Beginning attempts at a PIC-powered LED die

I thought a good step up from the blinking LED would be to make a die with an LED display, having seen someone do something similar.

I wasn’t sure from that diagram how the PIC was driving the LEDs, but I am sure that a naive implementation using 7 outputs would be a bit wasteful. And dull. I wondered if  it might be possible to output a 3-bit binary number between 1 and 6 and use the external electronics to determine which LEDs should be lit, based on the outputs. This might not be the best way to do it but it seemed fun.

I started by writing out a table of inputs and outputs, assuming a die consisting of 7 LEDs  laid out like this:

Each LED has a number, making a table with 3 inputs and 7 outputs:

I1 I2 I3   O1 O2 O3 O4 O5 O6 O7
0  0  0    ?  ?  ?  ?  ?  ?  ?  INVALID
0  0  1    0  0  0  0  0  0  1
0  1  0    1  0  0  0  0  1  0
0  1  1    1  0  0  0  0  1  1
1  0  0    1  1  0  0  1  1  0
1  0  1    1  1  0  0  1  1  1
1  1  0    1  1  1  1  1  1  0
1  1  1    ?  ?  ?  ?  ?  ?  ?  INVALID

It’s pretty clear from this table that the problem is actually simpler, as many of the outputs mirror each other, and O7 mirrors I3:

I0 I1 I2   O1,6 O2,5 O3,4 O7
0  0  0    ?    ?    ?    ?  INVALID
0  0  1    0    0    0    1
0  1  0    1    0    0    0
0  1  1    1    0    0    1
1  0  0    1    1    0    0
1  0  1    1    1    0    1
1  1  0    1    1    1    0
1  1  1    ?    ?    ?    ?  INVALID

I was going to make a Karnaugh Map to simplify this further, but then realised that I’d have to make one for each output, which would be tedious, and that actually the simplifications are easy to spot anyway — all the more so because we can ignore outputs for 000 and 111 which the PIC will not generate.

O7 can be connected directly to I3, which is to be expected, since the middle LED only lights up for odd numbers. O2,5 is similarly simple, and can be connected directly to I0. O1,6 should be on when either I0 or I1 is high, and O3,4 should be on when both I0 and I1 are high. So: a couple of easy connections, an OR and an AND to implement! Neat.

I started sketching out a schematic for the circuit, but I don’t think I’ve got it right yet — either that, or my breadboard implementation is wrong.

I’m using 2 NPN transistors to form an AND gate for O3,4. I0 and I1 are both connected directly to O1,6, and I2 directly to O7. O2,5 is connected to I0 behind a diode, to prevent a high signal on I1 illuminating O2,5 via their shared connection on O1,6.

However: this doesn’t work properly. The transistor pair doesn’t stop the LED lighting up when I0 is low and I1 is high — the LED remains on, albeit dimmer. Also, the diode arrangement for O1,6 and O2,5 doesn’t work with the schematic above. When I replace the connection from I0 to O2 with another diode, it does work correctly. I also added D2 as an experiment — it doesn’t work without it — and I don’t really understand why that’s necessary either. I suspect voltage drop(s) are at fault, somehow.

I ended up going round in circles trying to get this working properly, and ran out of things to try. Am also not sure if this is the right basic approach or if I’m going down a blind alley. Might have another go at it another day, or perhaps start with a sucky 7-output wasteful (but functional) design, and iterate from there!

Edited to add: Obviously, now that I’ve simplified it down to 4 outputs, I wouldn’t need to make something with 7 outputs. Duh. And this seems like an awful lot of work just to have one less output. So I think this approach is a bit silly, and I’m just making work for myself. Next, I will try making something with 4 outputs, and get on with writing the PIC program. That said, it would be nice to understand why this circuit doesn’t work. Perhaps I’ll meet someone I can ask.