Fire Alarm system (soft ware design)

This light-sensitive fire alarm circuit is extremely sensitive, low-cost, and easy to assemble, even by a beginner. It uses an infrared phototransistor as a fire sensor.

The circuit diagram of the light-sensitive fire alarm is built around the L14F1 photo transistor (T1), BC547 transistor (T2), and NE555 timer (IC1).

T1 can sense both infrared and visible light. A NE555-based, stable multi-vibrating system is used for audible warning. Resistors R3 and R4 and capacitor C1 decide the frequency. Here, the frequency is around 272 Hz, which can be changed by changing the values of R3, R4, or C1.

When T1 senses light, T2 stops conducting. As a result, reset pin 4 of IC1 is held high. This causes the multivibrator to function and an alarm to sound in the loudspeaker LS1.