Many devices within a local area network (LAN), such as DSL and cable modems, routers, wireless access points, networked hard drives, printer servers, and printers, are often left running continuously even when not in use. This prolonged operation can significantly contribute to higher electricity bills. To address this, the provided circuit ensures that these devices are powered only when at least one designated host device (such as a PC or streaming media client) is active. This is achieved by integrating a relay into the AC line supply of the devices that need their power controlled, coupled with a driver circuit regulated by the host device through a two-wire bus.
Galvanic isolation is achieved using optocouplers. One practical way to set up the bus is by utilizing the spare pair of conductors available in the existing LAN cable. The circuit diagram illustrates a configuration with two controlling host devices (a streaming media client and a PC) and three network devices (a DSL router, a networked hard drive, and a networked printer). In this setup, assuming all media files are stored on the networked hard drive, the DSL router (for internet connection) and the hard drive power up when either the PC or media client is active, while the printer powers up only when the PC is in use.
Consider the devices in two groups: the first group comprises the DSL router and the hard drive, while the second group consists solely of the printer. Each controlling host device powers an optocoupler, ensuring isolation between the devices and the rest of the circuit. The relay circuit, positioned near the networked devices, is governed by the outputs of the optocouplers.
Efficient AC adapters, like modified cell phone chargers, power the relay circuits. In the provided circuit, a 5 V supply from the controlling devices drives each optocoupler. Host 1 (the streaming client) controls optocoupler IC1, while host 2 (the PC) controls optocouplers IC2 and IC3. Optocouplers IC1 and IC2 manage the networked devices in group 1: networked device 1 (the DSL router) is operated by relay RE1, and networked device 2 (the hard drive) is controlled by relay RE2.
Optocoupler IC3 manages the networked device in group 2, the printer, controlled by relay RE3. The connections between the optocouplers and the relay stages serve as buses for each group of devices. Devices within a specific group can be activated by simply shorting their respective buses, providing an easy testing method. Resistors R2, R6, and R10, located at the collectors of the optocoupler transistors, safeguard them in case power is accidentally applied to the bus. The supply voltages V1 and V2, illustrated in the example circuit diagram, originate from the mentioned AC power adapters and are utilized to power the relays.
Assuming the networked hard drive and the printer are situated nearby, a single AC power adapter can offer both voltages. Alternatively, adding a third wire to the bus for power distribution would enable all relays, regardless of their location, to be powered from a single supply. It’s important to note that network attached storage (NAS) devices like networked hard drives typically require an orderly shutdown process before power removal. Devices using Ximeta’s NDAS technology do not encounter this issue.
