At the heart of this ultra simple smoke alarm is H21A1, which is a photo-interrupter module consists of a gallium-arsenide infrared LED coupled to a silicon phototransistor in a plastic housing. The slot between the infrared sender(LED) and receiver (phototransistor) allows interruption of the signal with smoke, switching the module output from ‘on’ to ‘off’.
When the smoke enters the slot, the infrared beam falling on the phototransistor is obstructed. As a result, the phototransistor stops working and the transistors (T1&T2), wired as a simple darlington pair, conduct to activate the rest of the circuit.
When the darlington pair conducts, the gate voltage of SCR BT169 (T3) goes high and the SCR is fired. Consequently, the gate terminal of mosfet BS170 (T4) receives positive voltage through resistor R4 and the active piezo-sounder (BZ1) energises to latch! Visual indicator LED (D1) lights up instantly.
The buzzer sounds to indicate the presence of smoke, unless you switch off the circuit by disconnecting the 9 volt dc input supply. For optimum performance, adjust trimpots P1, P2 and the value of resistor R1.