Reverse Engineering an LCD - Page 1
Forum section: electronics / projects

author: MotoDan
author_rank: Contributor
timestamp: March 20, 2019, 02:49:15 am
content: I've got a custom segment style LCD that I need to have made by another vendor and do not have the original documentation for. I have already done some initial RE and know the bias type (1/4 multiplexed) and have a partial pinout for the 4 common pins. What I need next is the segment pin map (schematic) so that I can finish the drawings needed to have the display made.My first thought was to apply a voltage to each common pin and walk through the remaining segment pins, noting which segments light up. There must be more to this step than I anticipated because I am not having any luck getting any of the segments to turn on. I've tried common voltages ranging from 0 to 5V.Another thought was to de-laminate the two glass sections to see if I could generate the schematic that way. I only have one display and don't want to ruin my chance of mapping this one.The display is driven by a PIC with a built-in LCD controller. I don't have the source code for the PIC so can't go about it that way. The product that uses this display has been in production for years and there is no chance of altering the LCD's pinout at this point or changing the firmware.Looking for suggestions on getting this display's pinout defined out without having to pay the manufacture to do it.Thanks!

author: EEVblog
author_rank: Unknown
timestamp: March 20, 2019, 03:40:26 am
content: Quote from: MotoDan on March 20, 2019, 02:49:15 amCan you get the at the right angle under bright light and see the traces?

author: MotoDan
author_rank: Contributor
timestamp: March 20, 2019, 03:52:22 am
content: Hi Dave,No, I've tried, but can't quite see the traces regardless of the angle, lighting, etc.

author: IconicPCB
author_rank: Super Contributor
timestamp: March 20, 2019, 05:30:36 am
content: The display as You know works as segments over a common electrode field.Make up an oscilator ( cmos schmidt trigger inverter gate) pass it through a couple of inverter gates (connected in series) and use the two buffered outputs to drive plane and segment pins. You are now ready to seek out the segment pin combinations.Egg me if I am teaching a granny how to suck eggs. You can also egg me if you dont like my politics...its OK.

author: Gyro
author_rank: Super Contributor
timestamp: March 20, 2019, 10:01:13 am
content: Quote from: MotoDan on March 20, 2019, 03:52:22 amStrangely, breathing heavily on it (light condensation) sometimes helps.

author: Yansi
author_rank: Super Contributor
timestamp: March 20, 2019, 11:30:37 am
content: If you have the pic, then open its datasheet and see, where the LCD controller pins are assigned. (tell apart COMs from SEGs), then use another controller to test the LCD out, SEG by SEG, COM by COM.

author: amyk
author_rank: Super Contributor
timestamp: March 20, 2019, 12:03:05 pm
content: Quote from: MotoDan on March 20, 2019, 02:49:15 amHave you tried an oscilloscope? If the product is working then that's probably the best way to see the signals.When testing, connecting onto the LCD glass itself is a little tricky --- you want to make a good connection, but don't want to scratch the ITO. If it has a zebra strip then I suggest using that.Related:https://www.eevblog.com/forum/projects/hooking-into-segmented-lcd-display-to-read-displayed-values/https://www.eevblog.com/forum/beginners/how-to-hack-a-weighing-scale/

author: NivagSwerdna
author_rank: Super Contributor
timestamp: March 20, 2019, 12:21:19 pm
content: I have done this before.... apply low voltage AC to and work around the pins. Do not apply DC!!!!Try 3V peak to peak and ramp it up a bit if you don't get any joy.https://aardvarklabs.wordpress.com/2013/02/03/reverse-engineering-the-salter-ultimate-accuracy-scale/and subsequent pages

author: johnkenyon
author_rank: Regular Contributor
timestamp: March 20, 2019, 06:53:10 pm
content: Have you seen this circuit:http://www.circuitdiagramworld.com/test_circuit_diagram/Liquid_Crystal_Display__LCD__Tester_CD4047__2489.htmlLCD tester - one chip, three resistors, a capacitor, a 9v battery and no DC voltages to ruin your LCD.

author: StreetRooster
author_rank: Newbie
timestamp: March 25, 2019, 06:11:21 pm
content: Hi There MotoDanYep been there...I Have a 3 segment LCD display module, from a TADANO AML1 LMI system. to the best of my understanding they are obsolete.But i managed to make a circuit to test the lCD display at a cost of ZAR 20.The problem i found is the polarising film on the glass is old.I will place on the post the circuit i used with pictures.

author: MotoDan
author_rank: Contributor
timestamp: March 26, 2019, 11:41:25 pm
content: Thanks to everyone for their input. I was able to get some of the segments to light up using a 5V square wave across two pins at a time. Still not 100% sure about the segment pinout as the results were slightly ambiguous. I am fairly certain about which pins are the 4 Commons, but the I'm not getting the segments to come on as I had expected. Perhaps the other pins need to be at multiplexed accordingly?

author: NivagSwerdna
author_rank: Super Contributor
timestamp: March 27, 2019, 02:28:50 pm
content: If you mean 0...5....0....5...0.... then that has a DC bias and is evil.It should be pretty deterministic especially if you have actual pins... a bit tricky when fighting with a zebra strip and just glass.Mind you some dude made a video...*UPDATE*: You didn't mention frequency.... Somewhere between 30Hz and 100Hz is where you should be aiming.

author: MotoDan
author_rank: Contributor
timestamp: April 01, 2019, 04:29:42 pm
content: Well, just about ready to send out some RFQ's. Looking for any recommendations for custom LCD suppliers with favorable results and are willing to work with lower volumes.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: April 01, 2019, 05:09:32 pm
content: For reverse-engineering, low-voltage DC would probably be fine.