This circuit will trigger a relay when a preset temperature is reached. The circuit uses a NTC thermistor with a resistance of 47k at room temperature. The circuit is set in balance by adjusting the the 47k Potentiometer. Any change in temperature will alter the balance of the circuit, the output of the op-amp will change and Energize the relay. Swapping the position of the thermistor and 47k resistor makes a cold or frost alarm.
At 25 degrees Celsius a NTC thermistor resistance is approximately 47k. The non-inverting op-amp inputs will then be Roughly half the supply voltage, adjusting the 47k pot should allow the relay to close or remain open. To calibrate the device, the thermistor Ideally needs to be at the required operating temperature. If this is for example, a hot water tank, then the resistance will decrease, one way to do this is use a multimeter on the resistance scale, read the thermistors resistance and then set the preset so that the circuit triggers at this temperature.
Please note that if the temperature then falls, the relay will de-Energize. Temperatures if the environment changes rapidly, then the relay may chatter, as there is no hysteresis in this circuit.
Hysteresis, allows a small amount of "backlash" to be tolerated. With a circuit employing hysteresis, there will be no relay chatter and the circuit will trigger at a defined temperature and require a different temperature to return to the normal state. Hysteresis can be applied to the circuit using feedback, try a 1Meg resistor between the op-amp output, pin 6 and the non-inverting input pin 2 to give the circuit hysteresis.