OK, so I have access to an oscilliscope on loan.  :)

I need to watch the 120 VAC line, and also monitor the other side of an 
optoisolator, which is, of course, supplied from a +5 VDC circuit that 
shares no common ground with the AC side.

My question is, how do I do this on a single, dual channel instrument?

I seem to remember from college that I need to isolate the scope using a 
2 to three prong adapter, so I don't just short one leg of the AC line 
to ground through the test lead.  that would take care of the AC 
measurement.  The problem is, how do I also measure the +5 VDC side 
without making a common ground?  Can I (safely) tie the AC neutral line 
to the circuit's ground?  I see sparks and singed eyebrows in my future 
if I do that...

my other option is to trigger only on the zero-cross from the A/C line, 
so I could possibly get enough "noise" by using the +5 VDC and ground 
lines for the measurement, and trigger on the A/C line.  I'd get more 
useful data, however, by seeing both traces at once.

--Michael
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