou want an alarm circuit with Britishpolice siren tone, assemble the circuitshown in Fig. 2 on another generalpurposePCB and connect it to pointsA and B of the control unit shown inFig. 1 after removing the circuit on theright side of the dotted line. Use a 9V,500mA standard adaptor to power thecircuit.This circuit may be used as a securityalarm in banks, households andmotorcars.
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