How Lite can PewPew be?

Published on 2018-07-15 in PewPew Standalone.

After designing a cheaper version of the PewPew FeatherWing recently, I started thinking: what if instead of using a separate chip for driving the LED matrix, I used the free pins on the MCU? And instead of buttons use touch pads? And instead of a USB port use the shape of the PCB? I could get the BOM literally to a few parts!

One problem with that: I would need 24 pins to drive the bi-color matrix. Sure, I could add shift registers and decades and whatnot, but then I’m back to using additional chips, and with the price of the HT16K33, I may just as well use that. But what if I could use fewer pins? Are there tricks for that?

With charlieplexing I could drive a 16×8 matrix with as few as 12 pins (the formula is n²-n, which for 12 gives me 132 LEDs, four more than my required 128). But you can’t charlieplex matrices with common cathodes. Well, you can , but that’s not useful for square matrices. But what if I made my own bi-color matrix on the PCB? You can get 0603 bi-color LEDs cheaply — in fact cheaper than the equivalent matrix. Soldering them is a lot of work by hand, but if I got it fabricated the pick-and-place machine does all the work anyways. And it would then be flat. Like a business card.

Wait a minute…

With so few parts it could actually be cheap enough to use as a business card!

So I made this mockup:

The big black hole is for a CR2032 battery — it will be sunken into the PCB, to keep the thickness at a minimum.

I even started routing the matrix — for now I have routed 8×8 LEDs with 9 pins used, and 7 pins free. Now I have two options: route the other half the same way, using the remaining 7 free pins, and the two debugging pins (possibly with some jumpers to disconnect them for debugging), or redo the whole thing as a 12×11 matrix using only 12 pins, and then rearrange the LEDs back to 16×8. Of course the second option would be much better, but I’m not sure I have the stamina for that. If I go for the 2×8×8 approach, I might also separate the two colors — we will see how it goes.