Load detector Mains Switch

Posted on May 25, 2012

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).

Load detector Mains Switch
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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 This circuit is designed for 120V operation. For 240V operation, resistors R2 and R6 will need to be changed. A maximum of 5A can be used as the master unless the wattage of R1 is increased S1 provides a manual bypass switch. 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.

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