Photoelectric Smoke detector

An very simple smoke detector circuit can be constructed using a 555 timer circuit and a photo interrupter . The photo interrupter module is used as the smoke detector, while timer 555 is wired in astable configuration as an AF oscillator for sounding alarm via a loudspeaker.

In the absence of any smoke, the gap of photo interrupter module is clear and the light from LED falls on the phototransistor through the slot. As a result, the collector of phototransistor is pulled towards ground. This causes reset pin 4 of IC 555 to go low. Accordingly, the timer is reset and hence the alarm does not sound.

When smoke is present in the gap of the photo interrupter module, the light beam from LED to the phototransistor is obstructed. As a result, the phototransistor stops conducting and pin4 (reset) of IC 555 goes high to activate the alarm.
The photo transistor and LED must be housed inside an enclosure with holes to allow entry of smoke.
This smoke detector circuit project require a power supply between 9 and 12 volts .