Green USB Switch

The circuit described here is designed for use by laptop or notebook computers. It will automatically switch off all mains powered peripheral equipment including monitor, printer, scanner, TV tuner and USB hub etc when it detects that the notebook is switched off. The circuit is quite straightforward in addition to an optocoupler it requires a 12 V double-pole relay with mains rated contacts and a small power supply for the optocoupler. When the laptop is switched on 5 V appears at the USB socket, activating the relay and switching through the mains supply on K3 and K4.