Increasing 20x4 LCD contrast range

[bitrex]

Replaced the LCD and backlight assembly in my old Emu ESI-4000 sampler:

<http://www.vintagesynth.com/emu/esi4000.php>

Which after 25 years even the LED back lighting was starting to get
annoyingly dim.

Replaced it with one of these white-on-black units which works fine
and looks pretty good:

<https://www.buydisplay.com/arduino-black-lcd-20x4-i2c-code-character-module-display-high-contrast>

Except that the software-adjustable contrast range, which was probably
optimal for a black-on-yellow display, can't bring the CV up enough to
avoid some "blockies" shining through. I'm wondering if there's a quick
hack to get some more range. Or maybe it would be better to put a
resistor in line with the backlight supply to reduce bleed-thru.

The "Turbo" board on these adds some of the nicest digital filters there
were in a hardware sampler of the era:

<https://www.youtube.com/watch?v=zEg60WzYco4>

[Rheilly Phoull]

> <https://www.youtube.com/watch?v=zEg60WzYco4Do
Do you mean the pot that adjusts the light level cannot provide enough
contrast without the :blockies: showing ? All the units I have seem
pretty bright without the "Blockies".

[bitrex]

Yeah. It's a little difficult to capture this particular color scheme
correctly on a cell phone camera but at the maximum range of the control
it looks kinda like this:

<https://imgur.com/a/liLGsd9>

And unfortunately the contrast is controlled in software, probably via
PWM there's no direct connection to an analog adjustment pot.

I might swap it for an OLED display instead, or just go back to the
boring black on greenish-yellow. Hmpf.

[bitrex]

On 6/29/2021 9:27 PM, Rheilly Phoull wrote:

I got pic of it by turning off the HDR on my camera, it looks about like
this at the maximum excursion of the contrast control. A lil worse in
the pic than it does in person.

<https://imgur.com/a/xEbbFZ5>

[Rheilly Phoull]

Okay, I use the blue modules with the pot turned back until the blocks
are only a little darker than the background which isn't too bad.

[Rob]

bitrex <us...@example.net> wrote:
> And unfortunately the contrast is controlled in software, probably via
> PWM there's no direct connection to an analog adjustment pot.

There still will be some line with a varying voltage that doesn't do
anything else, and which you can try to pull up or down using a resistor.