specialising in real-time, real-life robotic animation

"Let us startle, astound, amuse and inform our fellow human travellers."

Articles.. more details of projects.

These articles are for general interest, and there may be inaccuracies, spelling errors and mis-saids included. Anything is possible. I am happy to discuss the content with you.

A system for charging 100 iPods at a time for the Arts Festival (back to Projects)

100 iPod charger

This project was one of two electronic challenges put to me for the Adelaide Festival of Arts, 2012. A show required that 200 G4 iPods be charged each day.

The system needed to avoid ganging together a myriad of USB hubs connected to many USB ports on a number of computers. Early in my research I discovered that there are many possible ways to convince a USB powered device to begin charging, and that the rate of charge could vary a lot depending on what the device "saw" when connected to a USB port.

Initial trials with an iPod showed that I could get a maximum charge of about 50mA when tying both the data lines to the 5V line using 10k resistors. Many variations of tying data lines high and low, using resistor ladders to supply varying voltages to the data lines were discussed on the www. I couldn't be sure yet how best to get the maximum charge rate to the iPod. Time was short, as I needed to get the pcb layout off to the manufacturers quickly to be sure of delivering the project on time.

I used to hand-build projects like this up to a few years ago, but no longer. It is so much quicker to populate a pcb (if you have the experience to design in all the versatility you might need), and it makes trouble-shooting so much easier. Also, you might end up with a pcb you can use for other projects. If the project can't afford a manufactured pcb, I won't do it (unless it is really small!).

When designing the pcb I allowed for all possible configurations of resistor layout for each of the USB connectors, as well a variable resistor pot for each of the two commoned data lines, as I wasn't sure at that stage what would be be best for this project, and anyway I wanted to build in flexibility for the future, when some other device might be connected for charging purposes.

So I designed the pcb so that 10 iPods could be connected to one pcb, which measures 200mm long by 30mm wide. I could fit two of these into a box. As you can see in the picture, each box could accommodate 20 iPods, 5 boxes and a black power supply box were required to charge 100 iPods at a time. That was all the budget could support. I think the biggest mistake I made with the production of the pcb was that although I had asked for ten USB A connectors spread along the length of the pcb, there were 12 placed there. They fitted in, but there is very little space between them. They will accommodate iPad USB leads, but nothing wider. I should have had two connectors taken out of the design to provide more room between connectors. Oh well, there's always version two. So much of this work is only ever a prototype. Albeit, one that has to meet it's requirements.

The power supply is a 5V switchmode unit. It needed very good regulation with a nice, clean, stable 5V so as to not confuse the iPods, because they are talking to it, after all. I couldn't get them to charge at more than about 50mA, but that was OK, as they would charge from flat in a little over two hours. So with 50mA per iPod, and ten iPods to each pcb, the power rails needed to carry 500mA. No problem. With two pcb's in a box, each box required 5V at 1Amp. OK. So the five boxes required a power supply to deliver a good, solid 5V at 5Amp. I gave the system plenty of headroom and went for a 20Amp supply, which wasn't much more expensive than a smaller unit, and I had to order it before I was quite sure what the maximum current draw would be, so that I could be confident all would operate satisfactorily.

Which it did.

I can be confident in that, as I didn't hear a squeak from the customer after delivery. And that's a sure sign. However, if there are any developments, I'll let you know.

And if you are interested, I have a very pretty little pcb that in fact can accommodate 12 USB A connectors (through hole), and any number of SMD resistor pads for you to play with.

Fixing a fake Taser for the Arts Festival (back to Projects)

12V strobe

The phone call went something like this.. "A foreign theatre company has a fake taser in the show, and it doesn't work. We need it tomorrow night for the Dress Rehearal. Can you fix it?" "Who knows?" says me. "I'll try, if I'm your best bet." The reply came, "Yes please."

The first step, I figure, is get my hands on the device and have a good look at it. It is an aluminium tube about 300mm long and 50mm in diameter. With a red button halfway down it's length. One end is stuffed with two AA battery packs, each adding up to 12V, and the other end has a disk with four glass tubes protruding from it. There are two little black boxes connected to the glass tubes, and to the batteries and switch. About a metre of a four wire loom is stuffed into a tangle in that half of the aluminium tube.

The glass tubes are in pairs. So it looks as though maybe a spark might jump between the two tubes. Maybe. I didn't know what the tubes were. LEDs, I know a great deal about, and digital circuits. Not much about this stuff. So on the way back to my studio, I drop into the local electronics shop, and ask what the glass tubes might be. A smart young fellow says immediately that they are strobe lights. Thanks, I say.. and do you sell them? No, but this company in Adelaide did, the last time I knew, he says.

The Adelaide company explained that no, they didn't carry this product anymore, but used to get it from a company in Sydney. Apparently the product is a spare globe for motor vehicle accessories. So I call the Sydney company.

Sorry, but I was too busy trying to find 12V strobe lights to think of photographing all this. I only got a pic of the strobe (see above) to send to the Sydney company who thought they might be able to help me. They are vague, and couldn't guarantee me overnight delivery. So, some lateral thinking was called for.

Hmmm... I hadn't found any globes, but I knew where they were commonly used. I need 12V strobe globes. They are used in vehicle accessories. So, checking the catalogue of the original electronics company in Adelaide, I see that they sell 12V flashing amber units to augment car alarm systems. A visit back to the shop, and yes, 4 in stock.. just enough for the two I knew I needed, plus some spares. In the meantime, I had done some research on the www to refresh my memory of how these globes work, so I knew how to replace the broken ones in the circuit.

So on the day of the dress rehearsal I replaced the two globes that were broken, tested the system, which now worked, stuffed all the wire loom back in the aluminium tube, and delivered it to the theatre for the night's performance. All they had to do was refit some clear acrylic tubes over the globes.

The next day I get a call, "Sorry, but the gun is broken again. The Director was mucking about with it before we put the tubes over the globes. Can you fix it before tonight's opening show?" So I pick it up, and discover that the other two globes were broken. Fine, I have two spares. I fit them, and whilst I'm at it, cut out most of the metre long excess wire loom, and resolder all the globes. I get it back to the theatre in time for the night's grand opening. All went well for the season.

I enjoy the challenge of solving problems that involve discovery, application of learned skills and experience, lateral thinking, and time management. So bring them on!

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