While building the 2nd prototype, there comes an opportunity to revive an idea I first imagined a while back, when doing my first 7 segments decoder. In this log, dated 20151024, I entertained this:
http://hackaday.com/2013/10/18/making-a-diode-matrix-rom/ (a comment there and a link to http://josh.com/notes/bitlybot/ make me contemplate a version with red LED instead of Schottky. I have many 0603 LED so it can replace SMA/SMB diodes but the output must be "level shifted" because low level would be around 2V)
Fast forward today, with this stockpile of russian parts, and a better understanding, this idea takes a new life... I think the diode matrix should be replaced by a LED matrix, just like in this video:
I have the decoder, the matrix structure, the display, and... a small bag of AL102 ! (see this other log)
The AL102 might be pushed to the limits, both current-wise and their reverse voltage, but this depend on how much I can undervolt the bidirectional Numitron filaments. Let's test...
Power supply : 5.6V, 15mA - Vnumitron : 2.9V - Vdiode: 0.53V - Vled: 2.13V
Power supply : 6.6V, 18mA - Vnumitron : 3.83V - Vdiode: 0.55V - Vled: 2.17V
So despite my fears, it's possible to power the LED array with the same power as the relays, the Numitron takes most of the voltage differential and the current doesn't increase too much. However the voltage is still quite low and the 6.6V power must be totally reversed, I can't use a single -3.3V/0V/+3.3V trick. One more relay is needed.
That's no big deal because this is only a demo unit, "for educational purposes" :-)
Driving the relays is not so simple, though. But there are 9 relays if a single 6V supply rail is used :-)
So there is one less relay compared to the intended version. This is possible because there is no need to adapt the input impendance (only one button is used, it's not connected to a data bus) but this comes at the price of adding series resistors to adapt the string lengths. In this case, since there is only one or two pieces, it's not critical, the actual power is not important (unlike the intended circuit that must be energy saving because there is one or two dozens).
And this is where I realise I might have made mistakes with the driving circuitry.
OK so here is the corrected circuitry :
so outputs 5 and 6 should be swapped, and i remember I had to rewire them in the last prototype. Fortunately it's very easy to do.
20171007: Damnit, I had unexplainable shorts and now I understand why : the metal case is not insulated !
I have packed the LEDs close to each others and didn't think they should not touch the neighbours...