Simple Smoke Detector

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

 Simple smoke detector circuit :

The photointerrupter module (H21A1) consists of a gallium arsenide infrared LED coupled to a silicon phototransistor in a plastic housing. The slot (gap) between the infrared diode and the transistor  allows the signal to be interrupted with smoke, switching the module output from "on" to "off"state.



When smoke enters the gap, the infrared rays falling on the phototransistor are blocked. As a result, the phototransistor stops conducting, and Darlington turns on the transistor to activate the buzzer and light up LED1. 

When the smoke in the gap is removed, the light from the IR LED falls on the phototransistor and starts to conduct electricity. As a result, Darlington stops conducting the transistor, and the buzzer and LED1 are turned off. 

For maximum sensitivity, please adjust presets VR1 and VR2. VR1 is used to control the sensitivity of the photointerrupter module, and VR2 is used to control the sensitivity of Darlington to the transistor.

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