This Fire Alarm Circuit using IC LM358 is simple but smart, as it uses a 10k thermistor and LM358 IC; also it watches heat using a thermistor and makes buzzer ring when danger comes.
Additionally, this circuit is cheap and is good for home, school projects and safety demos.
Circuit Working:
Parts List:
| Components | Value / Type |
|---|---|
| Resistors | |
| Resistor 4.7k 1/4 watt | 1 |
| Thermistor NTC 10k | 1 |
| Potentiometer 10k | 1 |
| Semiconductors | |
| IC LM358 | 1 |
| DC piezo buzzer | 1 |
| Power Supply +5V DC | 1 |
A 10k thermistor changes resistance when temperature rises and at normal temperature thermistor has high resistance; then LM358 compares voltage of thermistor side and voltage of 10k pot reference side.
When heat increases thermistor resistance goes down and voltage at INA+ pin 3 changes, then LM358 output goes high and buzzer turns ON and finally, 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 and 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:
- First, take all the parts as shown in circuit diagram.
- Next, pin 1 is OUTA pin connect this pin to buzzer on end and other end of buzzer connect to GND
- Then pin 2 (INA-) goes to 10k pot one end and other end goes to positive of 5V.
- After that, pin 3 (INA+) goes to thermistor voltage divider one end and other end of thermistor goes to positive of 5V.
- Also, pin 4 goes to GND and pin 8 goes to +5V
Conclusion:
To conclude, this project is for simple Fire Alarm Circuit using IC LM358, and when heat rises voltage changes and buzzer gives sounds.
Finally, its low cost project and good for beginners.
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