Sensitive Fire Alarm


Very sensitive. Detects Fire from a distance of 2 meters in a dark room. Beeps loudly till Fire ceases. Since it is detecting Infrared rays, it can be also used as a TV Remote tester.

MOSFET IRF Z 44 N is used as a switch to drive buzzer and LED. Buzzer and LED are connected to the Drain (D) of MOSFET through 100 Ohms resistor ( R3 ). Photodiode is connected in the Reverse bias mode with its Anode to the Gate ( G ) of MOSFET. Cathode of Photodiode is connected to the positive rail through 1 Meg resistor (R1). To prevent the latching of MOSFET, 1 Meg resistor ( R2 ) is connected between the Gate ( G ) and Source ( S ) of the MOSFET. Negative of battery goes to the Source of MOSFET.


Testing – Connect battery .Buzzer will beep if there is ambient sunlight in the room. Move to a place without much light. Ignite a lighter from a distance of 1 meter. Buzzer will beep and LED lights.

NoteThe circuit is very sensitive in a dark room. Since day light and light from fluorescent lamps contain infrared rays, circuit will be triggered. This can be solved by enclosing the Photodiode in a black tube with its tip exposed. LED lights, CFL and Fluorescent lamps will not trigger the circuit.