Here is a simple Fire Alarm Circuit designed by using 10K NTC (Negative Temperature Coefficient) Thermistor and LM358 IC. This circuit produces a visible light signal using LED when the temperature will raise beyond the thermistor’s threshold limit, and this threshold limit can be varied by using variable resistance. Fire alarm circuit is used as an indicator or warning circuit for excessive fire and temperature. This circuit can be used in offices, school, laboratory etc. to prevent and detect any disasters due to fire.
Connection for Fire Alarm Circuit using LM358:
We make a fire alarm circuit using 10K NTC thermistor, LM358, 33K resistor and a LED.
To connect the all components according to above circuit diagram.
- The inverting terminal (Pin 2) of LM358 is connected with one terminal of a 10K POT.
- One end of thermistor and 9V dc power supply are connected with another terminal of 10K POT.
- The non-inverting terminal (Pin 3) of LM358 and 33K resistance (To limit the current pass through thermistor) are connected with a 10K NTC thermistor.
- The output terminal (Pin 1) is connected with LED (Positive terminal).
- 9V dc power supply is connected with pin 8 of LM358and pin 4 is grounded.
Working Principle of Fire Alarm Circuit:
The main purpose of this fire alarm circuit is to turn on the LED when fire indicates. The LM358 is a low power dual operational amplifier integrated circuit, which is used in this circuit to detect high temperature. This is an 8 pin Comprising two operational amplifiers at low power. In this circuit we use 10K NTC (Negative Temperature Coefficient) thermistor as a sensing element. That means the thermistor resistance across its two terminals will greatly reduce when the heat or temperature increases, so it allows more current to pass through the LM358 IC. So, when the temperature goes beyond the threshold voltage the LED get sufficient current to turn on. The threshold limit of NTC thermistor can be varied by adjusting resistance value. When temperature decreases the resistance across thermistor’s two terminals increases accordingly. So, high amount of current cannot pass through LED. So, then the LED will not glow.
See the Video for Reference: