Author Topic: Multiplexing scheme of an LCD module  (Read 680 times)

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Online MitiTopic starter

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Multiplexing scheme of an LCD module
« on: August 28, 2019, 07:14:09 pm »
I have a device with an LCD that is defective. I have no chance or will to repair it but I'm thinking of making my own board for it while using the case, the LCD, etc. How do you figure out the multiplexing scheme of a 7 segment LCD? I have no idea how many COMs it has.
Thanks in advance!

Edit: The device is defective, not the LCD.
« Last Edit: August 28, 2019, 07:16:35 pm by Miti »
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Offline TomS_

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Re: Multiplexing scheme of an LCD module
« Reply #1 on: August 31, 2019, 12:44:29 pm »
Brute force.

Hook a suitable power supply to pins 1 and 2, if no segment comes on try 1 and 3, and so on.

Eventually you will find a common and can start to map pins to segments.

Perhaps the datasheet for the original hardware used to drive the LCD could give some hints as to which pins are commons.
 

Online MitiTopic starter

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Re: Multiplexing scheme of an LCD module
« Reply #2 on: September 07, 2019, 02:55:31 pm »
Perhaps the datasheet for the original hardware used to drive the LCD could give some hints as to which pins are commons.

Thanks Tom!
There's no chance to find a datasheet, just a blob of epoxy and few caps.
Anyway, I was able to map it using the FY6600 generator set to 50Hz square wave and 3V amplitude. Higher voltage and I see ghosts, lower voltage and i don't see anything. 3V seems to be the sweet spot where only the correct segments turn on. This is the device that I want to make my own PCB for.
https://www.banggood.com/WeiHeng-A08-50KG5g-Backlight-Digital-Handle-Luggage-Scale-Portable-p-954735.html?rmmds=myorder&cur_warehouse=CN
If you ask why, I bought one from BG and it was reading too high. I found the way to calibrate it, you attach the batteries while pressing both TARE and UNITS and it goes into calibration mode. You then set the temperature, the zero level and, I think, 20Kg but I'm not sure. Unfortunately while I was playing with the calibration mode and trying to figure out what's the upper calibration weight that it wants, it simply died. Probably it has bad code that overwrites some code area with the calibration constants in certain conditions, I'm just guessing. So now it won't power up.
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