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).
Automatic Load Sensing Power Switch Circuit diagram

Parts List
Notes
Automatic Load Sensing Power Switch Circuit diagram

| 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
- 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.









This feedback causes a hysteresis of 20mV that prevents oscillation when the input voltage changes slowly. IC1d is an ordinary inverter. For input voltages above 10mV, the SIGN output terminal will swing to almost the positive supply. When the input voltage is below –10mV, the SIGN terminal drops low, almost to the negative supply voltage. For input voltages between these two thresholds, the output voltage is well defined, too, because it stays at its previous level. This circuit is the perfect complement to the ‘+/– voltage on bargraph display’ circuit discussed elsewhere in this blog.




























