After I created the project page, I was thinking about the boards that require development : the alphanumerical module itself, of course, and the small test board that sends the clock pulses during testing.
Then there is the question of how to drive the 50 modules and display messages.
One obvious solution is to use a Raspberry Pi with some sort of web interface (driven by the #micro HTTP server in C for example) but the workshops are about electronics, not computing or programming !
Then it occured to me that it's totally fine to make a "semimanual" keyer that generates the required pulses when pressing on a given button...
Of course the "normal approach" is to use an Arduino (or Pi) to scan a keyboard, and transform the keycode into a 16-bits synchronous serial stream. But the kids wouldn't learn much and programming would confuse them a bit.
It's more interesting to generate the bitstream with hardware, particularly with a couple of 4017 and a large diode matrix : everything is exposed, with LEDs showing the paths of signals. This requires a significanly large PCB but this can become a fun subproject on its own for the last months of the workhop.
Of particular interest is the circuit that generates exactly 16 pulses when a button is pressed. I have not yet figured it out completely but it's going to be very interesting. The 4017 can indicate when all the pulses have been sent to inhibit the clock but a single press of the button must send exacty one character, regardless of the pulse duration.
Oh and I'll have to find nice buttons for the front plate, if possible DPST because a single press must both route the signals from the diode matrix, and start the oscillator.
Another approach doesn't use a 4017 but 74HC165 parallel-in shift registers. There is still diode ROM but it is driven "backwards" compared to the 4017 version.
A single push of the button will drive diode to 1) decode the data to send 2) start the oscillator. This way, only SPST buttons (classic !) are required. The kids can play with the diodes to generate the desired letter shapes. The '165 can latch all the 16 bits during a single clock cycle then shift them out later, when the button is released.