Main display

To be readable in the dark without glasses from about 0.5m distance, the characters should be around 50mm high.

There are a few options here:

• numeric or alpha-numeric LED segment displays, sized 2.24" or 2.3".  These are used in the nice Alpha Clock Five.  These displays are expensive (Digikey €5/pce) and not very standard.
• LED dot matrix 5x7 5mm, not standard, not cheap
• LED dot matrix 8x8 5mm, about €1.5/pce, 2088AS or 2088BS, common anode or cathode.
• LED matrix 8x8 3mm common anode, €0.60/pce, tests with a 5mm LED dot matrix with 3x5 pixel font (35mm high) showed me that it's readable without glasses from 0.5m distance.
• LED Panel P10 Red, AliExpress $12.68.  Definitely the cheapest 32x16 panel, but the size is 320x160mm.  It's too big.  There are LED modules with smaller LED spacings, but the outside dimensions don't change much.
• LED Panel P3 RGB, AliExpress €18.51.  192x96mm, 64x32 panel

After putting a lot of effort into making my own module for the 8x8 3mm LED matrix (see Tileable 8x8 LED matrix), I discovered that 32x8 LED matrices are on sale for close to no money (€2.93).  These modules use the MAX7219, which is controlled by SPI.  You could also buy an 32x16 LED matrix (€6.89)/pce).

Development is ongoing : two 32x8 display units are stuck together with tape.  I had some trouble writing data to the MAX7219 modules.  It seems that only bit banging SPI works.  Maybe that's because the modules are only powered with 3V3, while the MAX7219 requires at least 4V according to its datasheet.

All the libraries I tried (LedControl, LEDMatrixDriver, ...), had issues.  I adapted the arduino-Max72xxPanel to suit my needs.

Choosing font

The font should be easily readable without glasses.  So we want it as big as possible.

The fonts have been recovered from the Xronos clock.  That clock contains five different fonts of which four are shown here.  The character '8' is chosen, because it's one of the most difficult ones to read.  The two fonts on the right are easiest to read.  The rightmost font has been chosen because it looks nicer.