Archive for the ‘electronics’ Category

Shop Demo Software Shenanigans

Saturday, June 9th, 2012

Back in the 1980s, a friend of mine had some serious phun with BBC model B’s in stores that also sold software.

He would take a box of 5.25 inch floppy disks, all but a few of which was labelled “Watford Electronics Compatibility checker”. (Watford Electronics were a third-party supplier of peripherals; they made an improved disk system for the BBC, better than but slightly incompatible with the “official” Acorn upgrade and some software, especially games, would not work with it.)

So my friend would ask to “check” if a game would be “compatible” with his Watford disk system. Inserting the “compatibility checker” disk into the drive of a BBC computer and pressing shift+BREAK produced a fancy screen with a progress indicator; which then asked for the game disk to be inserted, thrashed the drive a bit, then asked for the checker disk again. After a series of such disk swaps came the dreaded announcement that the game was not compatible with the WE DFS. He would return the compatibility checking disk to the back of the box, and ask the shop assistant if he could compatibility-check another game. While the assistant was away fetching it, my friend whipped out the compatibility-checking disk from the front of the box (nobody ever noticed this blatant switch, which was done with no sleight-of-hand) and booted it up.

Again the compatibility-checking process would require several disk swaps, and again it would fail. And my friend would wander off, dejected, before the shop assistant could work out what had just happened right under their nose.

What had happened, of course, was that the “compatibility checker” disk contained a program that loaded itself into RAM, then copied the game disk sector-by-sector first to RAM, then to the “checker” disk (overwriting the original copy of itself into the bargain, but it was already running from RAM, and each of several disks in the box contained a copy of the same software.) As there was only 32KB of RAM and some of that was eaten up by the OS, copying a 100KB disk (yes, you read that right: 102 400 bytes) had to be done in stages.

An Idea Whose Time has Been and Gone

Friday, September 4th, 2009

Filament light bulbs — at least the 100W flavour — have been banned. The only question that needs to be asked is, what took the government so long?

Anyone who has ever tried generating their own electricity will know that these bulbs are stupidly wasteful. A 100W bulb kicks out some 95W of heat. It’s surprising really, what with the distinction that has to be drawn between (for example) strawberry flavour, strawberry flavoured and strawberry lest the consumer be unaware how little fruit the product contains, that they’re even allowed to call them “light” bulbs at all! Heat bulb would be a much more accurate description!

Every person who has bought a 100 watt filament bulb to use for general illumination instead of a 20 watt compact fluorescent bulb is needlessly pissing 80 joules of energy up the wall every second the thing is on, using up fossil fuels that won’t ever be available again. Add up all those 80s and you could probably take a whole coal- or gas-fired power station out of service.

Of course, the 20 watt fluorescent is still putting out 15 watts of heat, so there’s room for improvement, and there does need to be a recycling scheme set up — but since compact fluorescent lamps contain enough valuable materials to be well worth recycling if you can get enough of them together, this is pretty much inevitable.

I’m looking forward to seeing the next generation of LED-based lamps.

Zero Standby Current Timer

Sunday, July 26th, 2009

This is a timer for controlling LED lights powered from a solar-charged battery. It was built to illuminate a passageway between two terraced houses, which is used to move the rubbish and recycling bins from the gardens of both houses to the street on collection days. Power is supplied from a battery charged by a solar panel. To guard against battery wastage due to leaving the lights on, it was desired to have a time switching arrangement where a single push of the switch gives a fixed duration of light before switching off automatically.

The first intention was to use a mechanical time delay switch, but this was found to be a rather costly solution.

The lights themselves were modified from 3-LED, battery powered push lights obtained in packs of two from a pound store (I frequent such places, searching for anything with white LEDs to use in my experiments!) The modification was simple but fiddly; entailing some track cuts to rewire the LEDs from a parallel to a series circuit and bypass the switch, and a resistor change. The timer itself was built inside an MB2 plastic enclosure with a pushbutton switch.


Circuit operation is as follows:

A Campaign I Can Get Behind

Friday, May 29th, 2009

I’ve been saying this for a long time now. Those “child safety” covers for power sockets — invariably peddled by charlatans who claim to know more than the professional engineers who designed the British ring main system more than 60 years ago — do not work.

For one thing, they are totally unnecessary. BS1363, the standard for the 13 amp plug and socket,mandates internal safety shutters in every socket outlet. If you look at any nearby power socket — whether on the wall or on an extension lead, they all have to conform to BS1363 — you will see that the Live and Neutral holes are covered up. The only place you can insert anything is the Narth hole, which is safe to touch anyway. The insertion of a plug pin into the Earth hole retracts the shutters from over the Live and Neutral holes. And the lengths of the pins on the plug and the depths of the shutters are specified such that this only begins to happen when the body of the plug is safely covering the socket.

But “safety” covers can actually make things worse. If a cover is removed (and don’t underestimate what kids are capable of), it can be inserted the wrong way around, thus opening the safety shutters and allowing an object to be inserted into the live socket receptacle. This can’t be done with a BS1363 plug, because the dimensions are such that the Live and Neutral pins would collide with the faceplate before the Earth pin entered far enough to retract the shutters. But these covers are made of thin, flexible plastic and can bend out of the way just enough to fit in upside down.

Some “safety” covers even allowed room for objects to be inserted into the “Live” hole even while the cover was in place.

And what happens when direct sunlight shines on plastic? Answer, it becomes brittle. “Safety” covers used on sockets in a conservatory or any South-facing room may well perish and, in the worst case, break off — leaving the Earth pin firmly retained in the socket by the sprung brass contacts, and the Live and Neutral holes wide open.

See the campaign web site at

Found Sat-Nav

Tuesday, February 24th, 2009

The other day, I found a satellite navigation system in the street, complete with in-car recharger. Sheer merionesian curiosity forced me to switch it on and try it out. Nothing happened, because it was flat out of juice.

I did not have a suitable 12V power supply at work, but fortunately the charger ends in a standard mini-USB connector (like the one on a digital camera) and I was able to charge it from my computer.

Now, this thing belongs to somebody. I shouldn’t just keep it, even though it’s a Tom Tom One and eminently hackable (it’s running Linux underneath). So what am I going to do with it?

Microwave Oven Repairs

Thursday, December 11th, 2008

My microwave oven packed up last Friday. The turntable, fan, light and timer were all working, and leaving it unplugged for awhile had no effect, so it was something more serious than just a thermal cutout tripping. Connecting via a wattmeter showed only 300 watts or so of power being drawn, so the magnetron was not firing.

On opening it up, I discovered a fuse in the HT circuit which had blown. Fortunately, I managed to find a supplier of the required part online; then I found a local supplier in a proper actual real live shop. Having visited said real shop and obtained the part today, I fitted it. Now, fuses don’t usually blow for no reason; the magnetron could have been faulty, which would have seen off the fuse. But the only way to find out was to replace the fuse …..

Anyway, so far it seems to be working fine. There’s actually surprisingly little that can go wrong with a microwave oven. I’ll be posting an article about them here shortly.

The Proof of the Pudding

Friday, September 5th, 2008

I managed to obtain a suitable donor lamp for my inverter project: £3 from Wilkinsons, plus another £1.99 for the bulb (9W, 14mm. screw base). The hollow china base of the lamp will contain the inverter circuit. When all is finished, you won’t be able to tell that it isn’t an ordinary mains lamp — except for the fact that the power lead will have a 6.3mm. jack plug on the end (this being the connector on which I am standardising for my 12 volt distribution boxes).

Nice hollow base:  Plenty of room for circuit.

Nice hollow base: Plenty of room for circuit.


Up from the Ashes of Disaster, grow the Roses of Success!

Sunday, August 31st, 2008

I have a working fluorescent lamp inverter!

There’s such a thing as going too far

Monday, August 25th, 2008

Whilst searching for power MOSFETs to build a H-bridge, I noticed this 12V microwave oven for in-car use. And whilst the thought of solar-powered cooking did cross my mind briefly, I had to reject it as being ultimately rather impractical. It needs 20 amps even on its low power setting, which is enough to cane one of my 8Ah batteries in 24 minutes.

If I wanted hot food while out and about, I’d be more inclined to consider a Coleman stove. This one is powered by unleaded petrol. It’s well worth the initial investment; petrol works out cheaper in the long run than butane canisters (not to mention there being several different, incompatible types), and is more widely available.

Or if I just wanted to heat up a Cornish pasty, I’d probably wrap it in foil and stick it in the engine compartment!

Stuff that Doesn’t Work

Wednesday, August 20th, 2008

Behind every great inventor, goes the saying, is a pile of failed experiments. Here I’m going to document some of my cases of finding out the hard way what doesn’t work.