Very cheap LCD with spongy thing, how's it work?

Today I found an old First Alert timer that was in the shed, about to be discarded. So I salvaged the cabinet which was pretty nice, and the internal boards for the 12VDC supply parts. While taking it apart I also got an LCD:

From the 16 contacts on the board I suppose it's probably some kind of HD77480 variety. Unfortunately the timer's dead so I can't really test anything out. I guess it's probably another misc part that's not usable. But... I am curious to find out what I'm looking at.

There's no metallic contacts on the LCD itself. And no wiring. There was a black sponge (visible in the photos) sandwiched between the glass LCD and the board. If I tilt the LCD I can see very faint transparent markings in the glass that seem to have the same shape as the contacts on the board. On one side of the sponge I can also see contact markings, but on the other side nothing.

So, how does this work? I suppose the electrical current goes from the board to the display through the sponge... what exactly is that sponge made out of? Are there discrete conductors inside it, or is it one single conductive piece? And how does the current get to the LCD itself if it's all encased in glass, with no metal contacts anywhere?

The spongey thing is known as an "Elastomeric connector", and is made up of lots of essentially conductive lines. What that means is that if you compress it slightly and have it oriented correctly, it will only conduct in one direction. By having a long line of the stuff, you can connect multiple pcb pads to multiple non-solderable traces on a display such as the one you have. Each pad will connect to any pad on the display directly above it in the direction of the conductive material, but not to the adjacent pads as it won't conduct in that direction.

The display does have pads on it, but they are made of a transparent conductive material. I have found that it is possible to use conductive glue to attach wires to the displays rather than using the 'sponge'. You just have to be very careful with the quantity of glue you use and keeping the wires in alignment with the traces on the display.

Here is a good reference, with a picture of what the sponge looks like diagramatically:

From the 16 contacts on the board I suppose it's probably some kind of HD77480 variety.

Maybe so, but it's not some kind of HD44780 variety since those only have a 14-pin interface.


That display is just a passive lcd glass. With so few connections it 1:4 multiplex or more is possible, It has NO controller at all. You will need to generate all voltage levels yourself.

You could spend 20-30 hours to find out how to make it work, just for a single small lcd glass. I would not do it...

// Per.