Do not throw away!!

So, I was pulling apart an old car CD-changer unit (as you do), when my eye was caught by the printing on a tiny bit of circuit board…

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I wonder how many of these little scrap-looking PCBs were chucked out by accident before someone at the factory realised they were going to have to do something about it:

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Tee hee.

Hacking an old RAF Nimrod control panel

Bought one of these things off eBay for around £35:

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Don’t know what it’s for. Probably some sort of comms / radio display. The reason I jumped on it – behind that dark window are 8 of these alphanumeric LED displays:

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These little displays are usually around the £15-20 each. Loads of LEDs:

Loads and loads of tiny LEDs...

And a little controller chip on the back of them:

... and on the back, under a little crystal window, a simple controller chip.

Little but not simple (!) – check out all the gubbins [that’s a technical term] in there:

OK, not that simple a controller chip.

Before trying to get the whole panel working, I hooked up an Arduino to a single display to get it working:

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Once you’ve sussed out how to get one display working, figuring out what the connections to a full display panel should be is a lot easier, even if it doesn’t look it:

Too. Many. Connections. Needed.

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Got it working!

Getting an expensive piece of ex-RAF equipment to say naughty words is the sort of thing that gets me out of bed in the morning.

Now that I’ve got the thing working, I’m not sure what I want to use it for. Maybe a new trip computer for the Landrover… we’ll see.

Chinese 12V 6800mAh Li-Po battery mini review

Lots of these Li-Po battery packs on eBay, going for around £12 from various vendors:

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Nice hand-sized battery pack, with a coax plug and socket on flying leads, and a handy little power switch and LED. Comes with a somewhat underspecced charger (350mA – so, around 20 hours to charge).

The battery specs seem too good to be true, though. 6800mAh at 12V? Nearly 7Ah at a fraction of the size of an old fashioned gel battery. Hmm.


To test the battery capacity, I hooked it up to a little LCD telly via a Turnigy power analyser.

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The TV takes about 0.3A, so it ought to run for about 20 hours. Ha! To start the test, I charged the battery fully with the supplied charger:

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12.77V is a little high, but it’s straight off the charger; it’ll relax back to around 12.6V.

About 6 hours later, it was time to switch it off before the battery flattened itself permanently (10.8V is as low as you should go):

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So. Not really 6.8Ah at all, more like a third of that. I think I know what they’ve done, though (assuming they aren’t just out and out dishonest): to create a nominally 12 volt battery, they’ve strung 3 cells together in series – but then they’ve mistakenly added the Ah capacities of the cells together. Connect 3 cells in series, you add the voltages, but Ah stays the same as a single cell; connect them in parallel instead, and you add the Ah up but the voltage stay the same as a single cell. They’ve mixed it up.

The headline, therefore: these battery packs only have a third of their marked capacity. Caveat emptor etc.

The upside (!) is that the little charger that comes with it takes half the time to charge it.

UPDATE: A kind commenter, Unferium, notes below that Li-Po cells can be safely discharged to about 3V per cell rather than the 3.6V I used, which does change the results slightly. The voltage drops off very fast from 10.8V down, so you only get an extra 0.25Ah out of the pack. I think the headline stands 🙂


Anyway, let’s see what’s inside. Note: Lithium batteries can explode or burst into flames if mishandled, or if they’re faulty. Do not take one apart unless you know what you’re doing, or you’re an idiot. Thankfully, I’m fully qualified in at least one of those categories.

First, off comes the outer blue shrink-wrap:

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… revealing a stiff cardboard “case”. It comes apart easily to reveal:

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The actual battery of cells is just the silver chunk; the dark strip on the left is just dense packing foam. Shame they couldn’t just make the whole pack a bit smaller instead – I can’t see what benefit that padding does given that it’s only on one side of the cells. Even if something bad happens to the battery and the cells start expanding, they’ll blow up like a pillow, not out sideways. Ho hum.

Let’s pull the cells out:

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Yup, three Li-Po cells in series. Thankfully they’ve each got their own protection circuit (which disconnects the cells before they get overcharged, or so discharged they’re unsafe to charge again):

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The 3 cells are extremely securely glued together, so don’t try pulling them apart. If the foil envelope around a cell is punctured, they give off a strangely fruity smell and need to be disposed of safely in a neighbour’s bin. (Not the nice neighbours, the ones on the other side)

In use:

Despite their misleading label, they’re still useful battery packs, particularly for Arduino / microcontroller use. 2.2Ah at 12V is still plenty of juice for some projects. Here’s a little wireless monitor I rustled up to monitor our solar panels:

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It’s just an LCD, an Arduino and an nRF24L01+ radio module, all cable-tied to the battery:

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Lasts for a couple of days between charges, and it’s surprisingly robust. Easy to recharge with the charger that came with the battery (albeit a bit slow).

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They’re not as good as they say they are, but they’re still handy.


Stranded wires often need to go into screw terminals:

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But they’re not very secure connections, and you have to be careful that you don’t get stray strands of wire sticking out.

The answer to this extremely common problem that blights our civilisation?

Ferrules. Mmmm.

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So the next time you have to stick a stranded wire in a screw terminal:

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… slip a ferrule on the end of the wire:

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… squish it up with the nearest tool to hand:

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Here, I’m using some crimpers, but you can use pliers, teeth, an anvil – whatever’s to hand. Go wild.

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Now that’s a nicely terminated wire, if you know what I mean. You can snip the end off the ferrule with snips if it’s too long for your terminal.

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Look at that. Isn’t that better? Strain relief, no danger of stray strands of wire causing short circuits that you spend an hour looking for; it looks neater… too many advantages to list.

To sum up: Ferrules. Oh yeah. Available wherever ferrules are sold.


Razer Hydra base station teardown

razer-hydra-gallery-3 Just got hold of a Razer Hydra, a 3D motion controller system aimed at gamers. There’s a base station with a glowing green ball on the top that needs to sit directly in front of you, and two handheld controllers with buttons and joysticks on them. They constantly feed back their orientation and position to the computer, so you can wave them in the air or twist and turn them, and objects on the screen follow along. I’m not into them as games controllers (prefer the ole mouse and WASD meself) but with a bit of hacking and help I’m hoping to use them as motion controllers for my graphics work. Record my motions as I manipulate the controller and apply them to, say, a character’s head on screen. Quick and expressive way to animate secondary characters in animations. Nothing new, but this is dirt cheap – £80! – so well worth a punt, and not the end of the world if I can’t get it working.

The base station is way too inconvenient (and has too many wires attached) to be sat there in the middle of your desk all day. That’s precious real estate, and I’ve got a system for my various keyboards where I can slide ’em in and out under my monitor stand, and that round thing just doesn’t fit.

The base station seems to work just as well upside-down, though, so I’m going to try sticking mine under my desk. (The base station uses a magnetic field to sense where the controllers are, so this trick won’t work with metal desks, and possibly not with very thick wooden ones, but mine seems to work OK. So far.)

Damn thing is too tall, though,10cm-ish, and I’d keep knocking it with my knees. So let’s void its warranty.

Even knocking a few centimetres off would help; I know the coils that let it do its magic are housed in the black ball on the top, and I’m hoping we can get that off and mount it to the side of whatever’s in the bottom section. Slim the whole thing down.

Ahh yes, the old hide-the-screws-under-the-rubber-feet thing.

Continue reading “Razer Hydra base station teardown”

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… 🙂

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: