Introduction: FIRE SENSOR

Hello everyone!

Fire sensor is a sensor designed to detect and respond to the presence of a flame or fire. Here, it is a PIN diode based fire sensor that activates 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.

Step 1: Hardware Required

  • CA3140 OP-AMP - 1
  • CD4060 COUNTER - 1
  • BPW34 PIN photodiode
  • LED 5 mm - 3
  • 9V BATTERY-1
  • 0.22uf ceramic disk capacitor-1
  • 1M ohm resistor- 3
  • 1k ohm resistor - 2
  • 100-ohm resistor - 3

Step 2: Circuit Diagram

Circuit diagram of the PIN diode based fire sensor is shown above in the image. It is built around 9V battery, PIN diode BPW34, op-amp CA3140(IC1), counter CD4060(IC2), transistors BC547, a piezo buzzer and a few other components.

In the circuit, PIN photodiode BPW34 is connected to the inverting and non-inverting inputs of op-amp IC1 in reverse-biased mode to feed photocurrent into the input of the op-amp. CA3140 is a 4.5MHz BiMOs op-amp with MOSFET inputs and bipolar output.

Gate-protected MOSFET (PMOS) transistors in the input circuit provide very high input impedance, typically around 1.5T ohms. The IC requires very low input current, as low as 10pA, to change output status to high or low.

In the circuit, IC1 is used as a transimpedance amplifier to act as a current-to-voltage converter. IC1 amplifies and converts the photocurrent generated in the PIN diode to the corresponding voltage in its output. The non-inverting input is connected to the ground and anode of the photodiode, while the inverting input gets photocurrent from the PIN diode.

Step 3: Circuit Operation

Large-value feedback resistor R1 sets the gain of the transimpedance amplifier since it is in inverting configuration. Connection of non-inverting input to ground provides low impedance load for the photodiode, which keeps the photodiode voltage low.

The photodiode operates in the photovoltaic mode with no external bias. Feedback of the op-amp keeps the photodiode current equal to the feedback current through R1. So the input offset voltage due to the photodiode is very low in this self-biased photovoltaic mode. This permits a large gain without any large-output offset voltage. This configuration is selected to get large gain in low-light conditions.

Normally, in ambient light condition, photocurrent from the PIN diode is very low; it keeps the output of IC1 low. When the PIN diode detects visible light or IR from fire, its photocurrent increases and transimpedance amplifier IC1 converts this current to the corresponding output voltage. High output from IC1 activates transistor T1 and LED1 glows. This indicates that the circuit has detected fire. When T1 conducts, it takes reset pin 12 of IC2 to ground potential and CD4060 starts oscillating.

IC2 is a binary counter with ten outputs that turn high one by one when it oscillates due to C1 and R6. Oscillation of IC2 is indicated by the blinking of LED2. When output Q6 (pin 4) of IC2 turns high after 15 seconds, T2 conducts and activates piezo buzzer PZ1, and LED3 also glows. The alarm repeats again after 15 seconds if fire persists.

You can also turn on an AC alarm that produces a loud sound by replacing PZ1 with a relay circuitry (not shown here). The AC alarm is activated through contacts of the relay used for this purpose.

Step 4: Schematic and Layout Design

A PCB for PIN-based fire sensor is designed using EAGLE. The schematic and board-layout are shown above in the image.

Step 5: Sending Gerber Files to the Manufacturer

After exporting my GERBER files from EAGLE I'm uploading them on LIONCIRCUITS to get my board manufactured. I usually order my PCBs from them only. They provide low-cost prototyping only within 6 days.

Step 6: Fabricated Boards

I have received my board from LIONCIRCUITS and I am sharing my Gerber files with you in case anyone needs the board to be manufactured.

Step 7: Assembling and Testing

After assembling my board with components it looks like this.

Testing the circuit is simple. Normally, when there is no fire flame near the PIN diode, the piezo buzzer does not sound. When a fire flame is sensed by the PIN diode, the piezo buzzer sounds an alarm. Its detection range is around two meters.

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