Segmented LCD From Sunbeam PAC-215

I now have imperfect but working control of a 2-digit 7-segment LCD module salvaged from a Sunbeam PAC-215 electric blanket controller. I used an ESP32 for this initial run because I thought I needed voltage control over all ten pins. After a little bit of experimentation, I’ve found that I could probably get away with using 0V and 3.3V directly on each of the segment pins. For the common pins I need an additional option of the midway point 1.65V, but if it is true that an LCD uses very little current, perhaps I could supply that voltage with merely a voltage divider built from resistors? If true, I might be able to control this with something far simpler and less powerful than an ESP32. Perhaps an ATmega328 (Arduino)? Or a PIC16F18345, which has a DAC peripheral and already on hand. (If I’m going to buy a chip, I’ll take Joey Castillo’s suggestion and buy one with LCD control peripheral built in.) These are potential experiments for the future. For now, I want to wrap up work with this particular pair of salvaged LCDs which means leaving good notes for my future self.

I scribbled down this diagram when probing the pins. The pins were numbered before I knew what they did. Now that I know which are segment and which are common, I could label them differently.

I also moved to a zero-based counting system thinking it might be easier to match up to code. Color-coding the different common pins is an idea I picked up from Joey Castillo’s slides, but I wonder if that’s the best way to go. I can’t claim to be an expert on how to use color in a way that’s still accessible to the color blind, and coding common pin information in color risks losing data for scenarios like printing on a black-and-white laser printer.

Moving to letters for segments and numbers for common allows me to put them together as labels on each segment while reducing ambiguity. This is also friendlier to black-and-white printing, but it loses the visual delineation of color coding. Is this better? I don’t know as I see strengths in each. I couldn’t make up my mind on which to keep so I’ll keep both of these diagrams here as references for later.

With this successful milestone of working with LCD modules, I revisited a module that frustrated me earlier.

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