Well I've tested the seven-segment LEDs I had in the junk box and they were pathetically dim. That's not surprising, LED efficiency has been going up all the time and they now compete with CFLs for lighting.

So I threw my old displays out and will buy some new ones from eBay.

Hmm, browsing eBay for displays shows some 4 digit modules but only 4 connections, 2 for power and 2 for clock and data. What's with that? More research shows that they contain the TM1637 chip and can be driven with a serial protocol. Here's how to do it from Arduino.

If I use these displays which are cheap enough, I can dispense with the latch, buffer, anode transistors, current limiting resistors, and a whole lot of pain wiring it up.

Ok done. I have writen the bit banging code to drive these serial interface displays. Some back of the envelope calculations show that updating the display will take about 250 μs. The display only needs to be updated once every half-second, to blink the colon, and the tick interval is 4 ms, so there is enough margin. Actually since the display does its own refresh, I can even increase the tick period, but I'll do this only if testing shows that it's inadequate.

I also changed the font table because the mapping of bits to segments is different in this display.