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I've got a bunch of physical switches and circuits that act like switches (they're either connected to ground or they're just an open wire). Is there some sort of thing I can plug into my computer (ideally, via USB) that has a bunch of screw terminals, and I can attach wires to the screws and have the computer keep track of which circuits are closed and which are open?

Bonus points if the device also lets the computer open and close switches, too.

I don't even know what to google for.

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Hi @raldi, any luck with the below? Let me know if it's not what you're looking for. –  dsolimano Mar 31 '10 at 22:07
    
you can probably build your own device to do that via a serial/RS232 interface. if you don't have a hardware serial port on your computer you may have a header for it on the motherboard, or you could use a usb-to-serial adapter. software interfacing would be trickier, though you could use a microprocessor that speaks RS232 and write your own firmware for it to report on your switches... –  quack quixote May 5 '10 at 10:48
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2 Answers

I think "usb relay" might be a fruitful search term - for example, this seems to do what you want, though I don't know what your voltage/amperage requirements are like.

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I haven't used it (yet), but Microchip's MCP2200 USB-UART translator has 8 GPIO on it that can be polled using their API. It's about a $2 part, which is a very good price point for something that can interact with external hardware like that.

You should only need to use a few pull-up resistors to accomplish what you've described: when a connection is open, the pin will be pulled up to VCC and will be read as a logic high, and when the connection is pulled to ground, your pin will be read as a logic low.

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