Jun 152015
 

Electronic Thermostat Circuit using transistors

This electronic thermostat circuit allows us to control a room temperature, turning on and off a heating device.

How the Electronic Thermostat Circuit works?

The circuit uses a NTC thermistor (Negative Temperature Coefficient).

  • – If the ambient temperature is above the temperature seted with the potentiometer, the relay is not activated and the green LED lights.
  • – If the ambient temperature is below the temperature seted with the potentiometer, the relay is activated and the red LED lights.

Electronic Thermostat Circuit using transistors

In order to properly set the temperature level at which the relay is activated, the potentiometer should be carefully adjusted. To adjust the electronic thermostat, the NTC thermistor is inserted into a glass tube. The NTC thermistor ends must be welded to a pair of long wires.

Place the glass tube and a mercury thermometer in a container of water, which will be placed first in a refrigerator, then at ambient temperature and finally on a gas burner or similar. The temperature of the gas burner must not be high. (see the temperature limits of the mercury thermometer). In each case, locate the point where the red LED lights, gently manipulating the potentiometer to the maximum and marking on a screen (white card) behind the potentiometer, the temperatures readed on the mercury thermometer.

The operation of the circuit is very simple and can be understood by analyzing the states of cut-off and saturation of each transistor.

When the NTC thermistor resistance is very high (the ambient temperature level is low), the transistor T1 goes into saturation, as long as the adjustment made to the potentiometer so permits. If  transistor T1 goes into saturation, transistors T2, T3 and T4 go into saturation too, activating the relay. This relay has dual contact and each time it is activated, it makes two connections, one to turn on the LED and the other to activate the heater.

The C1 capacitor is used to prevent sudden changes on the value of NTC thermistor and the C2 capacitor is used to avoid sudden changes on the voltage when the relay is disconnected, so the transistor is not damage.

List of circuit components for the Electronic Thermostat Circuit

  • 3  10K resistors (R1, R4, R6).
  • 1  12K resistor (R2).
  • 1  6.8K resistor (R3).
  • 1  3.3K resisistor (R5).
  • 1  470K resistor (R7).
  • 1  2.2K resistor (R8).
  • 1  560 ohms resistor (R9).
  • 1  10K linear Potentiometer (P).
  • 1  10K NTC (Negative Temperature Coefficient) resistor.
  • 1  100nF capacitor (C1).
  • 1  47uF, 25V, electrolytic capacitor (C2).
  • 2  LED, 1 green, 1 red (L1, L2).
  • 2  2N2222 NPN transistors (T1, T3).
  • 1  2N2907 PNP transistors (T2).
  • 1  2N2905 PNP transistors (T4).
  • 1  12V Relay with double contact output.
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