Your computer and television set, along with other electronic appliances, can be safeguarded by a fire alarm that is based on an ultrasensitive photodiode. When it detects a spark or fire in the power supply part of the instrument, it quickly triggers an alarm and cuts off the power supply. The fire sensor in this device is a photodiode, and it promptly sounds the alert. In order to detect a fire, the circuit makes use of the

photovoltaic characteristic of the photodiodes. Infrared detectors often make use of photodiodes, which generate a photo voltage that is proportional to the amount of incident light or infrared rays that fall on them. Typically, 1V is created in the photodiode when it is forward biased by accepting the photons. Here the passive infrared rays from the spark or fire are used to activate the photodiode to generate the photo voltage Since the photo voltage is relatively modest, a very sensitive voltage amplifier is needed to activate the remaining half of the circuit. Here dual op-amp IC 741 (IC1) is used as a Schmitt trigger with hysteresis. It does so by transforming the input voltage signals coming from the photodiode into a signal that has been shaped. In order to provide IC1's inverting input (pin 2) with a voltage that is half of the supply voltage, which is 4.5 volts, resistors R1 and R2 combine to produce a potential divider. Resistor R3 connects the output of IC1 to its inverting input. Photodiode Based Fire Detector Circuit Fig. 1: Circuit of the photodiode-based fire detector The photodiode is wired so that it connects to the junction of the R1-R2 divider as well as pin 3 of IC1, which is the non-inverting input. In the standby mode, the voltage across the photodiode will be zero, and the output of IC1 will remain low. This occurs when the photodiode is exposed to darkness. The photodiode begins to conduct current

and the output of IC1 rises to a high level whenever there is a spark or a fire. Because of the hysteresis of the Schmitt trigger, the amplifier is capable of high levels of sensitivity. Even after the photodiode loses its ability to conduct electricity, the output of IC1 maintains its high state for a few seconds. When the output begins to rise, even a minute shift in voltage at the inverting input has no impact on the state of the circuit. This is incredibly beneficial to activate the alarm even with a single spark. Circuit operation SCR BT169 (also known as SCR1) is triggered into action by the high output from IC1, which in turn activates the relay. As soon as the relay becomes active, the power supply to the gadget will be promptly switched off. The SCR1 push-to-off switch will not release its latched state unless it is pressed. When SCR1 is closed, base bias is applied to T1, which causes it to conduct and activate alarm generator UM3561 (IC2). Zener diode ZD1 keeps the supply voltage for IC2 at a safer level of 3.1 volts. By connecting pin 6 of IC2 to ground, an alert for the fire department will be activated once a fire is detected. The signal that is sent to the speaker comes from the output of IC2, which is amplified by the transistor T2.

On a perfboard, the photodiode-based fire detector circuit can be constructed in a straightforward manner. It is possible to power it using a regulated 12 V DC supply. Connect the device to the mains supply wire of 230V by passing it through the normally closed contacts of relay RL1. Connect the photodiode to the insulated single- core wire using, and position it in close proximity to, the power supply unit. Keep the photodiode in an area where there is little to no direct light so as to reduce the likelihood of it accidentally setting off the circuit.