Many devices connected to a local area net-work (LAN) are left on continuously, even when they are not needed, including DSL and cable modems, routers, wireless access points, networked hard drives, printer servers and printers. The power consumption of all these devices can add up to a considerable fraction of one’s electricity bill. With the simple circuit described here we can ensure that all these devices are only powered up when at least one selected host device (such as a PC or a streaming media client) is turned on. We insert a relay in the mains supply to the devices whose power is to be switched, along with a driver circuit controlled from the host device over a two-wire bus. Optocouplers provide galvanic isolation. One way to implement the bus is to use the spare pair of conductors that is often available in the existing LAN cable. The circuit diagram shows an example con-figuration where there are two controlling host devices (a streaming media client and a...