Well crap - I bought that one I linked to, knowing the ad said "C-wire mandatory" - all the while thinking surely mine has a C wire or I can always find 24V on the attic unit.
Of course it doesn't. At least, not that I can tell - I guess it could have a mislabeled C-wire.
My old unit was battery powered, and has these connections labeled inside it, with wires attached:
G Y R O W2
plus these unused wires: Blu, Blk, Brn
Maybe there is 24v there, but which is the ground I can test each of them with?
Nothing at the thermostat is a ground. This is transformer fed AC not DC.
The C-wire is Common. It goes to the common side of the 24 Volt AC transformer in the system.
R, usually run on red wire, is the power side of the transformer.
C, usually run on black wire, goes to the other terminal of the transformer.
Remember this is AC, so current goes both ways. All the t-stat is doing is closing the circuits, when it wants a service like Heat, Cooling, or Fan.
This web page has a nice electrical diagram about halfway down:
http://www.how-to-wire-it.com/wire-a-thermostat.html
What you're doing when adding a Common Wire is giving the thermostat a closed AC loop that's always closed/on and connected to both terminals of the 24 VAC transformer so it can power itself off of that circuit.
All the other circuits, are "normally open" switches inside the t-stat, that the t-stat closes, to "ask for" or "request" things.
Remember the earliest simple thermostats were mechanical and when a coil of metal changed temperature, that would tip over a mercury switch... some mercury inside a glass vial that would make contact between the R and W when it got too cold in the room, and when it warmed back up, the mercury would tip the other way and open that circuit.
Make sense?
Also you want to mention exactly what kind of system you've got there, because if it's a heat pump it gets more complex. And because you say you have O and Y2 connected...
