This Simple Thermistor Based Fire Alarm Circuit is very small, simple and smart, as it uses a thermistor to sense heat fast; when temperature goes high the circuit wakes up and makes loud sound.
Here, this circuit uses no microcontroller or complex parts, but it uses only a few components and provides a working fire alert that anyone can build at home.
Circuit Working:
Parts List:
| Components | Values | Quantity |
|---|---|---|
| Resistors | 1k 1/4 watt | 1 |
| 560Ω 1/4 watt | 1 | |
| Thermistor 10k NTC | 1 | |
| Semiconductors | Transistor BC547 NPN | 1 |
| Diode 1N4007 | 1 | |
| Speaker 8Ω small speaker | 1 | |
| Power Supply +6V DC | 1 |
To begin with, the 10k thermistor and R1 resistor make a voltage divider.
At normal temperature, thermistor has high resistance and voltage at the divider midpoint is low; then D1 diode 1N4007 passes this voltage to the R2 base resistor.
After that, the Q1 transistor base gets very small current, so transistor stays OFF and speaker stays silent.
When temperature rises, thermistor resistance drops and voltage at midpoint becomes higher; then more current reaches the transistor base through R2 resistor.
Now transistor turns ON and speaker receives current and makes alarm sound.
Formulas:
Below is the Thermistor divider voltage formula:
Vout = Vin × Rfixed / (Rthermistor + Rfixed)
where,
- Vout is the voltage between Rthermistor and Rfixed.
- Vin is the supply voltage.
- Rthermistor and Rfixed make a divider.
As thermistor resistance changes, Vout changes; also the formula shows the midpoint voltage when the thermistor value changes with heat.
How to Build:
To build a Simple Thermistor Based Fire Alarm Circuit follow the below connection steps:
- First, gather all the parts as shown in circuit diagram.
- Next, connect 6V positive to upper side of thermistor and connect lower side of thermistor to midpoint.
- Then from midpoint node connect one side of R1 resistor to ground and from same midpoint connect anode of diode 1N4007.
- After that, connect diode cathode to one side of R2 resistor and connect other side of R2 resistor to base of Q1 transistor.
- Now connect Q1 transistor emitter to ground and connect Q1 transistor collector to negative terminal of speaker.
- Finally, connect positive terminal of speaker connect to 6V supply.
Conclusion:
Overall, this Simple Thermistor Based Fire Alarm Circuit is easy and small to build, as it uses very basic parts and still work good when heat goes high.
Also, it is a good project to learn sensor and transistor ON and OFF action and this circuit can improve later by adding buzzer, relay or better heat setting.