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This light sensitive fire alarm circuit is extremely sensitive, low-cost and easy to assemble even by a beginner. It uses infrared photo transistor as fire sensor.
Circuit diagram and Working:
Circuit diagram of the light sensitive fire alarm is shown below. It is built around L14F1 photo transistor (T1), BC547 transistor (T2) and NE555 timer (IC1).
T1 can sense both infrared and visible light. NE555 based a stable multi vibrator is used for audible warning. Resistors R3 and R4 and capacitor C1 decide the frequency. Here, frequency is around 272Hz, which can be changed by changing 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 makes the multivibrator to function and an alarm to sound in loudspeaker LS1.
Construction and Testing:
Single side PCB of the light sensitive fire alarm and its component layout is shown below. After assembling the circuit on a PCB, enclose it in a suitable plastic case(optional).
Download the PCB and component layout PDFs: click here