Mystery Desk Phone LCD

After learning some basics of driving small segmented LCD units from Joey Castillo’s Remoticon talk, I started digging through my pile for LCDs I had salvaged in past teardowns because I thought I would play with them later. “Later” is now.

I chose this as the first candidate because it had metal pins for connectivity instead of the elastomeric “zebra strip” commonly used for LCDs. (Like the LCD in a food thermometer I recently took apart.) I hoped metal pins meant it would be easier to work with. I vaguely remember salvaging this unit from a landline office desk phone using my paint-stripping heat gun. I have little memory of the rest of the device, but I clearly remembered this screen had a LED backlight whose plastic bits melted out of shape when I tried to salvage it. Rendered useless by my mistake, I discarded the backlight. I have better tools and skills to remove that sort of stuff now, but that’s history so moving on.

Like I said I wasn’t terribly meticulous when salvaging this LCD, so I’m missing a lot of information about it. I didn’t write down the make and model of the phone or noted any information about the circuit board. (Useful for determining ground pins and such.) And now I’m faced with essentially a black box as there were minimal markings on the LCD.

One corner had a logo with “LCD” in the middle. Could it be the logo of a manufacturer who believes that the fact they make LCDs is more important than their own name? Seems odd but stranger things have happened. (U.S. Robotics made no robots but was named because founders were fans of Issac Asmiov.) Next to that logo is a number 6334 that might be a model number but that is too generic for me to find anything online.

Another corner had information etched into the glass. I read them as:


The nature of this information implies something that changes rapidly instead of a make and model. 20091007 conceivably stands for a production date of October 7th, 2009. The second row may be a serial number. None of which, sadly, helped me find more information about it.

Below the etched layer, I think I see “1B” which is likely on the same layer as “LCD 6334” earlier.

I then decided to probe these pins with an “AC” signal as per Joey Castillo’s talk. I thought it would be helpful to solder them to a set of 0.1″ pitch breadboard compatible header pins. With nine pins, I had hoped this was a very simple segmented LCD status indicator. Maybe a few phone-appropriate icons like “on hold”.

Probing various combinations of pins, the most response I could get out of the device is captured in this photo, and only for a few seconds before it faded out. I see a “PM” that would be appropriate for a clock, and a single trapezoid that could be a 7-segment numerical display to go with that clock. But more significantly, I see hints of a large dot matrix display with four separate lines of seven segments each. The information visible here implies a far more complex display than what could be multiplexed with just nine pins. This is not a bare LCD with pins for an external driver, there must be a controller chip under the black blob of epoxy.

But if that is a controller chip, how did I manage to darken a few elements by probing with AC? A wild guess is one of these two pins (leftmost or rightmost) is a data clock pin. Absent actual connection to VCC or GND, the chip was running on the minimal power flowing on those two pins. After a few seconds in the absence of an actual ground, some internal element became saturated and stopped working. If I wait a few minutes for that condition to fade, I can reapply these pins and light up the same elements again. So the good news is that I don’t think I broke the LCD, but I certainly wasn’t running it under any semblance of control.

I don’t think I can do much more with this LCD right now. More skilled hardware hackers have techniques to tease out roles for these nine pins, but that is beyond my skill at the moment. I thought reverse engineering might be easier if I could probe the associated circuit board, so I pulled out an LCD still attached to its board.

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s