Basic LM358 Based Fire Alarm Using NTC

NTC is a variable resistor that changes its resistance with temperature. Usually, the resistance value decreases with the increase in temperature.

This circuit is so simple and basic. It is great for learning how an op-amp works as a comparator. This is mainly a voltage comparison-based alert circuit. It compares the output voltage of the NTC with the preset value. When the preset value is being touched or crossed, the buzzer buzzes.

To build this project, you need a very few components. Those are:

1. LM358 Op-Amp
2. 10k NTC
3. 10kΩ Resistor
4. 10kΩ Potentiometer
5. Buzzer
6. DC Socket
7. Printed Circuit Board (PCB)
8. 8-pin IC base (optional)

After placing all the components on the board, soldering needs to be done. That is why the Soldering Iron and Solder Lead are mandatory. You can also try on the breadboard. It is not required to build it on a PCB.

First, need to draw the schematic diagram of the circuit. Any software can be used. I have done it using EasyEDA.

The inverting pin of the op-amp is connected to the voltage-dividing node. This node has been created using a 10 kΩ resistor and a 10 kΩ NTC thermistor. The non-inverting pin has been connected to the potentiometer. This potentiometer sets the threshold value. When the temperature rises, the resistance falls along with the voltage. When the voltage is less than the threshold, the output of the op-amp goes high. It means the buzzer is on.

The schematic diagram has been transformed into the PCB design. After completing the design, the printing process begins. I have printed the board from a third-party shop. If you have a breadboard and are comfortable with it, then build it on the breadboard.

After placing all the components on the board, solder them to fix the circuit to the PCB. The project is completed. Now,

1. Power up the circuit.
2. Using a multimeter, check the voltages of the potentiometer.
3. Use the potentiometer to set a threshold value.
4. Bring fire or hot materials in front of the NTC.
5. The rising temperature will decrease the resistance, thus the voltage.
6. Finally, the buzzer buzzes up. The buzzer will go down again when the temperature falls and the resistance goes up beyond the limit.