Fig. 1 shows the author's prototype while Fig. 2 shows the MQ2 sensor module used for the smoke alarm. Circuit diagram for the alarm is shown in Fig. 3.
The circuit comprises a stepdown transformer (X1), bridge rectifier (BR1), 5V voltage regulator LM7805 (IC1), MQ2 sensor module (connected across CON2), op-amp LM358 (IC2), sound generator UM3562 (IC3), three transistors (T1 through T3), an 8-ohm 0.5-watt loudspeaker (LS1), and several other components. Capacitor C1, connected across the supply terminals, minimises voltage ripples and noise signals.
The circuit works on 5V DC, which it derives from the transformer X1. The 9V AC secondary of X1 connects to bridge rectifier BR1 for rectification, which then feeds the regulator LM7805 to obtain a stable 5V supply for the circuit. The glowing LED1 indicates the presence of smoke near the sensor module.
The heart of the circuit lies in the MQ2 smoke sensor module and the sound generator IC UM3562. The smoke sensor module used here has three pins, but you can opt for a four-pin module as well.
2: MQ2 sensor module
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