Electrical appliances accidentally left on in (holiday) homes left unoccupied for a short or a long period consume power unnecessarily and can present a fire hazard. Everyone will be familiar with those nagging thoughts, a few miles down the road from the house: “Did I remember to switch off the coffee machine? The lights? The oven?” Hotel rooms are often equipped with a switch near the main door which enables the power supply to everything in the room only when the plastic card (which might contain a chip or have a magnetic strip or a pattern of holes) that serves as the room key is inserted. The circuit idea given here to switch off lights and other appliances is along the same lines. The solution is surprisingly simple.
A reed contact is fitted to the frame of the main entrance door, and a matching magnet is attached to the door itself such that when the door is closed the reed contact is also closed. To enable power to the house, press S1 briefly. Relay RE1 will pull in and complete the circuit for all the AC powered appliances in the house. The relay will be held even after the button is released via the second relay contact and the reed contact (‘latching’ function). As soon as the main entrance door is opened, the reed contact will also open. This, in turn, releases the latch circuit and consequently, the relay drops out. The various connected appliances will thus automatically and inevitably be switched off as soon as the house is left. The circuit is principally designed for small holiday homes, where this mode of operation is particularly practical. Of course, for any circuit that deals in AC power line voltages, we must mention the following caution.
Caution: shock hazard!
Construction and connection of this circuit should only be carried out by suitably-qualified personnel, and all applicable electrical safety regulations must be observed. In particular, it is essential to ensure that the relay chosen is appropriate for use at domestic AC grid voltages and is suitably rated to carry the required current.