Using L121 integrated circuit manufactured by SGS THOMSON and some classic electronic components can be made a very simple temperature regulator.
This electronic circuit allows the desired temperature inside an enclosure to vary between two limits. Ambient temperature is sensed by a thermistor NTC (negative temperature coefficient) with a nominal value of 47 K at 25 ° C. The voltage drop on the thermistor is used for setting the level to one input of the operational amplifier L121. This threshold determines operational amplifier circuit output toggle the order of integration. P2 potentiometer adjusts the second input of the same operational amplifier and is used to prescribe the temperature at which the circuit enters into operation.
When the temperature inside the chamber increases, the thermistor resistance decreases, and will cause the voltage at pin 3 to decrease. After some time, the set of P2 is reached, leading to tipping logic circuit of IC1. This will determine entry into conduction transistor T1, which will act fan allowing access to cool air. Once the temperature will drop below a certain prescribed value P1, the control circuit will turn off the fan.
The circuit can be used to regulate the temperature inside a vehicle and require a supply voltage of 12 volts.