General question about powering an arduino

Ok so for my setup in Tempest SXR I plan on having a separate adapter so the Arduino will be powered on when the computer is off (unless I can figure something else out - I'll get into that later). If I have the 5v socket hooked up and the USB (I'll need the USB hooked up for a bit once it's first installed to adjust the coding) will it automatically switch to the best power source or will it fry something?

Also, can I build a circuit that will take 5v from the PSU and store it in a capacitor so the arduino will have 4 or 5 seconds of power after the computer is off so I don't have to keep it plugged into a separate power source all the time? I need it to stay on after the computer shuts off so that it will close the louvers. If I just power it off the PSU, the same 5v line controls the loop in the code, so it'll power down before it has time to shut the louvers. But when I power it on it'll work just fine.

So after typing all this, I think I'd like to go the capacitor route and just supply 5v to the arduino for 5 seconds or so after the computer is off. Can anyone suggest how I might do this or point me in the right direction?

Well, you could have the shutdown commands of Windows send teh command to teh arduino and have the louvers shut before the computer shuts off.

Check out Renderman's Motorized Madness mod to see this in action (not necessarily with an arduino)

I see I put teh instead of the. I'm in a teh mood apparently (none of them were typed on purpose except for this line)

In the main ATX power to the motherboard, there is a 5v standby line, you might like to use that.

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Originally Posted by crenn

In the main ATX power to the motherboard, there is a 5v standby line, you might like to use that.

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I would use this if it provides enough current. Maybe, have the main power for the arduino be this, and have one of the I/O pins be monitoring the power on a regular 5V line. When the regular 5V line no longer registers, then start shutting the louvers.

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Originally Posted by x88x

I would use this if it provides enough current. Maybe, have the main power for the arduino be this, and have one of the I/O pins be monitoring the power on a regular 5V line. When the regular 5V line no longer registers, then start shutting the louvers.

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The code he has currently already does this. Maybe have a capacitor so that the servo has enough power without relying on the standy line. Ideally putting the arduino into sleep mode until the computer turns back on.

exactly what I was thinking. the 5v from the psu stops @ the same time the computer shuts down, so the computer will shut off leaving the louvers open

I'd like to make a circuit with a capacitor so like crenn said it will supply a few secodns worth of juice to the Arduino once the computer powers off just so it will shut the louvers. I can then hook it to the 5v psu power so while the computer is on it will charge the capacitor.

I'm also thinking of making a custom board to mount the ATMega chip frmo the arduino so I can continue to use the adruino in other projects, and maybe combine this capacitor circuit into the same board. Anyone have any ideas? :?

I think I've found the solution

http://arduino.cc/en/Main/ArduinoBoardFio

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Originally Posted by SXRguyinMA

exactly what I was thinking. the 5v from the psu stops @ the same time the computer shuts down, so the computer will shut off leaving the louvers open

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As long as you don't turn off the switch on the back of the PSU, the 5V standby line will stay on. That's kinda it's whole point. So, if you just use that line and just don't throw that switch until after the louvers are closed, you shouldn't have a problem.

Alternately, if you want to go with a cap, try experimenting with a few different high capacitance caps to find out what you need. As long as you're only feeding it a straight 5V charging line it'll only charge to 5V, so you shouldn't have to worry much about the voltage rating of the capacitor.

well I never shut off the psu itself so I don't need to worry about that. you're talking about the green wire on the 24-pin connector right? will that have enough current to run the servo though?

optionally I can make one of these and just keep an eye on the battery. The thing is that the Arduino doesn't need power after the 3 or 4 seconds it needs to shut the louvers, so I wouldn't want it to sit and drain the 9v all night while the computer is off, hence the capacitor route.

As far as that goes, what if any special circuitry would I need for it? Or would I just hook a 5v line to the cap (presumably with a diode) and hook the cap right to the power and ground pins on the arduino? Or hook it to the arduino without the extra 5v line and let it charge from the 5v power on the arduino itself?