Home | Gadget | Fuzzy | Lecturer  | Power  | Transceiver



Find more type here...




Green USB Switch


Wolfram Winfera




According to the Energy Saving Trust, if you add up all the current drawn in standby mode by items such as stereos, TVs, VCRs and DVDs over a year in the UK alone, it amounts to 3.1 million tonnes of CO2 released into the atmosphere. This is without factoring in the current drawn by all the PCs, laptops and their associated peripherals left in standby mode. It is not necessary to spend a great deal of money or time to make a difference on a personal level. 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 opto coupler it requires a 12 V double-pole relay with mains rated contacts and a small power supply for the opto coupler. 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. The notebook’s USB socket is still available to be used as normal but it’s worth remembering that the optocoupler takes a few milliamps from the USB supply and this may cause a problem if a high-current device is plugged into the USB socket. In the case where the laptop has more than enough USB sockets it may be worthwhile using one of them solely for this circuit, the extension USB connector K2 would then not be required.



The circuit is mounted into a mains plug enclosure which provides a socket where the mains extension strip will be plugged into. With any luck there will be sufficient space to fit the entire circuit into the mains extension strip enclosure and save the need for a separate enclosure. The slow blow 6.3-A fuse (F1) protects the equipment plugged into the strip. In addition to the optocoupler and relay the circuit also has a ‘freewheel’ diode D1 and a relay driver formed by T1 and its base bias voltage divider network R2/R4. The two ‘snubber’ networks C1/R3 and C2/R5 reduce the possibility of arcing which can occur when the relay contacts open (especially with inductive loads). Capacitors C1 and C2 must be class X2 types which can handle mains voltage plus any spikes. The power supply consists of a small mains transformer (12 V, 50 mA), bridge rectifier and smoothing capacitor C3. The laptop’s mains adaptor itself can also be switched by this circuit when the laptop is fitted with its rechargeable battery which allows the computer to boot up without a mains supply. The entire USB switch circuit draws current even when it is off but this value is tiny compared to the combined standby current of all the peripherals. Note that parts of this circuit are connected to the (potentially lethal) mains supply voltage; it is essential to provide protection to ensure that nothing can accidentally make contact with these parts of the circuit. It is also important

to observe correct separation between parts of the circuit carrying low voltage and those carrying the high voltage. Please observe the electrical Electrical Safety guidelines which are reprinted in Elektor Electronics several times a year. The circuit is less suitable for use with desktop PCs because the majority of these machines supply 5 V over the USB socket even though they have been shut down via software. The only way to turn off in this case is to reach around the back of the machine and switch off at the main switch.