[solved] how does this 4 item LCD works?

Other Hardware Displays
1

Hello,
I have a LCD that's being controlled with some unknown microcontroller. The LCD displays 4 different icons, and there are 5 pins. I assumed that it operates on a simple discrete way (1 pin for ground, and putting a positive voltage on the 4 other pins displays the 4 other items.

But that doesn't seem to be the case. Regardless of what icon is being displayed, all 5 pins are at 1.5V when I measure with a voltmeter. When I scope it out, all 5 pins have a square waveform that's 2.8V peak to 0V, with a frequency of 40Hz. I'd like to reverse engineer this LCD so I can figure out what the microcontroller is displaying on it, but I'm not sure how to go about doing that.

2

Ah, so you have just discovered how a LCD works!

It will be damaged if you apply DC to it.

The driver is essentially an array of XOR gates with the backplane feeding one input of each. When the other input is HIGH, the output is the inverse waveform of the backplane and activates the segment. When the segment is driven with the same square wave as the backplane, the segment is off.

If you want to decode this, you need either XOR gates, or since it is only 40 Hz, use an Arduino to decode the XOR function.

3

Thanks! That was pretty naive of me!