Optical Smoke Alarm



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.

Circuit diagram of the optical smoke alarm
Fig. 1: Circuit diagram of the optical smoke alarm

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.

An actual-size PCB pattern of the optical smoke alarm
Fig. 2: An actual-size PCB pattern of the optical smoke alarm
Component layout of the PCB
Fig. 3: Component layout of the PCB

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


  1. Comment:hello sir I am student of engg I choose the topic optical smoke alarm in this project I am confuse , how many sensor are use in one circuit mean when we use in application in office,house etc and in one room how many sensor we can use.

  2. Insert a thin opaque object in the slotted area of the sensor( MOC7811), you will see the LED glow and hear the alarm sound from the buzzer.


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