So far the module is pretty simple :
- 10 relays decode the 4 digits
- Tens of diodes encode one signal out of 16 into a 7-wires bus
- The numitron displays the 7 digits.
I want to display 16 bits as 4 hexadecimal numbers. That makes 40 relays. An octal version would use half the number of diodes and relays, but requires one more digit (plus one semi-decoded digit), 5×5=25 relays instead of 40 is cheaper...
But what dictates the form factor is the size of the IV9 and the RES15.
The diameters are both approx. 10mm so the relays can hide behind the Numitron tube.
The tube's height is approximately twice the RES15's height so we can stack 2 relays to decrease the module's length.
The problem is the thickness of the diode array... The diodes should be soldered horizontally, which will use several millimeters.
The other concern is how to solder the tube and relays : a pair of dedicated vertical PCBs will work well but this increases the complexity. The pins can be bent as well and only one PCB is required but shorts could happen between wires...
16 outputs have a length of 2.56×16=41mm, which is approximately the height of the module. The 7 outputs must fit in the approx. 60mm of the module's length, and the diodes must be soldered horizontally, meaning they use more than 2.54mm.
For the diodes, I could easily use the 1N4148 that I have amassed lately but... that's not very Russian. Instead I'll pick parts from the #Clockwork germanium project : D9B and D311 seem to fit the requirements.
- D311 (Д311) : Russian Germanium Diodes, 30V 40mA
- D9B (Д9Б): Germanium, 10V 40mA
40mA is twice the maximum segment current so even if the ratings are inflated, we're safe :-)
Д311 is larger and thicker, it requires more leg room so the Д9Б is chosen. I can fit one Д9Б in 10mm, the Д311 needs 15mm...
In the end, if we exclude the PCB, all the parts will be Russian :-D