Temperature Alarm with Op-Amp Comparator

The circuit below is configured as a comparator. A fixed reference voltage at the non inverting input is provided by R1 and R2. The inverting input voltage is set by the other two resistor. The output goes to minus 12 volts and the buzzer is energized if the voltage at the inverting input rises above the reference voltage.

By swapping the preset and temperature dependent resistors, we can change the behaviour of the circuit. The temperature dependent one can be replaced by light dependent resistors.