Testing the Display

Phew! After all that board's been through, can it possibly work? The answer is a qualified “yes”...but it is a YES!

Once I had all the parts installed, I wrote a quick little unit test using a 16F628. The test just sets the clock to show 23:38, (11:38 p.m. 24-hour style), and lets you advance the time one minute per press of a button. Here we see the display after I've advanced it ten minutes to 23:48. Click the thumbnail for a larger image.

The digits ended up somewhat dim, but I'm not sure there's much that can be done about that. I did a fair amount of breadboard testing to try to find good component values, and at 1/4 duty these displays are just dim. It might work better if I were to use a higher voltage - but I propped the board upright and was able to read the display from across a lighted room, so it should be good enough. Putting a red filter over the digits helps, and while this picture of the display in a lighted room is not the best, it shows the difference:

Meanwhile, the colon is really bright. It could be fixed by putting in bigger current-limiting resistors, but I could also fix it in software just by driving it at a lower duty cycle. That's clumsy, but I've done worse. I was a video game programmer for a long time and am not proud about kludging.

One slightly concerning thing is that the 7805 gets hot after only a few minutes. That may just be because I'm testing this board with a 12 volt power supply, so it's having to dissipate 7 volts at significant current - even dim LEDs pull a fair amount. This might be solvable just by using a lower voltage wall wart. If not, I should probably bend the regulator out away from the board and heat-sink it.

All in all, a fairly successful exercise! To sum up,

It works!
The software, with the exception of one buglet that caused the “9” numeral to show up as a blank, worked on the first try. That never happens! This was a simple program, but it did have to know how to build up all the bit sequences for the numbers, shovel them out to the shift registers, and manage all the timing with an interrupt - so while easy it wasn't trivial.
The display is rock-solid, with no flickering or ghosting.