Frost Alarm

A simple thermistor triggered cold switch with adjustable threshold.

The thermistor used has a resistance of 15k at 25 degrees and 45k at 0 degrees Celsius. A suitable bead type thermistor can be found in the Maplin catalogue. The 100k pot allows this circuit to trigger over a wide range of temperatures.

If using a different thermistor then the control should match the new thermistor at its highest resistance, or be higher in value. The op-amp in this circuit is the ubiquitous 741. It may be catalogued as LM741, CA741 etc, all types will work. In this circuit it is used as a comparator. The non-inverting input (pin 3) is biased to half the supply voltage. The non-inverting input is connected to the junction of the thermistor and potentiometer. The control is adjusted so that the circuit is on when the thermistor is at the required temperature range. Once the thermistor is outside the temperature range its resistance alters and the op-amp output changes from near full supply to around 1 or 2 volts dc. There is insufficient voltage to turn on the transistor and the relay will not energise.

A slight amount of hysteresis is provided by inclusion of the 270k resistor. This prevents rapid switching of the circuit when the temperature is near to the switching threshold.