I started to work on the frequency counter again. Back in December, I found a small hardware design flaw. Specifically, I 'd forgotten that the open collector switch the ULN2803 Darlington array is logically negated. To be precise the problem is not with the ULN2803. It just do it's job, driving the "high current" cathodes of the display. The problem is that the ULN2803 is driven by a Johnson decade counter ( 4017 ). I use the ninth output for reading the jumpers. And here, I did not take into account that this output is not negated. As a result, I soldered the eight diodes with reverse polarity.I handled the problem from the software but didn't wanted to keep that way.First, put together the fix on a breadboard (diodes and a dip switch) and tied to my prototype.After that, I changed the code, which has been working for the first attempt (you may consider it as a miracle).
Finally, with some soldering rework changed the polarity of the diodes.
This prototype (at least the hardware) is complete . Now comes some work on the schematic and the final PCB design.