Friday, February 20, 2009

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).



Total Qty.
C1, C3210uF 35V Electrolytic Capacitor
C211uF 35V Electrolytic Capacitor
R110.1 Ohm 10W Resistor
R2127K 1/2W Resistor
R3, R411K 1/4W Resistor
R51470K 1/4W Resistor
R614.7K 1/2W Resistor
R7110K 1/4W Resistor
D1, D2, D431N4004 Rectifier Diode
D311N4744 15V 1 Watt Zener Diode
U11LM358N Dual Op Amp IC
Q112N3904 NPN Transistor
K11Relay, 12VDC Coil, 120VAC 10A Contacts
S11SPST Switch 120AVC, 10A
MISC1Board, Wire, Socket For U1, Case, Mains Plug, Socket


  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.



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