FROST ALARMM

Description
A simple thermistor triggered switch with adjustable threshold. It triggers
with cold temperatures so may be used as a frost alarm or cold temperature
switch.
Circuit
Circuit Notes
The thermistor used has a resistance of 15k at 25°C and 45k at 0° 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.