This month, in the penultimate of our four-part series of "perpetual" projects, we give details of a further three circuits that will find many possible uses in and around the home.

Loop Burglar Alarm: This circuit works on the principle that when a continuous electrical loop is broken, an alarm is triggered. The loop may include a thin wire snare, which would be snapped by an intruder or by the removal of an object through which the loop-wire is threaded. The "loop" could also include contacts which would be broken by, for example, a gate or a window opening.

Touch-Switch Door-Light: This circuit has several possible applications – among them being to light an entrance upon entering, to illuminate a switchboard during a power failure, or to help you find a keyhole on approaching a door at night.

Once the door-light is triggered, by a touch switch, it shines for roughly half a minute at a time. It could be used more than 10 times in the darkness before the regulator’s capacitor C1 is exhausted.

Solar-Powered Rain Alarm: This circuit is triggered by falling rain. When the alarm sounds, a touch of the finger puts it to sleep again for an hour or so.