Thermostat (ART405E)

   The general purpose thermostat shown in Figure 1 uses a high resistive thermistor as a sensor. Thermistors in the range of 47 k ohms to 220 k ohms can be used and in their function the value of R3 must be changed to obtain the desired sensitivity and trigger point of the relay.

The circuit can be adapted to work as a shield for microcontrollers.






   With the use of a transistorized amplifier stage the small variations of resistance of the thermistor are detected taking the circuit to the trigger point. The setting of this trip point is given by the trimpot connected to the comparator reference input. For greater precision a multi-turn trimpot must be used.

   The relay used is of the sensitive type with a coil of 50 mA at the maximum and voltage according to the one used in the power supply.

   If the sensor is somewhat distant from the circuit (more than 2 meters), shielded cable must be used with properly grounded mesh. The 100 nF capacitor at the thermistor input prevents transients or short pulses from causing the circuit to trip erratically.

   The power supply must be stabilized so as not to affect the trip point of the circuit. One possibility that can be analyzed for some applications is the use of a common diode inversely polarized as a temperature sensor.



CI-1 - LM339 or equivalent - used - comparator

Q1, Q2 - BC548 - general purpose NPN transistors

D1 - 1N4148 - general purpose diode


Resistors: (1/8 W, 5%)

R1 - 22k ohms

R2 - 2.2 k ohms

R3 - 3.3 k ohms

R4, R5 - 4.7 k ohms

R6 - 5.6 k ohms

R7 - 1.2 k ohms

P1 - 10 k ohms - trimpot multivolt



C1, C2, C3, C4-100 nF - ceramic or polyester



X1 - Thermistor - see text

K1 - 12 V x 50 mA Relay

Printed circuit board, shielded wires, power supply, welding, etc.


Circuit Bench