I found a bunch of these boards in my surplus bin, no idea where I got them but based on the date codes they date back to the late 70's. I finally decided to play with them a bit so I traced out the circuit and did some experiments. They have 4 LED's, +V and GND inputs but no other inputs. I hooked it up to a variable power supply and found that LED's 8 and 20 come on around 6V, at 9V LED 8 goes out and 20 stays on, at 12V all the LED's go out, and at around 13V, LED 14 comes on. The part that perplexes me is, what condition would cause LED 28 to come on? The element of the comparator it's tied to has it's minus input tied to ground, so based on my understanding of comparators, there should be no way for that element to turn on. Anyone have and ideas on this?