Here is a PIN diode-based fire sensor that sounds an alarm when it senses fire. Thermistor-based fire alarms have a flaw: the alarm only goes off if the fire is close enough to heat the thermistor. For longer-range fire detection, this circuit uses a sensitive PIN diode as a fire sensor. It can pick up visible light and infrared (IR) light with wavelengths between 430nm and 1100nm. So, it's easy for both visible light and infrared light from the fire to set off the sensor and set off the alarm. It also looks for sparks in the mains wiring and sounds an alarm if they don't go away. It is a great way to protect stores, lockers, and record rooms. PIN diode in the circuit, BPW34 is used as a light and infrared (IR) sensor. BPW34 is a two-pin photodiode with an anode (A) and a cathode (C) (K). The flat top of the photodiode makes it easy to find the end that is the anode. The anode is a small solder point to which a thin wire is attached. The cathode is the other small solder point. BPW34 is a small PIN photodiode or mini solar cell with a light-sensitive surface that makes 350mV DC when exposed to 900nm light. It is sensitive to both natural sunlight and firelight. So, it works perfectly as a light sensor. The BPW34 photodiode can be

used both with no bias and with reverse bias. When light shines on it, its resistance goes down. In Figure shows a diagram of how the PIN diode-based fire sensor works. It is made up of a 9V battery, a PIN diode (D1), an op-amp (IC1), a counter (IC2), two transistors (T1 and T2), a piezo buzzer (PZ1), and a few other parts. In the circuit, the PIN photodiode BPW34 is connected in reverse-biased mode to the inverting and non- inverting inputs of op-amp IC1 to feed photocurrent into the op-input. amp's The CA3140 is a 4.5MHz BiMOs op-amp with MOSFET inputs and a bipolar output. In the input circuit, gate-protected MOSFET (PMOS) transistors give a very high input impedance, usually around 1.5T ohms. The IC only needs a very small amount of current, as little as 10pA, to change the status of the output from high to low. In the circuit, IC1 is a transimpedance amplifier that converts current to voltage. The photocurrent made by the PIN diode is amplified and turned into the voltage that goes with it by IC1. The non-inverting input is connected to ground and the anode of the photodiode. The photocurrent from the PIN diode goes to the inverting input. Circuit operation Since the transimpedance amplifier is in inverting configuration, the gain is set by R1, which has a large value. When the non-inverting input is connected to ground, it gives the photodiode a low impedance load. This keeps the voltage of the photodiode low. The photodiode works in the photovoltaic mode when there is no outside bias. Feedback from the op-amp makes sure that the current through the photodiode is the same as the current through R1. In this self-biased photovoltaic mode, the photodiode's input offset voltage is very low. This lets a big gain happen without a big offset voltage at the output. This configuration is chosen to get a big gain when there isn't much light. In normal light conditions, the PIN diode's photocurrent is very low. This keeps the output of IC1 low. When the PIN diode picks up visible light or IR from a fire, its photo current goes up, and the transimpedance amplifier IC1 turns this current into a voltage that matches the output. When IC1's output is high, transistor T1 turns on,

which makes LED1 light up. This means the circuit has picked up on fire. When T1 is open, it connects reset pin 12 of IC2 to ground. This makes CD4060 start to oscillate. When C1 and R6 cause IC2 to oscillate, each of its ten outputs goes high one at a time. The blinking of LED2 shows that IC2 is moving back and forth. After 15 seconds, when the output Q6 (pin 4) of IC2 goes high, T2 conducts. This turns on the piezo buzzer PZ1 and makes LED3 light up. If there is still fire after 15 seconds, the alarm will sound again. By replacing PZ1 with a relay circuit, you can also turn on an AC alarm that makes a loud noise (not shown here). The contacts of the relay that is used for this purpose are what turn on the AC alarm.

Put the PCB in a small box in a way that makes it easy to connect the PIN diode BPW34 on the back of the box. Put the PIN diode in a good spot and cover it so that it doesn't get normal light or sunlight. It's easy to test the circuit. Usually, the piezo buzzer doesn't go off when there is no fire flame near the PIN diode. When the PIN diode detects a fire flame, the piezo buzzer sounds an alarm. It can find things up to about two meters away. It can also find sparks caused by a short circuit in the mains wiring.