Automatic Fidge Switch for Caravans

At last there is another project for caravan owners. They are probably all aware of the problems of a flat battery when they’ve forgotten to turn off the fridge after the engine has been switched off. When this circuit is installed between the fridge and (caravan) battery, these troubles will be a thing of the past. This simple, but smart, switch reacts to the increase in voltage that occurs when the engine is running. Whereas the normal battery voltage is 12 V, this increases to a minimum of 13.8 V when the alternator charges it. With a suitably configured comparator this voltage difference can easily be detected, and as can be seen, the circuit consists of little more than a 3140 comparator and a relay driver stage for switching the fridge.

Contrary to what is indicated by the circuit diagram, R4 should be 330 Ù, while R6 is better changed to 1 kÙ. Regarding the adjustment, turn P1 until the relay pulls in at 13.5 V. Next, P2 is adjusted until the relay is de-energised at 11.5 V (i.e., do not use P1 for this adjustment).