Diode based fire sensor that activates an alarm when it detects fire. Thermistor based fire alarms have a drawback; the alarm turns on only if the fire heats the thermistor in close vicinity. In this circuit, a sensitive PIN diode is used as a fire sensor for a longer-range fire detection.t detects visible light and infrared (IR) in the range of 430nm – 1100nm. So visible light and IR from the fire can easily activate the sensor to trigger the alarm. It also detects sparks in the mains wiring and, if these persist, it gives a warning alarm.

It is an ideal protective device for showrooms, lockers, record rooms and so on.PIN diode BPW34 is used in the circuit as light and IR sensor. BPW34 is a 2-pin photodiode with anode (A) and cathode (K). The anode end can easily be identified from the top-view flat surface of the photodiode. A small solder point to which a thin wire is connected is the anode and the other one is the cathode terminal.BPW34 is a tiny PIN photodiode or mini solar cell with radiant sensitive surface that generates 350mV DC open-circuit voltage when exposed to 900nm light. It is sensitive to natural sunlight and also to light from fire. So it is ideal for use as a light sensor.
BPW34 photodiode can be used in zero-bias as well as reverse-bias states. Its resistance decreases when light falls on it.