componentslcd
I took apart a broken iHome ip90. I now have the lcd screen, but I want to figure out how to use it in a project or something. I am used to getting nice instructions and tutorials, but obviously it's difficult to find something like that for this LCD. Is there a systematic way (without breaking my arduino) I can use to figure out which pin is which (it has a female bunched up jumper wire)?
Just trying to figure out how I can start using old electronic parts.
You identified the LCD as being multiplexed—which it very likely is—so the AC you're seeing is probably just some segment being sequenced.
A microcontroller toggling pins in succession will indeed produce AC, however calling "x V AC" that is an extreme red herring. It is better described as a multiplexed signal with such a frequency and some voltage (amplitude, not AC voltage as measured by a meter).
Atmel's app note AVR340: Direct Driving of LCD Using General Purpose IO may be of use to you. This portion describes the "AC" that you're seeing and shows why.
To energize a segment, the waveform to that segment must be 180 degrees out of phase with its COM waveform. The voltage difference between the segment and COM signals will therefore be typically 5 Volts AC, which causes the segment to become visible after 300-400 msec of refresh cycles.
To de-energize (turn off) a segment, the COM and segment waveforms should be in phase with each other while that segment’s COM is active, that is, not at 2.5 volts. So, a LCD with all segments OFF will have the segment inputs IN PHASE with each COM input. (COM3 was omitted from Figure 2-2, since only 4 channels were available.)
Based upon the labeling, I would guess that the LCD glass has a total of 27 segments which are multiplexed in a 3x9 arrangement. The easiest way to produce a map is probably to temporarily wire all 12 LCD wires to CPU output pins, and then write a program that will alternately write (011111111111) and (100000000000) to the display (at 1kHz or so) until you've observed what segments are connected to the first pin, then (101111111111) and (010000000000) until you've mapped out what segments are connected to the second pin, etc. When driving the display, you must make certain that amount of time the outputs spend in any state is about the same as the amount of time they spend in the all-bits-complemented state; failure to do this may cause temporary damage to the display within seconds [meaning the display will look "odd" for awhile, but may eventually return to normal], and destroy it within minutes.
Best Answer
LCD's are almost always custom, with no datasheets available. This is especially true of LCD's that you find in consumer electronics gear. If you can't find the information you need with a quick Google search then I'd give up while you're ahead.
I choose to never use any LCD that I don't have the complete datasheet for (not just the datasheet for the controller chip on/in the LCD)-- and I've used a lot of LCDs! While it is possible to reverse engineer the LCD and figure out how to use it, it is often very time consuming and not worth the effort/bother/etc.
Sorry for the pessimistic outlook, but I've learned this the hard way and hate to see others repeat it.