A Slug-Controlled Power Switch

Posted on Feb 6, 2014

This page describes the hardware and software design of a printer power switch controlled over USB from my Linksys NSLU2, aka Slug. The unit can, however, be controlled from any Linux box, and can switch anything, not just printers. My NSLU2 acts mostly as a file and print server. I can go for weeks without printing anything, so I want to keep th

A Slug-Controlled Power Switch
Click here to download the full size of the above Circuit.

e printer switched off when I`m not using it (it takes about 4W while idle, which must be more than 99% of its total energy consumption). But it`s upstairs, and I don`t want to have to go up and down stairs once to switch it on and again to collect my printing. So I decided to get a power switch. There are plenty of power switches out there. Mostly these are expensive things intended for server rooms (see this example ) with RS232 or Ethernet interfaces; USB would be more appropriate for the Slug. One cheap USB switch is this one which has been interfaced to a Slug, but it only seems to be available with continental European sockets. So I decided to build my own. For USB control I decided to use the same USB module that I chose for SlugTerm, using the FT245BM USB-to-Parallel chip. I use just one of its 8 output bits to switch the mains power using a triac. Here`s a photo of my breadboard prototype: I needed to learn a bit about triacs before building the mains side of the circuit. There is more than you could ever need to know in this document from ON Semi. Here is what I learnt: Triac optocouplers come with various different sensitivities; the K3020P that Maplin sells is the least sensitive and takes 30mA. Futurlec sell more sensitive versions (the K3023P needs only 5mA) which could be driven directly from the USB chip. Choose R6 based on the optocoupler`s peak surge current: there will be a short time when it has turned...

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