RC Networks, They Matter!

After a long hiatus, I’m finally dusting off some projects and getting back into them, well, one in particular, the sous vide controller. I knew there were board errors from the prints I ordered early last year, I had even started to fix some of them. I got the boards and components and eagerly assembled one. Once I powered it on, there was one immediate problem. The LCD contrast with a diode that I wrote about previously, well, it’s not giving so constant a contrast on this board. Maybe it’s a poor diode, maybe I’ll build a bench power supply to vary the voltage across different diodes and see how they respond.

After putting some code on the chip, I found another problem. The push button on the encoder would not work. I got nothing at all from it. From probing the board and the resistors, I discovered that the button was active high, but my idle resistors were also pull-up. Tsk tsk. Ah well, that’s an “easy” fix, just scratch out some traces, re-route (in this case a power rail) around them, no problem, and connect the common side of those resistors to GND instead.

That got the button working, but not the encoder. Long story short, I played with the hard-to-see LCD for a while and then set it down for many moons. Oh that’s right, I also had to cut the common freewheel diode leg from the transistor array because apparently that’s not supposed to connect to “common”. Oops.

Fast forward, and now I dust it off, scratch my head, and try writing some simple encoder and button tests onto it. When they failed, I turned to my schematics, and scratched my head again. What I noticed actually pretty quickly is that the encoder legs were active LOW, but the push button active HIGH! *sigh* “I see why I did that,” in a way at least. I had to tie the push button active high because it shared with the common anode on the encoder LEDs. And it would seem I found that out after I made the quadrature active low.

So, I fixed all of that. I’m determined now to get this board to work. Of course I’ll make another run with corrections, but only after I find all the corrections. A few more trace cuts, more jumpers, and I got all the inputs working. Well, kinda… The rotary worked, a little bit. But not nearly fast enough. First I sped up the polling and that helped, only a little. Then I went full on PCINT driven, and no more improvement was to be found. At this point  I was beginning to suspect the RC network was too slow.


I plugged those values into units (not that that would be particularly needed in this case, but just the same, I highly recommend units!) and looky there. Fall time of 15ms! Yikes.

So I replaced the capacitors with some 15pF I found at Ra-Elco, and now it works flawlessly.

Moral is, don’t make board mistakes.
Moral is, you’re probably going to make board mistakes.
Moral is, don’t just pick component values because … whatever? Ya, don’t do that.

One thought on “RC Networks, They Matter!”

  1. Awesome! Its truly remarkable post, I have got much clear idea about from
    this piece of writing.

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