Simple Heat Sensor Circuit using a Single Transistor

This Simple Heat Sensor Circuit uses only one transistor and one special resistor like thermometer.

When it get hot this resistor changes to how it work because of which the transistor sees this change.

Transistor work like door and if temperature changes it opens or closes for electric.

So the circuit can detect when something is too hot.

Circuit Working:

Parts List:

This circuit uses BC547 transistor like common emitter amplifier.

Transistor base connect to middle pin of preset resistor.

Emitter connects to the ground and collector connects to LED with 470 ohm resistor in series.

Two side pins of preset resistor connects one side with 4.7k NTC thermistor and other side connects to ground.

Preset resistor help to set heat limit point.

NTC thermistor work like heat sensor when temperature go over set point and it turns ON BC547 transistor.

Then LED and buzzer turns ON and indicates that temperature is too high.

Circuit work with 12V DC power.

Formulas:

NTC thermistor is used as heat sensor in many circuits to feel temperature change.

NTC thermistor is special resistor when temperature goes up and resistance go down.

This makes it good for checking temperature.

Steinhart Hart Formula:

This formula help to know temperature from thermistor:

1 / T ​= A + Bln(R) + C(ln(R))³

where:

• T is temperature in Kelvin (K)
• R is resistance of thermistor at that temperature
• A, B, C are values from thermistor datasheet

Simple version of formula also look like:

T = 1 / A+Bln(R) + C(ln(R))³

where;

• T is temperature
• R is resistance and
• A, B, C are numbers from datasheet.

Note:

To get correct temperature use right A, B, C from the thermistor datasheet.

Sometimes calibration needed to make it more accurate.

Using this formula with real values the NTC thermistor can work good as temperature sensor.

Just change the resistance into temperature using this method.

How to Build:

To build a Simple Heat Sensor Circuit using a Single Transistor one need to follow the below mentioned steps:

Circuit Design:

• Connect middle pin of preset resistor to base of BC547 transistor.
• Connect emitter of transistor to ground.
• Connect collector of transistor to long leg anode of LED.
• Short leg cathode of LED connects to one side of 470 ohm resistor.
• Other side of resistor connects to positive side of power source of 12V DC.
• One side pin of preset resistor connect to 4.7k NTC thermistor.
• Other side pin of preset resistor connects to ground.

Set Temperature Limit:

• Turn preset resistor to set how hot it should be before LED and buzzer turn ON.
• Turning it change voltage on transistor base so circuit turn ON at different temperature.

Testing:

• Give 12V DC power to circuit.
• Watch LED and buzzer as they should turn ON when it gets hot enough.
• If not right then turn preset resistor more to adjust.

Final Step:

• Check for all wires and parts are fixed well.
• If required then put name label on circuit for future.

Note:

• If following these steps one can make heat sensor circuit work good.
• Be careful with wires and parts and ensure all are connected in correct way to stay safe.

Conclusion:

To conclude this Simple Heat Sensor Circuit using a Single Transistor and one thermistor works simple and good to check temperature change.

Because thermistor change with heat and transistor can make signal stronger this small circuit can show temperature change with LED or buzzer.

It easy to build so it is good for students, hobby people or small projects where one needs to watch a heat.

Also one can change parts or adjust preset to make it work for different heat levels.

References:

Temperature Sensing with NTC Circuit

Thermistor-Based Temperature Sensing System