How many times have you used your car demister and forgotten to switch it off? Lost count? This means that the next time you switch on the ignition, the rear windscreen will warm up whether you need it to or not. The warning light usually found in the switch is not much help and is easily missed.

If you have the headlights switched on too plus a few of the high-current devices, which seem to have become part of modern motoring, this imposes a considerable load on the car charging system. In extreme cases, it can exceed the output of the alternator. The battery will then run down at a rate needed to make up the difference.

The Demister One-Shot described here is an easy-to-build add-on circuit which puts the heated rear windscreen under automatic control On pressing a pushbutton switch, the heating element operates for some preset time between 6 and 50 minutes approximately then switches off.