Automated alarm circuits
- 1. circuit ideas Automated Alarm Circuits s.c. dwiv edi Pallabi Sarkar and Normally, when the door is closed, Anirban Sengupta reed switch S1 is closed, transistor T1 conducts and the monostable multivi- When the door is opened, reed T wo alarm circuits are presented brator (IC1) remains in standby mode switch S1 gets disconnected, T1 stops here. One produces bird-chirp- with ‘low’ output at pin 10. conducting and low-to-high pulse at pin ing sound and the other British police siren tone. Fig. 1 shows the circuit of the bird- chirping-sound alarm unit along with the circuit of the control unit. Fig. 2 shows the circuit of only the British police siren tone generator, which has to be integrated with the control circuit portion of Fig. 1 at points A and B to complete the circuit diagram of auto- mated alarm. The control unit is built around ICs CD4047 and CD4027 (as shown on the left side of the dotted line in Fig. 1). As mentioned earlier, it is common to both the alarm circuits. IC CD4047 (IC1) is wired in positive-edge-triggering monostable multivibrator mode to set and reset IC CD4027 (IC2). The output pulse width of IC1 depends on the values of capacitor C2 and resistor R3 connected to its pins 1, 2 and 3. Fig. 2: Alarm circuit that generates police siren tone Fig. 1: Alarm circuit that generates bird-chirping sound 9 6 • F e b r ua ry 2 0 0 9 • e l e c t ro n i c s f o r yo u w w w. e f y m ag . co m
- 2. circuit ideas 8 of IC1 triggers the monostable and a T2 (BC548), which enables the alarm ing sound. short-duration positive pulse of about circuit. For the chirping-sound alarm gen- 10 seconds is available as Q output at The output at point A is used to erator, assemble the circuit shown in pin 10. At the same time, complementary enable the alarm tone generator circuit Fig. 1 on a separate general-purpose output Q goes low at pin 11. The output (on the right side of the dotted line) PCB and enclose in a small box. And if from IC1 is used to set and reset IC2. consisting of two 555 timer ICs marked you want an alarm circuit with British IC2 is a low-power, dual J-K mas- as IC3 and IC4. The R-C network de- police siren tone, assemble the circuit ter/slave flip-flop having independ- termines the frequency of the sound shown in Fig. 2 on another general- ent J, K, set, reset and clock inputs. produced. The triangular waveform purpose PCB and connect it to points The flip-flops change states on the of the astable multivibrator is taken A and B of the control unit shown in positive-going transition of the clock out from the junction of pins 2 and Fig. 1 after removing the circuit on the pulses. IC2 is wired such that its Q 6 of IC3. This waveform is fed as the right side of the dotted line. Use a 9V, output turns ‘high’ when reset pin 4 control voltage at pin 5 of IC4 through 500mA standard adaptor to power the receives a high pulse. When set pin 7 resistor R18. The output received from circuit. receives a high pulse, Q output goes pin 3 of IC4 is fed to the base of transis- This circuit may be used as a secu- low and Q output goes high. This tor T3 to drive an 8-ohm loudspeaker rity alarm in banks, households and lights up LED2 and drives transistor (LS1), which generates the bird-chirp- motorcars. w w w. e f y m ag . co m e l e c t ro n i c s f o r yo u • F e b r ua ry 2 0 0 9 • 9 7