I've got a clock display I just salvaged... I can't find any datasheet because it's off a cheap off-brand clock. It's labeled "Longtek LY-6055L-S". It's got 30 wires in a few ribbon cables out the bottom. Does the average display simply have 1 power wire and the rest turn on the lights when they're grounded? Or 1 ground and the rest power? How's this thing work? I'd like to just use it to display a number or two, nothing complicated. 
Thanks!
It would help if you posted a picture, I don't think there is enough information without.
you can check it out here:
Did it glow green?
It looks like an LED seven segment display.
These normally have a common anode (sometimes cathode) for each digit and then a common wire to each segment.
Take the 5V from your Arduino, place a 500 ohm resistor in series and then apply that and ground to various combinations of contacts at random to see what lights up when.
yes, it's green. So it's not an lcd? I'll poke around at it and see what I can do... Anyone have a trick for stripping ribbon cables?
Here's a sketch of what it's able to do on top, and what's it's really doing on the bottom.
What I did was find the wire that needs to be grounded, (the one on the far left) and then I grounded that and attached a wire to 3v on the arduino (5v's too much). Running the wire along all the contacts one by one, the only ones that light up are a few dots and "t7t". The weird part is that I got all of them to light up one by one the one time, and now it's not working all the way. Any ideas?
5v's too much
Agreed. That is what the 500 ohm resistor is all about.
Possibly 3.3V is also too high, depending on the LED. That resistor is important.
You will likely find that there is not just one wire that needs grounding, but one for each digit. The last one on the left maybe controls the extra dots. Maybe the next one in is for the first digit.
nope, I've had got a 470 ohm resistor in the power line the whole time, unless that's not enough?
If you're messing around feeding the supplies into the LED module with no resistor, you are also undoubtedly shorting the supplies in some combinations, and that will not be particularly healthy for the voltage regulator.
each segment meaning EACH number? It's odd that this number of them are working then if only one wire is grounded...
each segment meaning EACH number?
No a segment is a bar, there are seven of them that make up a digit (number). Each digit is likely to have it's own common connector where as segments are wired up common across all digits.
Your LED module has 4 DIGITs plus some other symbols.
Each DIGIT is made of 7 SEGMENTs. The dots and things are also separate SEGMENTs.
You will likely find that there is a "ground" pin for each DIGIT and a "power" pin for each SEGMENT. (possibly vice versa)
To make it work, you have to ground and power various pins in combination.
so, each digit has it's own common ground, and then every segment has it's own anode? Why is it lighting up in this pattern if only one is grounded?
plus I have another similar clock display. What would be the correct method of determining pins this time?
so it won't damage the thing any more to put the 3.3v with 470 ohms on every combination? Reverse polarity or shorting out the two while experimenting won't damage arduino?
alright, I'll go give this all a try and see what I can do 
well, I DID try the 5V with no resistor a while ago, but didn't leave it on there for even a few seconds. The led that was supposed to glow green had glowed yellow, seemingly under strain. Therefore, I deduced that unprotected 5v was too much. But now I just found out that the resistor I was using was defective! (who knew?) so I replaced it with an identical one, and just now went back to testing. There seem to be 3 or 4 grounds, and I found the ground that needs to be connected to have a matching "7:t7" to fill in the "t:7t". A little more probing around should reveal what grounds the dots each use. I'm going to draw a diagram of it so I don't forget what's what :
Thanks for the walkthrough!
almost done decoding this cryptic thing I'll post my findings and see if it makes any sense.
here's another sketch:
I made a list of what I found so far with the 30 pins...
"nothing attached" means that there's not lead from it to do anything.
"ground #x" means that hooking that to ground and poking around lights things up.
"gnd x - [letter] light" means when the pin stated is grounded and power is put through the pin, the letter (on the picture) lights up.
Pin 1: ground #1
Pin 2:
Pin 3:
Pin 4:
Pin 5: gnd 1 – A light
Pin 6:
Pin 7:
Pin 8:
Pin 9: gnd 1 – G light
Pin 10: gnd 1- J light
Pin 11: nothing attached
Pin 12: gnd 1 – K light
Pin 13: gnd 1 – E light
Pin 14: nothing attached
Pin 15: gnd 1 – R light
Pin 16: gnd 1 – O light
Pin 17: gnd 1 – Q light
Pin 18: gnd 1 – W light
Pin 19: gnd 1 – Z light
Pin 20: gnd 1 – AA light
Pin 21: nothing attached
Pin 22: nothing attached
Pin 23: nothing attached
Pin 24: nothing attached
Pin 25: nothing attached
Pin 26:
Pin 27: gnd 3 – BB light
Pin 28: ground #3
Pin 29: ground #2 – same as gnd 1
Pin 30: gnd 1 – L and M light
Lights missing: B, C, D, F, I, H, N, S, P, T, Y, V, X
I'm trying to fill in the blanks, need to solder some wires in to be able to easily test the remaining ones. I know that they lit with some combination just a minute ago, but I can't find it now : Is it just me, or is there almost no pattern to this thing's activity?