Working of the circuit is simple. The alarm generator buzzer or Horn is activated by an SCR and its triggering is under the control of the transistor T1. In the Armed position, Switch S2 is closed and ...

At the heart of this ultra simple smoke alarm is H21A1, which is a photo-interrupter ... conduct to activate the rest of the circuit. When the darlington pair conducts, the gate voltage of SCR BT169 ...