This optical smoke alarm (Fig. 1) is built around opto-interrupter MOC7811 (IC1) and dual op-amp LM358 (IC2).
When thick black smoke enters in the gap (slotted area) of the opto-interrupter sensor (IC1), its internal photo transistor stops conduction. As a result, output of the dual op-amp goes low. So transistor T1 stops conduction and T2 starts conducting. This makes the piezo buzzer (PZ1) sound an alarm and LED1 to glow.
Potmeter VR1 is used to control intensity of the internal LED of MOC7811. Potmeter VR2 is used to control the reference voltage at pin 3 of IC2.
The circuit works off a 9V battery.
Construction and testing
An actual-size, single-side PCB for the optical smoke alarm is shown in Fig. 2 and its component layout in Fig. 3. After assembling the circuit on PCB, enclose it in a suitable box. Install MOC7811 at a proper location for monitoring smoke or fire.
Fix CON1, LED1 and piezo buzzer on the front side of the cabinet. VR2 is used for reference setting. Connect a 9V battery to CON1.
Download PCB and Component layout PDFs: Click here
Pradeep G. is B.Sc. (Physics) and a regular contributor to international magazines. He is also a small-business owner in south India