A very simple temperature indicator circuit can be built by interconnecting a single transistor, a diode and a few other passive components.
Using Transistor as a Heat Sensor
As we know that all semiconductors have this "bad habit” of changing its basic characteristics in response to ambient temperature changes.
Especially basic electronic components like transistors and diodes are very much prone to their case temperature variations.
The change with their characteristics with these devices is typically in terms of the passage of voltage through them, which is directly proportional with the magnitude of the temperature difference surrounding them.
Using a Transistor (BJT) as the Temperature Sensor
In the present design a diode and a transistor are configured in the form of a bridge network.
As both these active parts have identical properties as far as ambient temperature changes are concerned, they both compliment each other.
Using Diode for Creating a Reference Voltage
The diode is place as the reference device while the transistor is connected to perform the function of a temperature sensor.
Obviously since the diode is placed as the reference, it has to be placed in an environment with relatively consistent temperature conditions, otherwise the diode will also start changing its reference level causing error in the indication process.
A LED is used here at the collector of the transistor, which directly interprets the transistor conditions and therefore helps to show how much temperature difference is taking place around the transistor.
LED Indicates the Temperature Change
The LED is used to get a direct indication of the temperature level sensed by the transistor. In this design the diode is placed at the ambient temperature or at the room temperature which the transistor is placed or attached to the heat source which needs to be measured.
The base emitter voltage of the transistor is effectively compared with the reference voltage level produced by the diode at the junction of D1 and R1.
This voltage level is taken as the reference and the transistor remains switched OF as long as its base emitter voltage remains below this level. Alternatively this level may be varied by the preset P1.
Now as the heat over the transistor starts rising it base emitter starts rising due to the altering characteristic of the transistor.
If the temperature crosses the preset value, the base emitter voltage of the transistor exceeds the limit and the transistor starts conducting.
The LEDs gradually begins to illuminate and its intensity becomes directly proportional to the temperature over the transistor sensor.
Caution must be maintained, not to exceed the temperature over the transistor above 120 degree Celsius, other wise the device may get burnt and damaged permanently.
The proposed simple temperature indicator circuit can be further modified for making it switch an external appliance ON or OFF in response to the sensed temperature levels.
How to Calculate the Temperature Thresholds
I will discuss it in my forth coming articles. The resistor values of the configuration is calculated using the following formula:
R1 = (Ub - 0.6)/0.005
R2 = (Ub - 1.5)/0.015
Here Ub is the input supply voltage, 0.6 is the forward voltage drop of the BJT, 0.005 is the standard operating current for the BJT.
Similarly, 1.5 is the forward voltage drop for the selected RED LED, 0.015 is the standard current for illuminating the LED optimally.
The calculated results will be in Ohms.
The value of P1 may be anywhere between 150 to 300 Ohms