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:

Nearly done:

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... 🙂

The most useful thing a computer controlled mill could do for me is make circuit boards. It's a nice thing to get started with, anyway. Here's the first one I milled:

Oh dear. Doesn't look very good. The mill was gouging too deeply into the board, but that's up to me to correct in the software. The more serious problems are backlash related. The circular pattern on the board has flat edges on the left and right where there's too much slop in the leadscrew mechanism. Tell the mill to move right 10mm, then left 10mm and the head should be exactly back where it started. If the nuts and leadscrews are too loose then some of the motion gets lost in the mechanism, leaving you with flat-sided circles, or cuts in the wrong places - you can see that the square pads near the bottom ought to be evenly spaced, but they're not.

I tightened up the mechanism a little bit and had another go:

Much better. Not perfect, but getting there. (Ignore the circular cutouts - they were there beforehand.) A few more tweaks and I finally managed to get rid of most of the backlash.

There's still a tiny bit of slop, but the error is small enough for me to start making more complex boards:

The nice thing about carving PCB designs with a mill rather than using the traditional acid-etching process, is that I can get the mill to cut the board out as well as just carving the pattern on the PCB:

If you're ever in a posh car or on an expensive motorbike, you'll sometimes notice that when you turn the ignition on, the speedo and rev counter dials do a quick self-calibration, moving their indicator needle all the way round the dial and back to zero again.

Instead of using a traditional meter mechanism (a simple coil and magnet), they use a tiny computer-controlled stepper motor. Here's one removed from a dial:

The metal shaft sticking out used to have a little plastic indicator needle on the end. Inside, they're more like a watch mechanism than a traditional stepper motor:

The tiny black cog in the middle is magnetic, and sits in the round gap in the metal frame just to its left.

Power up the two coils in the right sequence, and it drives the tiny cog round, in turn moving the other cogs which move the indicator needle.

These are more traditional stepper motors, though still very tiny:

I dug them out of a tiny camcorder. The shorter one controlled the focussing lens, while the longer one controlled the zoom. The little chip on the left is an Atmega168 - similar to the chip in an Arduino. With luck the chip'll have enough power to drive the motors directly. Not sure yet what I'm gonna get the motors to do, exactly, but whatever it is it'll be tiny and very cool. Yeah.