Fire detector made with photodiode

This is an ultra-sensitive fire detector that uses a photodiode as a fire sensor. Once a spark or fire is detected in the power supply part of the equipment, the fire detector will immediately sound an alarm and immediately cut off the power supply.

The attached figure is a fire detector circuit. The voltage generated by the photodiode used in the infrared detector is proportional to the incident light or infrared light falling on it. Since this voltage is very small, a high-multiplier voltage amplifier is required to drive the rest of the circuit. The dual op amp chip 741 (IC1) in the figure is used as a Schmitt trigger with hysteresis effect. It converts the output signal of the photodiode into a pulse signal with a certain waveform. R1 and R2 form a voltage divider to give half of the power supply voltage to the inverting input pin ② of IC1, and R3 is a feedback resistor that feeds the output back to the inverting input pin.

The photodiode D1 is connected between the junction of the voltage divider R1 and R2 and the in-phase input pin ③ of IC1. When there is no light, the voltage across the photodiode is zero. At this time, the output of IC1 is low. When a spark or flame suddenly occurs, the photodiode is turned on, and the output of IC1 becomes high. This high potential can be maintained for a few seconds even after the photodiode is turned off. The high potential of IC1 is not affected by slight changes in the potential of the inverting input pin. Therefore, this is conducive to the triggering of the alarm circuit.

The high potential of IC1 is used to trigger the thyristor BT169 (SCR1) to push the relay RL1 to close. After closing, the fire appliance is immediately disconnected by the relay contact. SCR1 remains in the latched state until the switch S1 is pressed (disconnected). In addition, after SCR1 is turned on, the base of transistor T1 is biased and turned on, which in turn activates the alarm generator chip UM3562 (IC2), and the voltage regulator diode ZD1 keeps the power supply voltage of IC2 at a safe 3.1V. Connecting the ⑥ pin of IC2 to the ground can make IC2 sound a fire alarm. The output of IC2 is amplified by T2 to make the speaker Ls1 sound an alarm.

Connect the photodiode with a single-core shielded wire and place it close to the power supply of the appliance. To avoid false triggering of the circuit, the photodiode should be placed where direct light is minimal.