Simple Fire Alarm Circuit Using Thermistor And LM358

In this tutorial, I will show you how to make a simple fire alarm circuit that detects nearby fire in a small radius and the sensor activates the op-amp LM358 to produce the output Sound through a buzzer. 

A fire alarm circuit is a part of the security system which helps in detecting or preventing essential damages in shopping malls, flat houses, shops, offices and many more 

There are many types of small fire alarm that exists like the LM358 alarm, NE555 timer alarm, LM341 alarm, transistor type alarm, and a few more, in this project, I will going to use the LM358 op-amp ic. I will also show you the detailed circuit schematic, circuit connections, circuit working, video tutorials, and also its applications.

If you know more about how fire alarm works then check out Fire Alarm Fundamentals


How to make a Simple Fire Alarm Circuit

Circuit Diagram

Components Required

  • 10 KΩ Thermistor
  • LM358 Op-Amp
  • 4.7 KΩ Resistor
  • 10 KΩ Potentiometer
  • Buzzer
  • Jumper Wire 
  • Breadboard 
  • 5V Power Supply

Circuit Connection

The design of the simple fire alarm circuit schematics and components are mentioned above. So let’s see the connection we make.

First, connect the 10 KΩ potentiometers to the inverting terminal of the LM358 IC. Anyone end of the potentiometer terminal is connected to +5V, another one is connected to GND and the middle terminal is connected to the pin. 

We will now add a 10 KΩ thermistor and 10 KΩ resistors as a potential divider. The output of this potential divider i.e. the junction point is connected to the non–inverting terminal of the LM358 op-amp. 

Then we have chosen a buzzer for this project to make the alarm. So, connect the output of the LM358 IC to the buzzer directly. 

after this pin 8 will be directly connected to +ve and pin 4 will be directly connected to -ve of the LM358 IC.

Working Principle of Fire Alarm Circuit

The design of the simple fire alarm circuit connections is discussed above. We will now see the working principle of the simple fire alarm circuit. Here the main component for detecting the fire is the 10 KΩ thermistor. If the temperature increases, the resistance of the included thermistor decreases, and the output of the voltage divider increases. 

Since the output of the voltage divider is given to the non–inverting input terminal of the LM358 IC, its value will become more than that of the inverting input. As a result, the output of the op-amp becomes high and it activates the buzzer for alarm.

Application of Fire Alarm Circuit

Fire Alarm Circuits are very useful in homes, offices, schools, labs, industry, etc to detect any disasters due to fire accidents. 

Fire Alarm Systems can work as sensing security features like a smoke detectors, intruder alerts, motion detectors, gas detectors, etc.

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