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Help Building a USB-Controlled USB Outlet?

Discussion in 'General Electronics' started by [email protected], Jan 2, 2008.

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  1. Guest

    I am a novice and would like to build a 120V AC outlet which can be switched
    on and off by USB. I'm not sure what all I will need and want to make sure
    that I'm not trying to do something much harder than it sounds. My end goal
    is to have a single plug where I could plugin a lamp, TV, power strip, etc
    and have the power to that device controller by a PC via USB port. Now I've
    seen some devices out there like X10-compliant home automation controls and
    there has always been a severe limitation in one way or another. I would
    love the pulg to be grounded but it doesn't have to be if that is a
    complicating factor.

    It seems to me that I need a relay which can switch the 120V on/off and then
    need a USB controller to control the relay. And I have seen USB controllers
    which have serial interfaces on them but then how to get from a pin or two
    of a serial port to the relay is where it breaks down in my head. I'm not
    sure what I need between the two or if that would even work. I can handle
    the software part of it once I get a hardware solution idenfitied.

    Can any of you pros offer advice to get me going in the right direction?

  2. DJ Delorie

    DJ Delorie Guest

    Easiest way I can think of - get a FTDI 232R chip, hook up DTR to an
    opto-isolated triac driver, to a triac, to the power.

    Then, at least on Linux, you can toggle DTR just by changing the baud
    rate to 0 (dtr off) or anything else (dtr on) via "stty" or the
    termios() functions.

    The FTDI chips also have some GPIO pins you can use; in theory you can
    control six outlets with one chip that way (four GPIO, plus DTR, and
    RTS). You'd need to use their DLL and API to do that, though. Not a
    big deal if you know how to do any programming; the API is pretty

    You can even get a vanilla USB to Serial adapter cable, and use DTR
    off that.

    I have some triac circuits here:

    But the opto-triac-power circuit is pretty common; you should be able
    to find schematics all over the web. I think the triac data sheets
    usually have schematics too.

    Beware - messing with 120v power is dangerous. This isn't a
    beginner's project, although I think most beginners successfully pull
    it off anyway. I know I did :)
  3. TonyR

    TonyR Guest

    Have a look at
  4. You can use phidgets ( for software-controlled switched output via USB.
    You'd write your own program in C, .net, VBA, etc., and they give you the API to control the
    phidget output. I don't know if there are isolated outputs, but you could use an
    opto-isolator with a triac to switch the 120V power.

    Do be careful -- you're messing with the 120 V main, and your computer and other gear is
    grounded. It'd be a good idea to power it from a ground-fault circuit while you're
    developing it.
  5. CDESC

    CDESC Guest

    I'd suggest to hook-up a PIC 18F2550 (or something similar) to a solid state
    relay. You can have plenty of outlets controlled separately with only one
    USB input. Pros; well the USB stack is provided by microchip (many compilers
    provide their own as well) and it's going to be a breeze to control with a
    custom software (C#).

    I can give you a few quick lines on where to look and what to do if you need
  6. Electro


    May 24, 2008
    i was having a same problem but after reading the solutions now its fixed. Thanks
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