LCD driving reverse engineering

[TimB100]

Hello People

I'm trying to reverse engineer a LCD panel. The job is to take an off the shelf balance and replace the main PCB so I can add more functions. Like 232 RF USB etc.

On the board there is the usual "blob" handling the weight conversion and LCD driver. I want to reuse everything except the main PCB, so I will use a pic, HX711 for the weighing. HT1621B for the LCD driving.

The issue I have at this time is trying to work out what the pins are on the LCD.

I have scoped the signals and am getting a variety of wave forms. I took pictures of them (below) I think I can identify the commons and the non driven. But have another 2 that seem odd. 1/2 drive 1/3????

So my question is can you tell what they are? And do you have any pointers to how to use the HT1621B to drive the lCD?

Many thanks

Tim

[Lukas]

I had to do the same for the pluto watch project: https://github.com/carrotIndustries/pluto#reverse-engineering-bare-glass-lcd-panels

I couldn't make sense of the drive waveform as well, so I did what's described above. This however requires making a test jig for the LCD panel.

You could also try changing what's on the display and look for waveforms that don't change.

[TimB100]

Hi Lukas

Thanks for the post. Is there potentially an issue if I get the common lines wrong? If I wire it up as per what I think then drive the other lines and I get my pins wrong does it go bang?

Cheers

Tim

[TimB100]

Hi Lukas

It seems your github post answered my questions. Big thanks!

Tim

[amyk]

You can use a current-limited supply to test LCD connections. The segments are effectively capacitors and will draw far less than 1mA. Brief uses of DC will not cause any damage, but long-term may cause electrolytic degradation.

[TimB100]

Thanks amyk

I also found this

http://ww1.microchip.com/downloads/en/AppNotes/doc2530.pdf

It explains the signals I have been seeing.

Tim

[engineer_in_shorts]

I am not sure of the original size of your existing LCD, but I would consider swapping the LCD for a dot matrix graphics type that is the same visible area.
That way you can display more information then the original design for your 'extended' functionality, and less time reverse engineering.
The only thing to consider is the power consumption.

[TimB100]

Hi engineer_in_shorts

Yes I could convert to a Graphics LDC but that would increase the current and is not actually needed. The one included fits and once I made a long flat desoldering bit was easy to remove.

There is also a standoff designed to hold the current LCD I can use. Its not meant to be an all singing and dancing device just need to add some coms features not in the original.   

Cheers

Tim