This simple smoke detector is a highly sensitive but inexpensive device. It uses a Darlington-pair amplifier employing two NPN transistors and an infrared photo-interrupter module as the sensor. The circuit gives an audio-visual alarm whenever thick smoke is present in the environment.

Sensor Base Smoke Detector enters the gap, the IR rays falling on the phototransistor are obstructed. As a result, the phototransistor stops conducting and the Darlington-pair transistors conduct to activate the buzzer and light-up LED.

When the Smoke Detector in the gap is cleared, light from the IR LED falls on the phototransistor and it starts conducting. As a result, Darlington-pair transistors stop conducting, and the buzzer and LED turn off. For maximum sensitivity, adjust presets are used to control the sensitivity of the photo-interrupter module.