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Automatic AC Power Switch


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?” 


Automatic AC Power Switch-Circuit Diagram
Automatic AC Power Switch Circuit Diagram

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 in 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  powerline 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 volt-ages and is suitably rated to carry the required  current.




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