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RedCircuits Smart (original here)



R1, R2: 100R 0.5W Carbon Film Resistors C1: 100nF 630V Polyester Capacitor D1 to D6: 1N5408 1000V 3A Diodes (See Notes) D7: TIC225M 600V 8A Sensitive Gate Triac (See Notes) Three 2-way std vertical terminals 5.08mm pitch (Optional) A commercial trailing socket to be modified or a self-made box with several sockets.

Purpose of the device

This circuit consists of a Trailing Socket (also called Extension or Distribution Socket) or similar device where two, three or more sockets (depending on the box dimensions and on constructor's needs) will be powered when current flows in the Control Socket.

For example: if an electric drill is connected to the Control Socket, the Switched Sockets will be powered each time the electric drill is running. In this case, a lamp could be connected to a Switched Socket and will illuminate when the drill is operating.

Circuit operation

Six back-to-back power diodes are connected in series to the Control Socket. The current drawn by the device plugged into this socket when in the on state, flowing through the diode chain, causes a voltage drop of about 2V. This voltage, limited by R1, drives the Gate of the Triac D7 which, in turn, will switch on the output sockets.

C1 and R2 form a so called "Snubber network", helping to eliminate switching transients generated by inductive loads.


• The circuit is sufficiently small to be embedded into some types of commercial trailing sockets, or a box with a number of sockets can be made at will. • The diode types suggested in the Parts List for D1 to D6 will allow an appliance of up to about 500W power to be plugged in the Control Socket. Use BY550-800 diodes for up to 800 - 1000W. • For less power demanding appliances, 1N4007 diodes will allow up to 200W power. • The Triac type suggested in the Parts List for D7 will allow a total power available to the Switched Sockets of more than 1000W. If you intend to drive loads of more than 500W total, please use a suitable heatsink. • Wanting to drive less powerful loads, you can use for D7 a TIC216M (up to 800 - 1000W) or a TIC206M (up to 500 - 600W).