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What we really need is a tiny UPS, of sorts. We'll be hooking up a solar cell and a battery to a single board computer. Currently, that SBC is a custom Pic32 device, and it does it's own UPS and voltage monitoring duties.

I've been tasked with trying to replicate all of its features with off the shelf products... and for the most part I've succeeded. But I don't currently have any way to switch between two sources of juice, or monitor when they're getting low.

These guys have something:

I really like it, the price is well within the budget. We might even work it in though it does 12V and I'll probably be using 5V... there are enough engineers on hand to figure out something. But I'd still have no idea what the voltage was for the PV or battery. I was hoping that there was some simple little USB multimeter thing that I could use to monitor this with, but I can't seem to come up with anything. I've found all sorts of cool hardware, but nothing that will help us.

Does anyone know of anything?

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How about an arduino ? It's got A-to-D ports on it and a pile of inputs as well as outputs. A little more expensive than what you pointed out (though not much, and I think the more highly-integrated ones get cheaper). Plus, the hardware is open source so available from all over the place.

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I think you're on to something with the picoUPS. It seems like a good product, and if I'm understanding you correctly, you now need a device to monitor your battery and bus voltages. pjz's suggestion about the Arduino boards looks pretty good too. The Arduino Duemilanove looks like it will be about right. It has six analog inputs which you can use for monitoring your DC voltages. The downside is that it isn't a fully-integrated solution. You will have to experiment with programming it, and then you'll need software on the computer it's attached to for reporting or logging the results. You will probably find software available which is "almost right," which should save a lot of heartache. Good Luck!

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Software on the computer side isn't a problem, we're going with a fairly full linux load. I'm more concerned with a product that we can just order 500 at a time though. My only other issues are the size, though probably unrealistic on my part, I was hoping for something that wasn't much bigger than the USB plug itself, with just 3 or 4 lead wires coming out of it. More important though is a device be powered off of USB for simplicity's sake. I'll definitely investigate further though. – John O Jun 18 '10 at 12:54
The volume may be an issue, also. I'm not sure the Arduino folks can get 500 units out the door at the drop of a hat, but I may well be wrong. I'm sure they would like the business, though! Also, look into the Arduino Nano. The spec sheet claims that it can be USB bus-powered, and it is quite a bit smaller than the Duemilanove. It will require soldering to connect the analog lines, though. – Jesse Jun 18 '10 at 13:18

It looks as if this place offers a 4 channel voltmeter (or will soon): and voltmeter.htm

Still a bit larger than I'd like, but not much.

Too bad I can't find one of the micro-UPS boards that actually monitors its own voltages via USB.

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