Flipdot Display!
About a month ago JonW sent me a link to flipdot display on eBay. We decided to put an offer on it and go half, and got it for (I think) £90ish. On the drive down to pick up the display, we ended up coming back with another flip dot display (quite a bit smaller) and four LED matrix displays - so six displays in total! I managed to get it running pretty quickly from my laptop with an RS485 adapter and pyflipdot, but wanted something a bit more permanent for it at the hackspace.
The display itself wants 24v and thanks to AJ's stockpiling of bad 3D printers we have a nice surplus of 24v supplies. That led me to wanting to put a DC-DC converter on board. In the end, the board does the following:
- R7FA4M MCU, so that it's compatible with the Arduino Uno R4.
- MAX485E chip for talking to the display via RS485.
- A small transistor and mosfet circuit to control the display's lamp brightness.
- An input for an LDR to measure the ambient lighting.
- A WizNet W5500 for Ethernet.
- A buck converter circuit to step the 24v down to 5v.
After my issues with the W5500 controller on the PoE board, I added a BS138 mosfet to level shift the 3.3v from the ethernet controller to 5v for the Arduino. The WizNet chip is 5v tolerant, but 3.3v can be seen as "low" if the MCU is running at a tad above 5v. Interestingly the PoE board was the first time I'd had this problem, despite using the W5500 on a number of boards.
The RS485 stuff is pretty simple so I wasn't too worried about that, and I was able to breadboard the lamp control pretty easily to pick values and get something working well. The annoying part was the buck regulator circuit. There were some lessons learnt…
On the first version of my board, I used electrolytic capacitors to filter the output of the inductor. That was pretty stupid because of the high frequencies involved. This was slightly annoying because I knew this and didn't think it through properly while designing the board. I also had a very common reference schematic, because it's the same regulator chip used on the Uno R4 boards. I blew up about 10 of these chips before getting something that worked. On V2 I used multilayer ceramic caps - as I should have done all along. Hopefully it doesn't blow up again.
As a side, the failure mode of the chip was quite fun. It would form a dead short and get so hot it'd melt the solder. This was quite useful for removing the broken component, as I could pick it off with tweezers.
Anyway, without further ado, here's the display.
Also, since I went to a panthers match today it was good timing for a very panthers-photo.
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Comments and feedback are welcome by email (aaron@nospam-aaronsplace.co.uk).