Fire Alarm Circuit using IC LM358

This Fire Alarm Circuit using IC LM358 is simple but smart.

It uses a 10k thermistor and LM358 IC.

It watches heat using a thermistor and makes buzzer ring when danger comes.

This circuit is cheap and is good for home, school projects and safety demos.

Circuit Working:

Parts List:

A 10k thermistor changes resistance when temperature rises.

At normal temperature thermistor has high resistance.

LM358 compares voltage of thermistor side and voltage of 10k pot reference side.

When heat increases thermistor resistance goes down.

Voltage at INA+ pin 3 changes.

LM358 output goes high and buzzer turns ON.

And when temperature is low buzzer stays OFF.

Formulas:

Below is the thermistor voltage divider formula:

Voltage on thermistor is:

Vthermistor = Vcc * (Rfixed / (Rthermistor + Rfixed))

When temperature goes up, Rthermistor goes down.

So Vthermistor becomes higher at LM358 input.

Comparator rules:

If INA+ is more than INA− then output goes high.

If INA+ is less than INA− then output goes low.

Example with 5V:

If Rthermistor = 10k and Rfixed = 10k then,

Vthermistor = 5 * (10k / (10k + 10k)) = 2.5V

If heat drops Rthermistor to 5k then,

Vthermistor = 5 * (10k / (5k + 10k)) = 3.33V

Now input voltage is higher so comparator output becomes high and buzzer turns ON.

How to Build:

To build a Fire Alarm Circuit using IC LM358 follow the below steps for connection:

• Take all the parts as shown in circuit diagram.
• LM358 has two op amps.
• We have used only the first op amp.
• Pin 1 is OUTA pin connect this pin to buzzer on end.
• Other end of buzzer connect to GND
• Pin 2 (INA-) goes to 10k pot one end and other end goes to positive of 5V.
• Pin 3 (INA+) goes to thermistor voltage divider one end.
• And other end of thermistor goes to positive of 5V.
• Pin 4 goes to GND
• Pin 8 goes to +5V

Conclusion:

This project is for simple Fire Alarm Circuit using IC LM358.

When heat rises voltage changes and buzzer sounds.

It is low cost and good for beginners.

References:

fire detection system with lm358 using arduino and labview.I want help regarding if my circuit is correct or not?VI