I’ve got a few of these little LED displays knocking about. They’re the same kind I used a couple of years ago on my Landrover speedo thingy. They’re not too tricky to use with a microcontroller, but you’ve got to connect at least half of the 26 legs on them to display anything meaningful.
It’d be a lot simpler, wiring-wise, if you could treat them like a serial display; then you’d only need 3 wires to bring them to life – power, ground and serial data. I realised that you could actually fit a surface-mount Atmega [Arduino] chip between the pins on the back if you were careful, so I designed a little “backpack” circuit that you can solder the display to, which handles all the various connections and leaves you just to give it power and data to display.
The schematic isn’t particularly interesting (email me if you want it) but it was simple enough to lay out. I’m starting to find that when designing boards to go in tight spaces, it’s best to be flexible with which pins connect to what; go with what’s easiest to layout, then you can untangle it all in the software. Here it definitely made sense to work out which Atmega pins were going to end up nearest to the display pins first – so you end up deriving the circuit diagram from the layout, rather than the other way round.
Action shots: first, milling the board:
And roughly chopped out with the bandsaw:
Components and display soldered on – holding the circuit up to the light makes it much easier to see if you’ve accidentally bridged solder over two pads:
It’s pleasingly slim from the side 🙂
I programmed the Atmega with my little spring-loaded ISP clip thingy and tried sending a word for it to display:
The camera doesn’t do it justice – it’s bright and contrasty in real life. With only three connections needed now I can build the displays into things without having to worry about getting a ribbon cable in there as well now… Plus I can always program scrolling messages etc straight into the backpack itself – instant light-up geek badges, just add a battery… 🙂