Last night I was messing around with another experimental wireless Renard-type controller and it *kind-of* worked, and then it didn't.... and then it did... and then it didn't at all and wouldn't come back to life. I thought the chips maybe lost their firmware, so I reprogrammed them.... then maybe the XBee radio pooched-out, so I reprogrammed that, but no, no blinky.
So I turned to my oscilliscope for some answers. The nice square wave I got out of pin 2 of the Xbee radio confirmed it was receiving the signal and processing it, so I knew the XBee was okay. Testing pin 5 on the PIC16F688 showed the same square wave, so I knew the data was getting to the PIC and there were no breaks in the circuit from the XBee to the PIC. Testing pin 1 of the PIC confirmed there was 5vdc there so I knew the board's power section was working; testing pin 2 showed the 18.432mhz clock signal was there, so I knew the crystal was working fine. Testing pin 4 on the PIC I found there was no ZC signal. Hmmmm....
So I took a look at the H11AA1 chip... The A/C side of the chip showed a nice 60hz wave, but nothing on the DC side at the PIC. This smacked of either a bad chip, a short or a no-connect. I pulled the H11AA1 chip and made sure there was nothing inside the socket that would cause a short, flipped the board over and didn't see anything obvious, so I replaced it with a new H11AA1. Powered it up and... no change.... still no ZC.
Hmmm... time to go looking for no-connect issues. Flipping the board over and this time with the lighted magnifier, I found that one of the leads of a resistor adjacent to the H11AA1 chip looked like it was free-floating in the solder pad. AHA! Maybe I missed that one! I reflowed some solder there, powered the board up and ZC was back on pin 4 of the PIC. Problem solved.
The point of this diatribe is that without the oscilliscope, it would have been a lot harder to trace the problem down to a specific issue because a DVM isn't generally quick or sensitive enough to catch it.
For those of you who plan to build a lot of your own gear or who want to experiment with designing your own, I heartily encourage that you consider adding an oscilliscope to your workbench. It doesn't have to be super-expensive because if all you're looking for is the existence of a signal and don't intend to measure the specific timing, most cheap 'scopes would suffice. I purchased a Rigol DS1052E for about $400 last year and while it's only a 50mhz 'scope, it does everything I need it to do and has more capability than I'll probably ever need. Others whose jobs require faster sampling speeds and more functions will likely chime in on this thread, but for the electronics hobbiest, almost any 'scope would be a terrific addition to your diagnostic tool set.