I am a member of the local Round Table and each Christmas we take Santa and his sleigh to the streets of Shirley collecting money for local charity's and give children a chance to meet Santa. This year with Covid restrictions we decided to offer the ability to donate by txt message or online but needed a means to advertise this while driving down the road.

At first we considered just a standard A frame sign on the roof rack but decided something more eye catching was in order. After a bit of discussion we decided a scrolling text sign mount to either side would be ideal but quickly ruled out purchasing one of a suitable size to be read at a distance on the grounds of cost.

This left us with the only option to make them (to be honest I was not to upset as it sounded like a fun project). I immediately thought of using the 75hub panels salvaged LED panels from Project MC2 Pixel Purse's, have used them for several escape room props in the past but needing 24 this seemed an extreme option so I first look for other purchasing option. I soon realised I had left it a bit late to order LED panels from Aliexpress and deciding they were just too expensive elsewhere I accepted that stripping down 24 hand bag was in my future.

Other design requirements were it should be water proof, run on 12v from the cigarette lighter socket and be able to be mounted to a standard roof rack.

I will own up some of the pictures were taken out of sequence or later when I disassembled the signs as I was rushing at the time I built it.

Supplies

These are the supplies required to build one sign although I build a pair so it could seen from either side of the street.

12 x 75hub LED panels (in my case salvaged from Project MC2 Pixel Purse's)

11 x 16 Way SMT IDC Sockets

1 x 12v to 5v Buck Converter

1 x Raspberry Pi

1 x Adafruit RGB Matrix Bonnet for Raspberry Pi

1 x SDcard

12 x 16 Ways IDC ribbon cables

Power Wire

Wago Connector

Power Connector

Crimps

Waterproof Box

Cable Gland

Polycarbonate

Plywood

Pine

Screws

Silicon Sealant

Foam tape

Waterproof Butyl Rubber Aluminium Foil Tape

Step 1: Salvaging the Panels

Having decided to salvage panels from the pixel purses, (all 24 of them) it was time to stick some music on and start stripping them down. I really didn't realise how long this would take, just taking them out the boxes and flat packing the boxes for recycling took half a day and resulted in an awful lot of cardboard!

Removing the panels was simply a matter of removing a few screws, unplugging the controller and snipping the power wire from the pcb. One big advantage of using salvaged panels is that they have a really nice defuser that I have not seen for sale elsewhere (you just need to file the tabs off so they can sit side by side.

Not of use for this project but I also ended up with 96 AA batteries which I am sure will come in useful at some point.

Step 2: Adding the Output Connector

The one big disadvantage of using salvaged panels is the don't have the output connector for daisy chaining. Soldering on these connectors was a pretty simple task but once again with so many to do took longer than I expected.

Step 3: Power

First stage was to make up a power harness to both power the panels and the Pi / bonnets.

For the sake of expediency and easy of disassembly I created individual tails and then linked them together with Wago connectors.These were connected to the power output from the bonnet.

The bonnets and pi's are power from a 12v to 5v buck converter that I had intended to plug into the socket on the bonnet but due to an oversight I ordered too small a box for them to fit into so had to desolder the connector directly solder the wires in place. This had the added advantage of making it impossible for the connector to vibrate loose.

The power itself was taken from the rear cigarette lighter socket via a small junction box and switch.

Step 4: The Code

As we used the Adafruit RGB Matrix Bonnet we just used the The LED-matrix library from Henner Zeller like they did. Rather than repeat it, all be it poorly I would recommend you look at the Adafruit article for a how to.

https://learn.adafruit.com/adafruit-rgb-matrix-bon...

Step 5: Prototype Assembly and Testing

With all this completed I setup a 8 panel prototype, it worked but there was a problem. I had hoped this would be enough but soon realised that more panels would be needed for the sign to be easy to read at the required distance. I decided that I would go for 12, the limit of what the budget could allow.

Step 6: Making a Waterproof Enclosure

The biggest challenge of making the enclosures was ensuring they are water tight but could still be serviced and the panels removed afterwards for use on future project but be returned to use for next year.

The overall dimensions of the sign where limited my the size of the polycarbonate sheet (1200 x 600) from which I needed to build two signs. Ideally I would of used a slightly larger sheet as it was slightly shorter that ideal for 12 panel sign but it was close enough to get away with.

A frame was made of pine the sized to fit 12 panels side by side. At this point I painted the frame all round to help the prevention of water ingress. The polycarbonate was cut to size and a series of holes drilled around the outside. A generous bead of silicone was applied and the panel screwed in place being careful not to over tighten the screws.

The rear cover was made made from some left over phenolic ply from a previous project. As I wanted to be able to remove this in the future rather than bed it on with silicone I use foam draft excluder tape to form a gasket.

The Pi and bonnet sat in small waterproof box I attached the the rear of each sign so the could be access easily without needing to disassemble the whole thing.

The whole sign was raised off a mounting board by a couple of wooden blocks, rather stupidly in a rush I didnt pilot drill then and they split, they did however hold up for the two weeks they where in use.

Step 7: Assembly

Assembly is simply a matter placing the panels in the frame, and screwing them in place using two clamping strips. Make sure the arrows on the panels all line up and daisy chain the panels using the IDC cables making sure you daisy chain the in the direction of the arrows.

Once thats done you just need to add the extended power cables built earlier, seal up and you are ready for a final test.

At this point I got a little nervous that the silicone sealant and foam tape were not going to be sufficient to water proof the enclosures so I added rubber tape.

Step 8: Final Test

I this point it was pretty much completed so was time to test, I set a current limit on the power supply just incase I have made a mistake somewhere, but surprise surprise it just worked first time! I would point out that the power consumption was much lower than I expected and a 10A buck converter was massive overkill!

Step 9: In Place on the Vehicle

The signs were mounted to the roof rack using a U bolts designed to mount a roof box. This meant they could easily be removed for repair or maintenance if need be. Taking a picture of the signs on proved a little difficult but you get the idea!

Step 10: Future Changes & Improvements

All in all I am pretty happy with how it turned out, the signs operated just fine for the two weeks they were in use with one exception. On one of the nights the SD card ejected from the pi when we went over a speed bump. I am not sure if this was because it was not inserted properly in the first place or just bad luck but in future I would probably add a drop of hot melt glue to make sure that didnt happen again.

Improvements wise it has been suggested that maybe a way of changing the the displayed text on the fly might be a good idea, possibly showing when a donation is made. I will probably add this before next years sleigh run.