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Description

Attempting to build a device using the Arduino that will allow me to remotely restart a cluster of desktop computers that I have. The device works by momentarily connecting the RESET button jumpers on a desktop's motherboard. Two multiplexers are used so that 8 desktops can be connected to it at once. One multiplexer controls the reset button ground and the other controls the reset button positive terminal. Two multiplexers are needed to connect the same mother board ground to the same mother board positive.

Problem

The problem I am having is that the device works when connected to my laptop USB, but restarts the computer when I disconnect power. The device does not work at all when I connect it to the USB of my lunux server, instead seeming to send some kind of very weak signal to the mother board RESET leads that causes the mother board to freeze rather than reset.

I think the problem has to do with my limited understanding of the nuances of electrical flow through these devices. I feel like there must be some kind of backflow or small leakage that is causing disruptions for this use case. As you can see, I've added diodes connecting the power. This significantly lessens the chance of the motherboard restarting when I disconnect the power, but it still happens sometimes. Also, the device still only works when connected to my laptop USB and not my linux server USB. I feel like there is perhaps some configuration of diodes, capacitors or different types of switches that should be used. Such as a relay or MOSFET instead of the multiplexers?

I did try completely disconnecting the yellow wire before disconnecting the power, but this did not help. It seems that a signal is leaking from the mother board ground to the mother board positive through some other channel. The only thing that works is to physically disconnect one of the mother board leads from the board before disconnecting power. Connecting the mother board positive or ground to the arduino's positive or ground also does not reset the mother board. This implies to me that a signal is somehow leaking directly from one mother board lead to the other through the system.

Example

In the example below, only one desktop motherboard is connected for testing purposes (via the yellow and blue wires). The LEDs on the right side are for "debugging" purposes so I can see which control leads are actually delivering charge to the multiplexers. The green wires connect like control leads to each multiplexer (so A,B,C, and INHIBIT are the same for each one) and the yellow wire connecting the two multiplexers connects the "OUT" of the ground multiplexer to the "IN" of the positive jumper wire multiplexer.

enter image description here

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  • \$\begingroup\$ Do you have any bypass capacitors on the DC lines? Edit: Also, is the RESET signal HI or LO? Also, as rawbrawb says, a schematic is the best. \$\endgroup\$ – scld Oct 10 '13 at 19:43
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    \$\begingroup\$ You did put effort into preparing this. This is good. But there really is no substitute for a schematic, well labelled and labels referenced in the write up to be able help. I suspect that "the mux does work the way that you think it works" to quote a famous movie, but I am not willing to spend the time to work through your dense explanation to be sure I am right. \$\endgroup\$ – placeholder Oct 10 '13 at 19:44
  • \$\begingroup\$ Short is indeed sweetest when a thousand words are put into a picture.. er well.. schematic LOL \$\endgroup\$ – Andy aka Oct 10 '13 at 20:01
  • \$\begingroup\$ How are schematics prepared? Is there a software program that works best for windows? Does not have to be free. \$\endgroup\$ – Chris Dutrow Oct 10 '13 at 20:08
  • \$\begingroup\$ @Chris L - I don't have any bypass capacitors. What are these? What are they used for? I did just by a bag full of capacitors at radio shack. I don't know if the reset is HI or LOW. How would I test this? \$\endgroup\$ – Chris Dutrow Oct 10 '13 at 20:10
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Two multiplexers are needed to connect the same mother board ground to the same mother board positive.

I may be misunderstanding what you've done but it sounds like you're trying to use the multiplexers as if they are physically like this inside:

enter image description here

But, in fact, they're more like this inside:

enter image description here

The point being that the multiplexer is not actually connecting a particular input directly to the output, like a physical switch would do, but rather, a buffered version.

If you want to physically connect the reset switch positive pin to the motherboard ground, you can do that with, for example, an opto-isolator for each motherboard:

enter image description here

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  • \$\begingroup\$ Ah ok, so it sounds like this is not the proper use-case for the multiplexer. Do you think a MOSFET or relay might work for this use case or must it be an opto-isolator? Thanks so much! \$\endgroup\$ – Chris Dutrow Oct 10 '13 at 21:11
  • \$\begingroup\$ @ChrisDutrow, a MOSFET, BJT, relay, etc. could be used. The nice thing about the opto-isolator is that, like with a relay, each motherboard is isolated from your circuit and each other. But, it's simpler circuit than a relay. That may or not be important in your application but it is something to consider. \$\endgroup\$ – Alfred Centauri Oct 10 '13 at 21:29
  • \$\begingroup\$ Just wanted to post an update. I tried a MOSFET that I had sitting around. It had the same problems as the multiplexer though. This may be because the MOSFET I used is designed to carry significant amounts of power though, so perhaps it had more leakage than a better suited MOSFET. Next I tried a relay from radio shack (OMR-C-105H), it works in all situations. I could not find any opto-isolators at radio shack, but I ordered some off ebay. Are there places that are better than radio shack to buy electrical components like these? \$\endgroup\$ – Chris Dutrow Oct 11 '13 at 19:58
  • \$\begingroup\$ @ChrisDutrow, for a start, try Digi-Key. \$\endgroup\$ – Alfred Centauri Oct 11 '13 at 20:01
  • \$\begingroup\$ @ Alfred Centauri - I have ordered from Digi-Key before. This is kind of a dumb question, but is there like a "type" of store that I'm not aware of that I can walk into and buy these types of components in a pinch? The only such store that I am aware of is Radio Shack, but their selection is limited and their mark-up is huge. \$\endgroup\$ – Chris Dutrow Oct 11 '13 at 20:04

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