Category Archives: Uncategorized

h’s Laser Dymo Clock

A quickie project. Two stepper motors, a UV laser, some luminous paper, an Arduino, a couple of ULN2803 drivers for the motors, and a “daisy wheel” printed onto transparency film.

After playing with some luminous paper and a laser pointer (UV ones work best – red ones don’t work at all), I thought of making a little laser display board thing. I thought about using galvos to draw the characters on, but the mechanics of it would have turned it into a bigger project than I’ve got time for at the moment, so this mini-projector approach will have to do.

Next step is to reduce all the electronics onto a single PCB to mount on the back of the wooden board…

Other thoughts:

  • There’s a tradeoff between how bright you can make the text, versus how often you want to update the display. You can make the text brighter and last longer by lluminating it for longer, or, as this clock does, by going back over the same text a few times, but if you then want to write new text over the old, you’re still left with the ghost of what was there before
  • There’s lots of things you could use this idea for. Because the text starts fading as soon as it’s written, it’s ideal for showing any ephemeral data. A clock was obvious, but a continuous stream of headlines from an RSS feed could be cool, too
  • You could stick a large sheet of luminous paper on a wall (possibly in a frame), and then have a laser write the messages or draw a clock face and hands from the other side of the room
  • Because the paper doesn’t respond to many wavelengths of light other than the blue/UV end of the scale, there’s nothing to stop you using red and green LEDs to illuminate the workings a bit better
  • Rather than using a disc of paper to write on, you could stick lots of strips of luminous paper together to form a long band, and run it round pullies for a more ticker-tapey effect

Intelligent LED display backpack

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… 🙂

Clamping…

Up until now I’d been using scrap bits of metal to act as clamps, holding my PCBs down to be milled:

Not exactly convenient. I needed a better way to fix workpieces down for the mill to work on, but you never know what sized bits you’re going to be dealing with in future, so I thought I’d make a perforated work bed. I bought a chunk of white acetal (an engineering plastic) and stuck a bit of graph paper to it, then used it as a guide to drill holes at regular intervals.

I used a countersink bit to take the sharp edges off the holes, then used an M5 tap to thread them all.

Finally, I designed some little clamps and cut them out of black acetal.

Using them I can easily clamp down just about any size of PCB to the bed for the mill to work on, plus it’s much easier to clamp other flat things down too. Here’s a bit of clear acrylic being worked on:

Total cost: just under £10 for the plastic plus a handful of M5 bolts. Nice.

New mill – cutting circuit boards

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:

Tiny stepper motors

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.

LED Boost driver

A friend found a stack of discarded LED assemblies in a skip, and threw them my way. Amongst the other bits, there were several Dialight Lumiled strips – nicely machined aluminium backplates with 6 bright white 1-Watt LEDs mounted on top. They’re designed to be run from a proprietary power supply that delivers around 19.2 volts at a tightly controlled 350mA. So, not the sort of thing you can wire up to a car battery.

I figures this was as good a time as any to start embarking on analogue electronics – gotta dip your toe in the water at some stage. So, after several hours of wading through various “solutions” on Farnell’s website, I came across a tiny little chip that claims to be designed specifically for driving blocks of 6 LEDs like this. The datasheet showed an example circuit along with the specifications of the extra components needed, so I ordered the bits and designed a little board.

It’s a boost driver, which means it can produce a higher voltage output than the voltage you put in. Power in and out are roughly the same, though; so it takes more current input than the current it can provide at the output.

I designed the PCB in Eagle, and used a free plugin/script to convert the pattern into G-code for my mill to understand. Gotta write more about my mill another time – for me, this is where it shines. It took about 5 minutes to carve out this little circuit, much much faster than etching a board in acid, the old way.

Soldering the components on was a little bit fiddly but doable. I find it helps, when soldering multi-legged components like the little SOT3-6 driver chip, to wet all the pads on the board with solder first, using solder wick to remove all but the thinnest little layer.

The board worked first time. Which shouldn’t have been surprising, given that it was the manufacturer’s design, but I never seem to imagine things will go that easily… And the little driver circuit is small enough to mount inside the lights’ cases, too, so I’m going to have to make up a batch of them.

Solar power for emergency use

I’ve had lots of questions from friends about how to set up their own emergency solar system, so that if there’s a power cut / apocalypse / etc they’ll still be able to have lights, radios and phones working. So I’ve put up a page with FAQs and a few example solar setups, from around £250. Cheaper if you already have a car battery lying around – the bare bones is really only about £150. It’s all stuff bought from eBay, and it’s a tiny fraction of what most people charge for a solar starter kit. Check out http://www.shopwiki.co.uk/l/Solar-Technology-80W-Solar-Panel-Kit—Starter – even Maplin are trying to charge £600 for a £110 panel plus a £20 charge controller. And there are shops asking for upward of £800, relying on the fact that people don’t understand how simple they are to use.

So, do it yourself! Don’t get ripped off by buying a kit (unless you buy it from me, in which case why not buy two?).

Check out http://howiem.com/solar for more info. Any questions, drop me a line and I’ll add it to the page.