Saturday, June 22, 2024

Optical Smoke Alarm

An optical smoke alarm (also called photo-electric smoke alarm) works using the light scatter principle. The alarm contains a pulsed Infra red LED which pulses a beam of light into the sensor chamber every 10 seconds to check for smoke particles. This optical smoke alarm (Fig. 1) is built around opto-interrupter MOC7811 (IC1) and dual op-amp LM358 (IC2).

When a fire breaks out smoke will enter the optical chamber through the opening vents. When thick black smoke enters in the gap (slotted area) of the opto-interrupter sensor (IC1), its internal photo transistor stops conduction. The smoke particles cause the Infra red light to be scattered onto the photodiode light receptor. 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 controls the intensity of the internal LED of MOC7811. Potmeter VR2 is controls the reference voltage at pin 3 of IC2.

The circuit works off a 9V battery.

Circuit diagram of the optical smoke alarm
Circuit diagram of the optical smoke alarm

Optical smoke alarm construction and testing

An actual-size, single-side PCB for the optical smoke alarm and its component layout is given below. After assembling the circuit on PCB, enclose it in a suitable box. Install MOC7811 at a proper location for monitoring smoke or fire. Optical technology makes the alarms less prone to false alarms from cooking fumes.

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An actual-size PCB pattern of the optical smoke alarm
An actual-size PCB pattern of the optical smoke alarm
Component layout of the PCB
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

The project kit is available at


  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.


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