January 25, 2013
about 2 years ago
I’m sure you’ve gone a different way by now, but i’m putting this here for other people looking for an answer.
The deal with HD44780/compatible LCDs is that the contrast voltage (sometimes referred to as the “bias” or “Vee” in datasheets) should be 4-5.5V BELOW Vcc/Vdd. That’s why at 5v the screen is basically blank for most of the range of your pot and then the last 1/5 or so seems like a hair trigger. Unfortunately at 3.3V this means your visible range will be between NEGATIVE 0.7V and NEGATIVE 2V. Bummer!
What you need is an inverting charge pump to supply a negative voltage from your 3.3v.
I’m not sure about sparkfun’s LCDs, but most HD44780 LCD modules have an unpopulated SOIC-8 footprint and spots for a couple capacitors. You would need to populate the SOIC footprint with a 7660 charge pump (available from Maxim or Intersil) and the caps with 10uF. I don’t see that footprint in the picture but i guess it’s possible it might be under the sticker. If not you should be able to make a breakout and provide the -V through the Vo pin (not tested by me.) I don’t know if SF will let me post a link on my first post but there’s a schematic of all this on some blogger’s (not mine) site:
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