Automatic Load Sensing Power Switch

This circuit will automatically switch on several mains-powered "slave" loads when a "master" load is turned on. For example, it will switch on the amplifier and CD player in a stereo system when the receiver is turned on. It works by sensing the current draw of the "master" device through a low value high wattage resistor using a comparator. The output of that comparator then switches on the "slave" relay. The circuit can be built into a power bar, extension cord or power center to provide a convenient set of "smart" outlets that switch on when the master appliance is powered (turn on the computer monitor and the computer, printer and other peripherals come on as well).

Schematic

Parts

Part Total Qty. Description Substitutions C1, C3 2 10uF 35V Electrolytic Capacitor C2 1 1uF 35V Electrolytic Capacitor R1 1 0.1 Ohm 10W Resistor R2 1 27K 1/2W Resistor R3, R4 1 1K 1/4W Resistor R5 1 470K 1/4W Resistor R6 1 4.7K 1/2W Resistor R7 1 10K 1/4W Resistor D1, D2, D4 3 1N4004 Rectifier Diode D3 1 1N4744 15V 1 Watt Zener Diode U1 1 LM358N Dual Op Amp IC Q1 1 2N3904 NPN Transistor K1 1 Relay, 12VDC Coil, 120VAC 10A Contacts S1 1 SPST Switch 120AVC, 10A MISC 1 Board, Wire, Socket For U1, Case, Mains Plug, Socket

Notes

1. This circuit is designed for 120V operation. For 240V operation, resistors R2 and R6 will need to be changed.
2. A maximum of 5A can be used as the master unless the wattage of R1 is increased
3. S1 provides a manual bypass switch.
4. THis circuit is not isolated from the mains supply. Because of this, you must exercise extreme caution when working around the circuit if it is plugged in.