LED Message Marquee

I came into possession of an LED message marquee in a state of disassembly. I have two of the modules pictured to the right. I have no idea how they work. In this post I'm going to figure it out!

Backside of one of the modules.
The high current and data traces are clearly defined.
Note at the top the barely visible pins and
the hand soldered jumper at the bottom.

So to start out, lets look at the pins. There are 16 pins connected to various traces. The pins are about halfway down the board. We can pretty clearly identify data and high current busses.

Pin	Signal/Power	Notes
1	Signal	This doesn't appear to connect to anything. It just runs from one end of the PCB to the other.
2	Signal	This is tied to pin 10 using an external hand soldered jumper wire. Very suspicious.
3	Power	Tied to one of the pins on the LED modules.
4	Power	Also tied to LED module pin.
5	Power	Also tied to LED module pin.
6	Power	Also tied to LED module pin.
7	Signal	Routed to half the chips. Possibly an enable or clock
8	Power	Also routed to half the chips. VCC or GND
9	Power	Tied to pin 8
10	Signal	See pin 2. This leads into one IC and terminates. This IC is then connected to the next IC down the line, and so on. My guess these are shift registers and this is the data line.
11	Power	Connects to every IC on the board. VCC or GND
12	Power	Tied to 11.
13	Power	Tied to LED module pin.
14	Power	Tied to LED module pin.
15	Power	Tied to LED module pin.
16	Signal	Similar to 1.

So based on this, I can probably assume a few things. This is a serial in LED message marquee. There are 7 power traces connected to each LED module. So 3, 4, 5, 6, 13, 14, and 15 clearly control which row is active. Shift one row of data in, turn one LED row on for a fraction of a second, turn off the LED row off, shift in the next row, then turn the next LED row on, repeat! Pin 2 is somewhat puzzling; If pin 10 possibly carries data, what is pin 2 doing?

Based on just looking around I could probably take a couple of guesses and get it working, however I have some concerns. If I reverse these pins it could probably draw huge current and let the smoke out of something. Lets take one apart and see if we can refine what we're doing a little better!

I haven't desoldered anything before. I tried using some wicking solder braid, but I couldn't get it to flow reliably. I ended up using a hot air gun to heat the solder and a vacuum solder removal tool to remove the solder. Suffice it to say, I butchered this board pretty badly.

The peeling solder mask betrays my skill

Table breaking definition

So there are two different types of ICs, and some real beefy resistors. One is a darlington array, and the other is a shift register. Pretty close to what I was expecting! My guess is these modules are pretty old. There are LED driver ICs available that will do all of this in a small SMD package. At least 10 years?

Pin	Purpose
1	Neighbor data carrier. See 2
2	Data input; Data you want to have appear at the start of the display goes onto this line. This is interesting. All of the shift registers are connected in a way that the entire display is one big shift register and this connects to the start of the whole register. This also explains pin 1 and 16; Those lines can carry data for other optional modules. I like it!
3	LED Anode. This is going to be interesting. I need to be able to source large amounts of current for each row. I don't have enough mosfets to do this, and the darlington arrays I have only sink current. =(
4	LED Anode
5	LED Anode
6	LED Anode
7	Clock
8	Ground
9	Tied to pin 8
10	Data. Due to where this is broken out, putting your data on this line will cause it to start halfway down the display. While this pin is broken out, it was never be driven directly!
11	VCC
12	Tied to 11
13	LED Anode
14	LED Anode
15	LED Anode
16	Neighbor data carrier. See 2

So based on this pinout, I was able to get it to display some stuff.  As I mentioned, I don't have enough components to actually run the entire display yet. In the photo below I've shifted some data in and hardwired pin 15 to VCC. I'm running this at 5V and its pulling just below one amp.

This is actually the second module I didn't disassemble; pin 1 and 2 are swapped

I've ordered additional components from Digikey, so I should be able switch the various row controls on and off. Once I get my parts I'll write another blog post about actually displaying something useful.

I'm not sure what I want to use these for. I think it would be really cool to build a GPS clock! It would give me practice laying out a PCB and I could probably put some SMD components on there to improve my soldering skills.