Simple Heat Sensor Circuit using a Single Transistor

This simple heat sensor circuit uses just one transistor and a special resistor that acts like a thermometer.

As things heat up, this resistor changes its behavior, which the transistor picks up on.

Depending on the temperature, the transistor acts like a gatekeeper letting electricity flow through or blocking it completely.

This way, the circuit can tell if things are getting too hot.

Circuit Working:

Parts List:

An arrangement involving a BC547 transistor configured as a common emitter amplifier is utilized.

The base of the transistor is linked to a central arm of a preset resistor.

Ground is connected to the emitter, while the collector is coupled to an LED through a 470 ohm resistor in series.

The outer arms of the preset resistor are interconnected with one arm connected to a 4.7k NTC thermistor and the other arm grounded.

This preset resistor serves to calibrate the temperature detection threshold.

Acting as a temperature sensor, the NTC thermistor triggers the BC547 transistor when the temperature surpasses the predefined threshold.

Consequently, the LED and buzzer are activated indicating that the temperature has reached the maximum limit.

The circuit is operable with a 12V DC power supply.

Formulas:

In a variety of circuits, an NTC thermistor can be used as a heat sensor to sense temperature changes.

An NTC thermistor may be used as a heat sensor using the following simple formula and explanation:

Resistors known as NTC thermistors have a resistance that drops with temperature.

They are advantageous for temperature sensing applications because of this feature.

The Steinhart-Hart Formula

The Steinhart Hart equation describes the resistance R(T) of an NTC thermistor:

1 / T ​= A + Bln(R) + C(ln(R))³

where:

• T is the temperature in kelvin K.
• R is the resistance of the thermistor at temperature T.
• A, B, and Care coefficients specific to the thermistor and can be determined from the thermistors datasheet or calibration.

Practically speaking, the following formula provides an approximation of the connection between an NTC thermistors resistance and temperature:

T = 1 / A+Bln(R) + C(ln(R))³

where;

• T is the temperature in Kelvin K, R is the resistance of the thermistor, and A, B, and C are constants determined from the thermistors datasheet or calibration.

Note:

For precise temperature calculations, make sure the coefficients A, B and C in the formula match those listed on the datasheet for your NTC thermistor.

To account for any departures from the ideal behavior that the formula describes, calibration could be required.

With this method, you may utilize an NTC thermistor as a heat sensor in an efficient manner by applying certain datasheet derived coefficients or established formulae to translate the resistance of the device into a temperature value.

How to Build:

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

Circuit Layout:

• Connect the base of the BC547 transistor to the central arm of the preset resistor.
• Connect the emitter of the transistor to ground.
• Connect the collector of the transistor to the anode longer lead of the LED.
• Connect the cathode shorter lead of the LED to one terminal of the 470 ohm resistor.
• Connect the other terminal of the 470 ohm resistor to the positive terminal of the power supply.
• Connect one outer arm of the preset resistor to the 4.7k NTC thermistor.
• Connect the other outer arm of the preset resistor to ground.

Temperature Detection Threshold Adjustment:

• Adjust the preset resistor to set the desired temperature detection threshold.
• Turning the preset resistor will change the voltage at the base of the transistor altering the threshold at which the circuit triggers.

Testing:

• Apply the 12V DC power supply to the circuit.
• Monitor the LED and buzzer to ensure they activate when the temperature surpasses the set threshold.
• Adjust the preset resistor as necessary to fine tune the temperature detection threshold.

Finalization:

• Secure all connections and components properly.
• Label the circuit if necessary for future reference.

Note:

By following these steps, you should be able to construct the temperature detection circuit successfully.

Remember to exercise caution when working with electronic components and ensure that all connections are made accurately to avoid any potential hazards or malfunctions.

Conclusion:

In conclusion, a heat sensor circuit using a single transistor and a thermistor offers a basic and efficient means of detecting temperature changes.

By leveraging the temperature sensitive properties of the thermistor and the transistors amplification capabilities this circuit can reliably indicate variations in temperature through an LED or buzzer.

Its simplicity makes it accessible for hobbyists and small scale applications where temperature monitoring is essential.

Additionally, with appropriate adjustments and component selection this circuit can be tailored to suit specific temperature detection requirements.

References:

Temperature Sensing with NTC Circuit

Thermistor-Based Temperature Sensing System