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).
Simple Automatic Load Sensing Power Switch Circuit
Parts
| 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.
