Remote Control for Network Devices

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  PC) and three network devices (a DSL router,  a networked hard drive and a networked  printer). We will assume that all the media  files are held on the networked hard drive.  The DSL router (to provide an internet connection) and the hard drive are to be powered up when either the PC or the media client is powered up; the printer only when the  PC is powered up.

 

Circuit diagram :

Remote Control for Network Devices Circuit Diagram

 

We can think of the devices as being in two  groups, the first group consisting of the DSL  router and the hard drive, the second just the  printer. An optocoupler is powered from each  of the controlling host devices: these ensure  that the devices are isolated from one another  and from the rest of the circuit. The relay circuit, located close to the networked devices,  is controlled from the outputs of the optocouplers. The relay circuits are powered from  (efficient) mains adaptors: modified mobile  phone chargers do an admirable job.

 

In the circuit shown a 5 V supply from the  controlling devices is used to drive each optocoupler. Host 1 (the streaming client) drives  optocoupler IC1, host 2 (the PC) drives opto-couplers IC2 and IC3. Optocouplers IC1 and IC2 both control the  networked devices in group 1: networked  device 1 is the DSL router, switched by relay  RE1, and networked device 2 is the hard drive,  switched by relay RE2. Optocoupler  IC3  controls  the  networked  device in group 2, namely the printer. This is  switched by relay RE3.  The connections between the optocouplers  and the relay stages can be thought of as a  kind of bus for each group of devices. The  devices in a given group can be switched on  by simply shorting its bus, and this gives an  easy way to test the set-up. Resistors R2, R6  and R10 at the collectors of the transistors in  the optocouplers protect them in case power  should accidentally be applied to the bus.

 

The supply voltages V1 and V2 shown in the  example circuit diagram are derived from the  mains adaptors as mentioned above and are  used to power the relays. We have assumed  that the networked hard drive and the printer  are located near to one another, and so it is  possible to use a single mains adaptor to provide both voltages. Another possibility  would be to add a third wire to the bus to  carry power: this would allow all relays, wherever they were located, to be powered from  a single supply.  It is worth noting that network attached storage (NAS) devices such as networked hard  drives normally require an orderly shutdown  process before power is removed. Devices  that use Ximeta’s NDAS technology do not  suffer from this problem.

 

Author : Werner Rabl - Copyright : Elektor