You know that feeling you get when you put a whole bunch of system components together, and they work perfectly the first time? Yeah, me neither. At least, not tonight. First, I installed the MOSFET driver shield board on the Arduino Mega, and discovered that a clever enhancement I had thought up not only prevented the board from seating properly, but shorted out the power supply. Turns out I had forgotten that a section of the shield board sits flush against the ground shield of the Arduino's USB power, so you can't install any components there. I got fooled by the existence of component holes in a place where no components should go. So there were solder blobs and wires that shorted out on the USB shield, and prevented the shield from fully seating into the Arduino:

Removed the offending components and solder, and it fit with no shorts:

Fortunately, it was possible to re-route the offending connection so that it was well clear of the USB shield. I'll have to fix that in the instructions.

Then, the LED that was supposed to light up, didn't. Turned out that my Arduino IDE installation was corrupted on my main computer, and wasn't properly uploading the testing software on the Arduino. Switched over to a different computer, and the software worked, but the LED still didn't turn on. Finally found that one connector that should plug in snugly and securely was loose and floppy, and when I temporarily fixed that, the LED lit up. Yay. Haven't had that problem before, so I'll have to clip the wires off the connector, re-wire a new connector, and see if I can figure out what caused the problem. At least I know the system will do what it's supposed to do now.

And so to bed.