I’ve done this for a couple of years. I even built a little breadboard so I could test all the signals in and out of the board on the bench. Hard to see in the picture but there is an LED strip there that lights when a transistor is on. The DIP switches are for setting the BCD inputs. I can sequence through all the transistors and make sure they are all working correctly, the relay picks and contacts are good, etc. Very handy.

Good job on your build.

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Billc479 Yes, I power the breadboard with the SDB +5 volts. The first thing I check is the +5v and HV, if those pass then I connect up the breadboard tester and test all functions of the board.