Originally published by Paul Stenning in Electronics in Action, July 1994

• Diagrams and Schematics

Introduction
If you've got a computer system, do you forget to switch off the printer sometimes? The problem is the number of separate power switches to turn off each night - PC, monitor, printer, modem, table lamp, radio - the list goes on! Normally these are connected to one of those four way trailing sockets, which will be plugged into one wall socket. If that socket is accessible, the equipment could be switched off there. In my case the socket is in the corner, behind the desk.

So what's the solution? This Mains Switcher will allow you to turn all the equipment on or off, by operating the switch on just one item. When you turn off the switch on the PC, the monitor, printer and other bits are powered down automatically. You can choose which item controls the system, by plugging it into the master socket - so you can use the table lamp to control everything if you want.

The unit works by sensing if current is being drawn from the master socket, if so it powers up the other (slave) sockets. The load on the master socket can be anything between 20 and 500VA, and the load on the slave sockets can be between 60 and 1200VA. None of the equipment used requires modification.

The unit may be useful for controlling hi-fi equipment, or your electronic test equipment. At Christmas time, you could use your indoor flashing tree lights to flash the lights on the tree outside! The options are limited only by your imagination.

The item connected to the master socket must be switched on the primary side of any internal transformer. Some modern electronic equipment has the power switch on the secondary side. Since these will continue to draw a small current from the mains when supposedly switched off, they will not control this unit correctly.

One important safety point. Although this unit removes the power from the equipment being controlled, IT DOES NOT PROVIDE ISOLATION FROM THE MAINS. Before working inside any piece of equipment connected to this unit, it must be unplugged.

The Works
The full circuit is shown in figure *. The unit operates by sensing current from the master socket. D8, D9 and D10 act as the sensor and will drop about 1.8V when current is drawn. D7 carries the current on the opposite half cycles. The resulting 1.8V half cycle pulses charge C3 via D6. This capacitor will retain sufficient charge to hold TR2 on for about 200mS after the controlling load is switched off.

When TR2 is on, TR1 will also be switched on. This turns on triac TR3, which will then power the slave load. C4 and R9 form a snubber network to ensure the triac turns off cleanly with an inductive load. LED D5 indicates that the unit is operating.

C1 and D4 are effectively in series across the mains. The resulting -15V pulses across D4 are rectified by D3 and smoothed by C2. R2 limits the switch-on surge current, and R1 rapidly discharges C1 when the unit is unplugged. D1 will light whenever the unit is plugged into the mains; the current on the opposite half cycles are carried by D2.