All items stored in a deep freezer will thaw out if, for some reason, the temperature inside the freezer rises to the thaw point. However, a freezer monitor alarm can warn you of the rising temperature before the thaw point is reached.
This simple circuit is powered by regulated 9V and built around a few discrete components that are readily available in the market. Current consumption of the circuit is only a few milliamperes in idle state. So you can also use a 9V 6F22/PP3 type compact battery for powering the circuit.
Germanium transistor AC125 (T1) acts as the temperature sensor. For sensing, the monitor circuit uses the leakage current of the transistor, which rises with temperature, and is adjustable from –18°C to room temperature.
Working of the circuit is simple. At normal temperature of the freezer, the resistance between the emitter and collector of transistor AC125 is very high and therefore the voltage at the gate terminal of SCR BT169 is too low to fire it. As a result, the rest of the circuit is disabled. Relay RL1 does not energise and hence the buzzer remains silent.
For more detail: Freezer Monitor Alarm
