While I managed to get a specific LCD work with both 3.3V and 5V, it was an edge case. It was not transferable to other LCD manufacturer's displays and it may have shortened the display's life. To that end I changed the specifications to use a 3.3V LCD. This display is actually a 5V display with a voltage inverter and divider on board that provides -2V to VO.
PanelOne circuit board view from the back with 3.3V LCD
Close up of the -2V circuit on LCD
This -2V means that with 3.3V supplied, the voltage drop is 5V which the display driver chip needs to run the LCD pixels.
For reference here is the Schematic again, now on version 2.1, with the -2V on board annotated:
So the contrast pot on the PanelOne circuit board is acting as a variable resistor setting VO between 0 and -2V, and hence the contrast between "off" and "on" pixels on the LCD.
For a purely 3.3V use there would be no requirement for the 5V-3.3V LDO and the 4050 level shifter for the SD card. However, I want to be able to use this on both 3.3V and 5V logic with minimal changes. For the display to work with 5V we have two options. The first is to use a 5V display rather than 3.3V, thus requiring only 1 PanelOne circuit board design. Alternatively we could use the same 3.3V display and disable the -2V on VO. The simplest way to do this is to remove the resistor R6:
Close up of the -2V circuit disable with R6 removed
I have left it soldered on at one end so it's easy to switch back and forth. Obviously a switch would be even better (cue email to LCD manufacturer to consider updating their design).
This design is now proven on both 5V (RAMPS) and 3.3V (Duet etc) electronics:
PanelOne 2.1 with RAMPS
PanelOne 2.1 with Duet
The connection to the Duet is still made with single pin connectors and the RepRap Firmware does not yet support an LCD screen although the community (and me!) are working on it.
As always our designs are open hardware (CERN OHL 1.2) - the latest KiCAD files are available on our Github. In addition the updated case is also shared in STL and OpenSCAD on github.
We will be using the PanelOne v 2.1 circuit board in our Kossel Mini kits once our current stocks of 5V-only PanelOne boards are used up. In addition we have listed variations on the webstore, where you can choose the 5V only version or the 3.3V version (which you can adapt to 5V using the resistor de-soldering method outlined above).