Simple Temperature Indicator Circuit using a Single Transistor

Building a basic temperature sensor is easier than you might think.

This post shows you how to make one with just a single transistor, a diode and a few other simple parts.

What is a Temperature Indicator Circuit using a Single Transistor:

A temperature indicator circuit using a single transistor is a simple electronic circuit that provides a visual indication of temperature changes.

The circuit typically uses a temperature dependent component such as a thermistor to sense the temperature.

This article will guide you through the construction details with essential formulas for optimal results.

Circuit Working:

Parts List:

Semiconductors including basic electronic components like transistors and diodes exhibit changes in their characteristics in response to variations in ambient temperature.

The alteration in their characteristics particularly voltage passage through them is directly proportional to the temperature difference in their surrounding.

The design involves configuring a diode and a transistor in the form of a bridge network.

Both components share similar properties concerning ambient temperature changes, making them complementary.

The diode serves as the reference device and the, transistor functions as the temperature sensor.

Positioning the diode as the reference device is critical.

It should be placed in an environment with relatively consistent temperature conditions.

Any deviation can lead to changes in the diodes reference level causing errors in the indication process.

A Light Emitting Diode LED is connected to the collector of the transistor.

The LED directly interprets the transistor conditions providing a visual indication of the temperature difference around the transistor.

The diode is placed at ambient or room temperature while the transistor is connected to the heat source being measured.

The base emitter voltage of the transistor is compared with the reference voltage level produced by the diode.

The LED serves as a direct indicator of the sensed temperature level.

As the heat over the transistor increases its base emitter voltage rises due to the changing characteristics of the transistor.

When the temperature exceeds the preset value the transistor starts conducting and the LED gradually illuminates with intensity directly proportional to the temperature over the sensor.

Caution

It is crucial to avoid exceeding the temperature over the transistor beyond 120 degrees celsius to prevent permanent damage.

Formula and Calculations:

How to Calculate the Temperature Thresholds

Resistor values for the configuration are calculated using the following formulas:

R₁ = U_b−0.6 / 0.005

R₂ = U_b − 1.5 / 0.015

here,

• U_b is the input supply voltage 0.6 is the forward voltage drop of the BJT and 0.005 is the standard operating current for the BJT.

Similarly, 1.5 is the forward voltage drop for the selected red LED and 0.015 is the standard current for optimal LED illumination.

The calculated results will be in Ohms.

The preset P1 can have a value between 150 to 300 Ohms for customization.

How to Build:

To build the simple temperature indicator circuit using a transistor and a diode follow the below mentioned connections steps:

• Collect all the necessary components mentioned above.
• Set up a breadboard and place the transistor, diode and LED on it.
• Ensure that the transistor is properly oriented.
• Connect the diode and transistor in the form of a bridge network.
• The diode serves as the reference device, while the transistor functions as the temperature sensor.
• Connect the LED to the collector of the transistor.
• This will be the visual indicator of the sensed temperature level.
• Connect the diode to create a reference voltage.
• The diode should be placed in an environment with consistent temperature conditions.
• Connect the preset to the circuit.
• This will be used to adjust the reference voltage level.
• Ensure that precautions are taken not to exceed the temperature over the transistor beyond 120 degrees celsius to prevent damage.
• If desired, modify the circuit to control an external appliance based on the sensed temperature levels.

Testing:

• Apply power to the circuit and observe the LEDs behavior.
• The LED should indicate changes in temperature becoming more illuminated as the temperature rises above the preset value.

Conclusion:

By following these construction details, you can build a simple and effective simple temperature indicator circuit using a transistor and a diode.

Remember to adhere to the provided cautionary measures and consider further modifications for enhanced functionality as discussed in the article.

References

Temperature sensor