A quick trip to Vetco this morning, and I got some 22uF ceramic capacitors to replace the electrolytics that I was using in Metric Blue’s circuit. The LED blinked on cue. Success!
Then the screen read test was getting error flash, but it turned out that I had inserted one of the photo resistor leads into the wrong row. With that fixed, the circuit read the data off the screen just fine.
But then only one of the servos would turn. I switched wires between the servos, but to no avail. The left hand servo would always turn, the right hand would always not.
As shown in the photo, I detached the servos from the circuit and attached them one at a time to an Arduino programmed to run a servo diagnostic test. It confirmed that the right hand servo was bad.
How did that happen? When I converted the servo to continuous rotation a couple days ago, I tested it and it was good. Maybe, in the time since, I connected wires wrong and blew out the internal circuitry. Anyhow, it looks like it will have to be replaced.
But the main thing is, the breadboard circuit appears to have passed test phase. Hooray!