I am still trying to get my laundromat watcher to work. I had it working fine with about a dozen sensors, but then I was using more power than the arduino could supply. So I changed the design to use an external power supply, and I combined the grounds but it doesn't seem to work anymore. Of course now I have lots more sensors so I didn't notice it wasn't working for awhile.
Basically I am wondering if I need to combine the +5V on the arduino with the 5v output of the power supply. I don't think I should do this but I don't know what else it could be! Ideas?
Try going with a 9VDC power supply of around 1-2 amp capacity, plugged into the Arduino thru the external connection (so its regulator supplies the Arduino power), then hook up a separate 7805 with a heatsink attached to power the sensors and whatever else. The 7805 with a proper heatsink should allow you to get 1A of current for your devices; if you need more, get a bigger current rated power supply, and parallel another 7805 with the first with a heatsink (or get a bigger heatsink and attach both to it). Stay around 9V for the power supply, though, so you aren't throwing off a ton of excess heat thru the regulator (varies based on input vs output voltage).
If that doesn't work (or is more complex than what you want), then I suggest purchasing a regulated switching power supply to drive everything; get one with 12V and 5V outputs (with large current capability on the 5V rail), run the 12V to the Arduino's external input, and the 5V to everything else.
For either of these, you should still tie the grounds together (though in the second example both should be tied together in the power supply, but it doesn't hurt to add it anyhow - most of the time).
