Simple Temperature Indicator Circuit using a Single Transistor

Its not as challenging as one may imagine to build a simple temperature sensor circuit.

The article describes how to create one using a single transistor, a diode and a few other basic components.

What is a Temperature Indicator Circuit using a Single Transistor:

Using a single transistor a temperature indicator circuit is a simple electronic device that shows temperature changes visually.

A thermistor or other temperature dependent component are often used in the circuit to sense the temperature.

With the use of important formulas for the best results this article will guide you through the construction details.

Circuit Working:

Parts List:

Semiconductors including basic electronic components like transistors and diodes show changes in their features in response to variations in normal temperature.

The change 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 parts complement one other since they have similar features with regard to changes in the surrounding temperature.

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

It is important to place the diode as the reference device.

The environment in which it is kept should have a relatively constant temperature.

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

A LED is connected to the collector of the transistor.

The LED directly reads 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 for 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 features of the transistor.

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

Caution

To avoid permanent harm the transistors temperature must not increase above 120 degrees Celsius.

Formula and Calculations:

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 ideal LED flashing.

The calculated results will be in ohms.

The preset P1 have a value between 150 to 300 ohms for modifications

How to Build:

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

• Gather all the above mentioned required components.
• Place the LED, diode and transistor on a breadboard.
• Make sure the transistor is set correctly.
• Create a bridge network by joining the transistor and diode.
• The transistor acts as the temperature sensor and the diode as the reference device.
• The LED should be connected to the transistors collector pin.
• This will act as the temperature level visual indicator..
• Connect the diode to create a reference voltage.
• The diode must be placed in an environment with reliable temperature conditions.
• Connect the preset to the circuit.
• This will help to adjust the reference voltage level.
• To avoid damaging the transistor take care to keep its temperature from rising above 120 degrees Celsius.
• If necessary adjust the circuit to use the measured temperature levels to operate an external appliance.

Testing:

• Turn on the circuit and watch how the LEDs perform.
• As the temperature rises above the set value, the LED will indicate temperature changes by getting brighter.

Conclusion:

By following the construction details one can build a simple and effective temperature indicator circuit using a transistor and a diode.

Do not forget to follow the warnings that have been presented and think about making required changes for better performance which the article discusses.

References:

Temperature sensor