HD44780 mod for 3.3V power supply!

[kgavionics]

Hi guys
I added an ICL7660 to a 1602 display based on the famous HD44780 to work with 3.3V ( contrast). It worked with no issues, but I have done the same procedure to another display  which didn’t work, maybe the fact that the 2 boards are a little bit different( there is 2 resistors R6 and R7 beside the chip which remained unpopulated)
I looked online, but couldn’t find any resources how to modify the second one to make it work with 3.3v.
Does someone have any suggestions how to fix that?
Thank you in advance

[Doctorandus_P]

...  which didn’t work, maybe the fact that the 2 boards are a little bit different.

Yes, it's quite likely that is the reason.

Fooling around this way is mostly hit and miss.
It can be nice for hobbying on your own lab, but don't sell those things to customers. The most common solution is to just use a 5V power supply and a small linear regulator for the 3V3 part.

[T3sl4co1l]

If it's not in the datasheet, it's not for you.  I've never seen DNP components documented, ever, on one of these modules.  (I don't recall a case where components were documented on any module, for that matter?)

ICL7660, is a good assumption, a common use case -- but you basically have to trace the entire circuit to know.  Or buy a premade 3.3V model in the hopes that they populated exactly these components, and cut whatever jumpers might be needed to make it work.  But without that, you don't really know where it connects to the blobs, and what's under them for that matter (real HD44780s, clones, or equivalents?).  So there's no design process here, it's just try and see, and hope it doesn't fail catastrophically.

Hmm, I forget what I did last time I ran one on 3.3V; maybe it needed negative contrast bias?  Easily enough added on the main board, assuming you aren't terribly strapped for space or anything.

Tim