srimech

"FabLab" opening in Manchester

2009-05-01T02:37:00.000-07:00

Some vague details have appeared in the press of a 'FabLab' opening in Manchester later this year. It sounds like a low-volume, general manufacturing plant. I'm imagining some CAM and 3D printers. This could be very exciting if it goes ahead.

News article: http://opensource.sys-con.com/node/940106

The main backers seem to be manufacturinginstitute.co.uk although there aren't many more details on there yet.

Electric pressure gauges

2008-11-09T14:01:00.000-08:00

Earlier this year I made a load average meter out of an old ammeter. But I've decided I want to use some even older technology, so I've been refitting some pneumatic pressure gauges so I can display information with them.

I got a load of old gauges from Empire Exchange, the little Mancunian goldmine that supplied the ammeter. Opening these reveals an arc-shaped copper pipe which deforms when inflated, driving a lever and gear which moves the needle. Now, ideally, I would control these using a computer-controlled variable pressure pneumatic source, but I don't have pneumatic mains in my house (not at 140 bar, anyway), so I've had to go for an electro-mechanical solution. It's a shame to cut the proper workings out of them, but I don't think they would have gotten any better life in their present form.

A servo motor is the obvious choice to get a controlled rotational movement. However, most servos only have a range of 180°. I need a range of 270° to use this meter properly. You can get servos with a greater than 180° range but they are quite rare, especially in the small sizes I need to fit into the gauge's body.

In order to get the 270° range I've used a rudimentary belt drive. I made a pulley for the gauge's output shaft by clamping the shaft into a dremel and using it as a lathe to turn down a larger plastic pulley to the right diameter.

The servo's output shaft is about 5.6mm diameter, so I made a pulley as close to 4.2mm as I could get it for a 1:1.5 enlargement. The pulley I made is rather rough, but no-one will see it in the finished product.

I've attached the servo to the gauge body with Araldite, and used polyester sewing thread as a belt, with two turns around both pulleys. The belt is tied to the servo motor's horn and fixed with glue at that point. Ideally, it would also be tied to the output shaft, but the tiny scale of the components make this kind of engineering impossible for me.

A real engineer would use a toothed belt or a gear drive for this - or make their own servo motor. But for a few hours' engineering in my kitchen, I'm happy with this solution.

I've tested this with a remote control system and it works very well. The biggest concern is slippage of the belt, but the exercise I've given it so far shows that it's actually perfectly repeatable - it doesn't slip at all.

The next steps are to replace the scale backplate with something like a 'load average' or 'spare memory' scale, and to rig up a servo motor driver so I can control it from a PC. Servos are very easily to control, just needing a PWM signal - a little easier than pneumatic signals in this day.

If you want to know why I bother to do this, you are probably not my target audience. :)

Opening a WRT150N

2008-11-09T13:37:00.000-08:00

Only a tiny number of people will be interested in this, but I recently had to open the case of a Linksys WRT150N router and found a few questions on the web asking how to do so. Doing this voids your warranty, of course.

First of all, find a T10 security torx driver. These are like standard Torx bits with a hole in the middle.

Start by flipping the router over and levering up the silver panel at the rear of the router. I'm calling the rear the side which the antennae are attached to. The rear half of this panel will pop out of the body, but the front is attached with hooks which will break if you try to lever up the front half first. Once the rear half is free, the front half will slide off easily.

This will expose four security torx screws. Remove these.

Now you can remove the top and side panels. Pull one of the side panels and its side of the top panel away from the router at the same time. The hooks which hold the side panels on should pop open one at a time without damaging anything.

Repeat this with the other side panel. You should now have a completely disassembled WRT150N, so you can get at the serial port connections you want to attach to.

Tea-making with gas and electricity

2008-08-10T04:58:00.000-07:00

B3ta recently asked whether it was cheaper to make a cup of tea using an electric kettle or a gas one. My gut feeling on this was that gas was cheaper, because gas is a lot cheaper than electricity per unit energy. I've done an experiment to see which is actually better.

Starting with gas, I put a mugful of water in a steel saucepan and heated it over a low flame. I can measure the amount of gas used using my gas meter. I've completely shut off my central heating boiler, including the pilot light, so no other gas is used during the experiment.

This took 0.0092 cubic metres of gas to bring it to the boil. I'm calculating the energy and cost based on my last gas bill, which explains how to convert from cubic metres to kWh - ultimately by multiplying by 11.0786. So I've used 0.102 kWh. I pay 2.88p per kWh for gas. USwitch.com reckons I could get gas for 2.86p from another supplier, so I'll use that. That makes a total of 0.29p.

Next I set up an electric kettle with a plug-in power meter. I've got to use half a litre of water to cover the element and make up the 'minimum' mark on the kettle, which is slightly more than the mugful I boiled in the saucepan. To boil this took 0.06kWh. The cheapest electricity I could find on USwitch was 11.21p/kWh, so this cost me 0.67p. Even given the inaccurate measurements in this experiment, gas is the clear winner.

But what if I've got friends round? I did a test with a large teapot full of water as well. This time, the saucepan took .281kWh for 0.82p, and the kettle took .140kwH for 1.57p. Still nearly double the price of the gas option.

Boiling speed is another factor - the gas saucepan takes a lot longer to boil than the electric kettle. I did some more experiments, using the gas turned up higher than I normally would. I expected this would waste gas, because a lot of heat escape round the side of the saucepan. However, it was barely any extra. There's probably a 'sweet spot' for the gas flow - too high would waste gas, and too low would take so long to boil that heat gets a chance to escape from the saucepan. Still, I couldn't get it as quick as the kettle. The kettle isn't a fast boil one - it's about 2.2kW. Even so, in all the experiments I measured time for, the kettle is twice as quick. So for the extra third of a penny it costs to boil the electric kettle, and the convenience of having an automatic off switch, I may well stick with that for the time being.

You'll notice that the kettle uses less energy despite costing more. Does this mean the kettle is more efficient? Not necessarily. The kettle is more efficient in itself than the stove and saucepan, but a lot of the electricity comes from gas in the first place. About 36% of our electricity comes from gas-fired power stations, and the best stations are about 58% efficient. Another 37% comes from coal, but I haven't found efficiency figures for those yet. If our electricity came entirely from gas, the kettle would use about the same amount of energy as the stove. To do a fair comparison you would also need to know how efficient the national electricity grid is, and how much energy is used in pumping gas around the country.

http://www.berr.gov.uk/files/file39569.pdf
http://www.the-eic.com/News/Archive/2006/Nov/Article2799.htm

The data in full:

Fuel	Quantity	Speed	Time (s)	Volume of gas	kWh	cost/kWh (p)	Cost (p)
Gas	1 cup	slow		0.0092	0.102	2.86	0.29
Gas	1 cup	fast	192	0.0100	0.111	2.86	0.32
Gas	1 teapot	slow	1376	0.0254	0.281	2.86	0.8
Gas	1 teapot	fast	506	0.0258	0.286	2.86	0.82
Electric	1 cup	n/a			0.060	11.21	0.67
Electric	1 teapot	n/a	213		0.140	11.21	1.57

The by-products of potato juice

2008-07-27T15:33:00.000-07:00

Today I have mostly been performing silly experiments in the kitchen. I like potato pancakes, but they are problematic to make because the moisture has to be removed from the potatoes before frying them. This usually involves dirtying a clean cloth, or trying to squeeze the moisture out by hand which doesn't work well. I've also tried using a ricer on raw grated potato which is quite effective.

I had a brainwave today - realising that I already had a device which does this job, namely a juicer. I pushed three potatoes through the juicer, and kept the pulp that normally gets discarded. Mixed with some chopped onion and seasoning, this makes really good, fine potato pancakes. I wasn't brave enough to try drinking the potato juice.

An earlier experiment to try separating freshly brewed coffee from its grounds using the juicer didn't work so well. It just tastes like soil.

12 volt mains

2008-06-05T15:21:00.000-07:00

If you like gadgets as much as me you probably have a large number of AC adaptors with power bricks cluttering your house up. A lot of these have different output voltages, but I have quite a few that deliver 12 volts, or claim to. Is it possible to replace some of these with a single transformer to simplify the wiring and increase efficiency?

The experiments I've done so far suggest that the first part is quite possible, but surprisingly, I couldn't run my appliances as efficiently with a single power supply than with individual bricks.

The appliances I used for this were my Netgear ADSL Wifi router, my 20" LCD TV, and my mini-itx based server computer.

The first step was to give each a common connector. I split the cable on each and attached a PC drive power connector on each side. Drive power connectors are cheap, meant specifically for 12V and can be plugged straight into an ATX power supply.

Having done that I reconnected them to their original power supplies and switched them all on, checking the power consumption for each. Measured at the mains socket with a plug-in power meter, I got: PC on idle: 28W. TV: 50W. and the router: 7W. Total 85W. Incidentally, if you repeat this experiment, it might be better to plug all those into a power strip and plug that into the power meter, in case the meter has some non-linear error.

Using the first attempt - an "L&W" 250W power supply used 48W just to run the router and PC - obviously less efficient. The TV wouldn't work properly at all - I got sound but no picture. A quick check revealed that the 12V output of this power supply was actually only raising 11.05V unloaded - which presumably is not enough to power the TV properly. The PSU also used 12W without any load at all.

I tried again with an older Opus PSU which had the same problem powering the TV. It also used 8W unloaded.

Next was a Morex PSU, only a 200W model for a micro-ATX system but this was evidently of better quality and managed to get the TV running. However, just the TV and PC drew 92W from the mains - over the limit before the router was connected. It also drew 11W when idle.

The last test was my bench PSU. This is a bit of a beast, rated at 40A continuous, but given its higher cost I thought it might stand a chance of being more efficient. No such luck though - with all three appliances it drew 110W, consuming 18W unloaded.

So, while it might make the wiring nearer if done properly, I've yet to find a power supply which will do the job more efficiently than using individual power blocks.

I suspect that computer PSUs are not the best choice for this job, as they are probably geared towards delivering a very stable smooth 5V and 12V rather than efficiency. The power bricks are not, so far as I can tell, very stable in their output voltage, probably relying on further conditioning inside the appliance.

As usual, I wouldn't recommend anyone else trying this for themselves - running things on too low a voltage could damage them, and not all PSUs output the voltage they claim - the router's power brick, for example, delivers 16V when unloaded.

Asus Eee 900

2008-04-30T13:51:00.000-07:00

Hooray! Despite making me jump through a few hoops to verify my identity, Dabs came through and delivered my Eee 900 today. I'm posting from it right now - the keyboard will take a bit of getting used to but it's a lot easier to type on than my previous UMPC. It'd be hard to give a negative review to something I've just spent 330 quid on but it is very good so far. It's a lot better built than I'd been expecting - it may look like a toy but it's very solid. Next step - apt-get emacs and pygame.

Toaster video

2008-04-17T15:21:00.000-07:00

I've been playing with my digital camera, trying to make a better video for the turbo toaster. The results aren't perfect, but still better than I expected for about an hour's work. The title and fade in was done with Kdenlive, which I found really easy to use. Pity that it doesn't seem to anti-alias the text properly when fading to the video clip. I brought up the colour a bit using mencoder with a few filter options. Anyway, there you go, flying toast in full motion video.

From the archives

2008-04-11T15:51:00.000-07:00

I've uploaded a few of my older posts from my wordpress installation - the LED message board modifications, the event report from Robot Wars 2003, and the Turbo toaster.

Load Average Meter

2008-03-19T14:54:00.001-07:00

I found this old milliameter in Empire Exchange in Manchester. It was built sometime in the 1960s. I then printed out a new scale for load average, turned the original scale plate around and glued my new scale onto the back, so as not to ruin the original one.

I've used a USB interface board from Maplin to drive it. The interface board has open collector PWM inputs, but the meter needs a bit more power than the board can supply - about 100mA on its smallest scale to go full scale. To get power, I've drawn off a drive power connector from the motherboard. The last component is a small perl script that does a bit of maths to convert the computer's current load average into a duty cycle.

Moving from Wordpress

2008-03-07T14:11:00.000-08:00

While I'm very impressed with Wordpress, maintaining my own installation is taking up a bit more time than I'd like. I don't write posts very often and Wordpress seem to come out with a new version more often than I blog. I've decided to move over to Blogger so Google can take care of the service. I'll either write a script to keep them in sync or move srimech.com over to a republishing system.

MUTR Micro rover

2007-12-08T12:57:00.000-08:00

I found this little tracked robot platform at mutr.co.uk. It's quite difficult to find tracked robot platforms (or even the components) cheaply, but this is very nice for the ~ ten pounds they charge. It contains the tracks, wheels, motors and gearboxes, an aluminium plate for the chassis, fixing screws and some three-way switches and wire. It needs a bit more than just assembly to get it moving - I had to fold and drill the aluminium and glue some of the wheels together, and I found the motor tended to rise out of the gearbox if I didn't tie it down with some copper wire.

Somewhere I have some infra-red proximity sensors and a camera board I'd like to attach to it to make it into a proper robot. It'll need a suitable battery and some motor controllers - the Electronize controllers I have at the moment are a bit too big for it.

MUTR sell a lot of useful stuff - I got some Nitinol memory wire at the same time, which is very interesting stuff. Their £5.80 default delivery charge is a bit annoying for small orders, but handily I have some colleagues who also buy from there to share costs with.

ZX Spectrum Laptop

2007-10-28T17:07:00.000-07:00

I've been working on this for quite a while. It's one of them 'casemods' that all the kids are talking about - the mind and display of a Libretto in the body of a ZX Spectrum. More details / more pictures

Busy

2007-07-18T12:31:00.000-07:00

I've not had much to post about recently, due to a rush at work, not being able to get to Newark Kit Car show, and being on holiday. I'm working flat out on a little retro computer project at the moment, which I'll detail later, but for the moment I'll just post this picture of a method I'm experimenting with to attach flying leads to flexible circuits.

Theremin!

2007-05-30T15:38:00.000-07:00

A few days ago at work we got some new computers in and decided to call one of them 'Theremin' since all the others were named after instruments and we were running out of ideas. A quick browse later and I found Art Harrison's web site on Theremins. This is built from his '2006 Minimum Theremin' instructions and I found it pretty easy to find all the components at Maplin. It's a simple design, meant more for demonstrations and science fairs than musical use - Art has designs for more sophisticated theremins on his site. I've no doubt reduced its musical quality by deviating from his design, but you can still get a tune out of it. I've left off the tone control - it's an preset on the board rather than a proper control - but you can alter the range quite adequately by adjusting the length of the telescopic antenna.

Dan's from-scratch barbeque

2007-05-20T15:39:00.000-07:00

Last weekend my friend Dan invited us all down for a barbeque. A generous offer anyway, but this had the added highlight that Dan had built the barbeque from scratch. It's a fine piece of engineering - perhaps not quite as stable as a brick barbeque but it served very well and cooked plenty of food for fifteen or so hungry guests over the weekend.

The construction materials are (so far as I remember) 1/2" steel bar and angle, some thin steel plate, steel mesh and those corner strips plasterers use - the closest thing most DIY stores will have to perforated angle. If anyone else has a go at this, my one piece of advice would be to avoid galvanised metal, as can give off some nasty fumes if heated. Compared to the quantities of ethanol usually drunk at barbeques though, it's probably a drop the ocean.

Dan also made a giant jenga set by cutting up some planed timber - a very quick and effective party piece. Pictures of that are in the gallery.

Sugar Fab

2007-05-10T16:30:00.000-07:00

EvilMadScientist's sugar fabber promoted quite a lot of discussion in our office today. Using sugar as a fabrication medium is inspired, but we wondered whether we could use something more accurate than a hot air gun to fuse the sugar. Using a high-power laser would seem ideal, but I think there's a lot of milage in using a lamp (or lightbulb) and a moving lens or mirror to focus heat on a particular area.

Ghetto 100Base-TX

2007-05-10T15:57:00.000-07:00

Networking Today's top tip: Don't slam your network cable between a door and a door frame. You'll end up cutting out a section of damaged cable and assembling it as shown. Although, if I do say so myself, the slight over-exposure makes for quite a nice photo.

Top Of The World

2007-05-10T15:49:00.000-07:00

attic Last weekend I went on a little exploration of my attic. There are two water tanks up there, quite a lot of wooden beams and lots of rockwool insulation. Not all of the rockwool is in the right place, so I've attempted to distribute it properly with a pair of sturdy gloves and a fancy dust mask.

Anthony Axford delivered a batch of quarter-boards of chipboard and some aluminium angle which I've used to board up some of my attic. Using the aluminium angle as guide rails I've been able to put down boards to create useful storage space while still allowing me to lift the boards up in case I need to do any electrical work in the future. The south side of the attic already contained quite a lot of wiring which I couldn't cover in case of overheating, but the north side was free of it so I've been able to create quite a lot of useful space there.

Happily, a quarter sheet of chipboard is exactly the width of the rafters in my attic. I didn't know that would be the case when I ordered them - it could be a coincidence but it's probably something deeply entrenched in ancient building codes that makes things a multiple of one foot sizes.

Tiny memory cards

2007-05-01T16:20:00.000-07:00

microsd 2GB!! On that tiny thing? Crazy. In the gallery I've got pictures of that next to an 8MB Playstation memory cartridge, which is about 100 times the volume. If I could find it, I'd have a picture of my 8" floppy disc drive next to it. Kids these days. Honestly. Tsk.

I've bought this for my new M3 DS Simply cartridge, which promises to allow me to put some homebrew software onto my Nintendo DS. I was introducted to it by a friend at work who has ScummVM working on his DS using the same cartridge, but I'm mainly interested in writing my own software for it. At the moment though, I have no idea what that entails - I'm led to believe the thing has a couple of ARM processors but apart from that I know nothing.

Bed #2

2007-05-01T16:12:00.000-07:00

A few weeks ago I thought it'd be a good idea to make myself a bed rather than buying one. After all, I'd get to make one to my exact specifications and it wouldn't look like anything you could buy from Ikea. In addition, I'd made a bed before and it had served me very well.

The thing is, when I say 'a few weeks ago' I actually mean 'over a year ago' and I've had the timber cluttering up my front room since then, and simultaneously been opposed to buying a proper bed because I'd already made that small initial investment. Since then other commitments kept appearing, such as fences blowing down and my employers who continually expect me to do the job for which I'm paid.

Last weekend I finally finished sawing, chiselling, filing and sanding the components and got the bed assembled, and to be honest it's not that great. My last bed - a bunk bed - creaked quite a bit but I was confident with it because I knew I had enough cross-bracing. The legs on this depend on the quality of the top joints and they're not that accurate, so in engineering terms it's a bit disappointing. Still, it is very solid. I'm mainly glad that all the timber is out of my living room.

Mechanical-digital clock

2007-04-21T14:22:00.000-07:00

mdclock/
I like mechanical digital displays. I had a clock radio with such a display but the existing case was a hideous 50s-style plastic effort, so for a while I've planned on making a new case for it.

While in the pub a few weeks ago, a friend came up with the idea of using copper pipe and plumbing fittings as a structural system. I've started making a frame and it's working out well so far. It'd be nice if they made three way corner plumbing connectors, but I suppose you don't have much call for those when you're connecting up taps.

The biggest challenge in making this is going to be making the mains components safe. There's a transformer for most of the radio, but the clock itself runs off a 250V AC motor. The case will have to be finger-proof and carefully earthed.

Look at the twisted-together connections in the photos of the original radio - they didn't always make things better in the old days.

Toaster modifications

2004-02-23T18:13:00.000-08:00

Every day we are lied to by mainstream media and Hollywood. This is especially apparent in the portrayal of toasters. Toasters in films and cartoons consistently launch hot grilled bread several feet into the air. However, real toasters like the one in your kitchen merely lift the bread by an inch or so after it's grilled. This is very disappointing, and with smaller grilled products, leaves you digging inside an electrical appliance with a spatula to remove your breakfast.

So, one morning I assembled a team to save the world. Omar, Phil and me descended on Winchester to find parts for our prototype.

In the hardware and charity shops of Winchester we found:
* 1 Rowenta TO-18 toaster
* 2 30mm springs

Construction

A brief warning: Do not do this. It involves inserting foreign bits of metal into mains appliances. These are not instructions.

The first job was to remove the original spring, although I kept it in case we wanted to use it in addition to the extra springs. I then clipped the new springs so the end formed a hook shape rather than a ring, and attached it in place of the original one. This gave quite a good improvement in the spring force, but it could be better. Happily, it was quite easy to attach the second spring between two other points on the chassis. The spring still releases without excessive force - this is important, because the toaster could stay on forever if the spring force gets so high it can't release itself.

So, we reassembled the toaster and tried it out without power applied. The first two test items - half a bagel and a slice of stale bread - got a reasonable distance out of the toaster, not quite as high as hoped but as good as we could do, at least until we got new springs. After this, we decided we needed to use an RCD before plugging it in. We didn't have one, so we left it and went to drink beer.

Four months later, I found the toaster in a box I'd used to move house, so went to buy an RCD and try it out.

Results

This time, the results were very pleasing. The picture above was the first take with my camera and the only one needed. Two slices of wholemeal bread on minimum toast setting made it at least three loaf-heights out of the toaster, almost hitting the base of the cupboard. Naturally, it landed nowhere near the toaster, but luckily not on the floor. The toast was tasty.

Robot Wars 2003

2003-08-24T15:02:00.000-07:00

Tuesday 19th August, 5:45PM

Greg: “We’re all sorted. I’m trying to sort out either a Ford pick-up or a Land Rover, but we’ve definitely got a vehicle of some sort.”

Jim: “Great, anything you can find with a decent boot size would be great. I’ll see you on Thursday night, then?”

Greg: “Friday night?”

Jim: “No, Thursday, we need to be in Nottingham on Friday.”

Greg: “Er…”

Cue frantic searching for Robot Wars’ phone number. I’ve got about ten minutes before they close and they’re travelling to the site tomorrow. Luckily, they’re in and are able to reschedule us for 9:30 on Saturday.

Friday 22nd August, 13:00

The weapon is only moving intermittently. With Greg arriving in a few hours, and no idea as to the cause of the problem I’m starting to feel very worried that is going to be a long day.

After a lot of inspection I find the BC108 transistors are getting hot, in fact too hot to touch. These are the first stage of a darlington driver and shouldn’t be handling any current to speak of. The base and collector legs of the transistors are connected via 10k resistors to other parts of the circuit. At 12V, these resistors could conduct a maximum of 1.2mA into each transistor, nowhere near enough to make them overheat.

After checking everything thoroughly the only thing I can think of is that the 12V linear regulator is generating lots of high frequency noise. I solder a capacitor across the output of the linear regulator to smooth it out, and the weapon works perfectly. However, whether I’ve actually fixed it, or that the transistors have just cooled off while I was modifying it remains to be seen. I’m confident enough that the interface will work for long enough to demonstrate the weapon during the tech check, though.

16:00>

Greg arrives with a small tank. Try to put the top armour on the robot which seems to have shrunk. After fifteen minutes of shoving, ratchet compression and hammering, all eight bolts go in. Ish. Set off for Nottingham. It's been three solid days of sawing, drilling and heavy lifting, so sitting in the passenger seat for an unhurried journey down the A6 is very relaxing.

Saturday 23rd August, 7:00

Eat uninspiring but functional breakfast at the hotel and delightedly discover that thieves haven’t broken into the landy and nicked the robot. I have nightmares of finding the robot on sale in the local second hand shop for 50 quid.

9:00 AM

We arrive at the hangar in RAF Newton. I feel somewhat glad that we’re not the only team whose manufacturing skills are not quite up to Razer’s.

Excuse the lack of pictures from here on - I was a bit busy

10:00

Cayene is sitting on the bench in bay 17. Briefly consider taking the top armour off to push one of the drive axle bearings back into place, but think better of it. Manage to push it back into place by sliding a plastic ruler between the wheels and armour.

10:30ish

Tech check performed; passes without any problems.

Time zero: called in

Alan Gribble checks the failsafe works OK. It does, but there’s something not right with the rear right speed controller. The zero area on the controller seems to have shrunk to nothing, so it’s hard to persuade it to stop moving completely, and it seems to be sticking in forward drive when it should be going in reverse.
The front right wheel is working correctly, so on some occasions we seem to have the wheels on the right hand side rotating in opposite directions; going nowhere fast.

T + 10 minutes

Push Cayenne into the loading bay. Realising that the link is on the wrong side, we have to rotate it to put the links in, but it won’t rotate back under its own power due to the same direction problem. Grab one of the spikes and rotate it to face the arena by hand. Drive out into the arena. Get called a tart by one of the tech crew for the message board.

T + 15 minutes

Up in the competitor booth, Cayenne decides to start playing and works perfectly. We drive over to the far corner of the arena and do a few test maneuvers, and spin on the spot a bit to turn the message board to face the judges. No problems. Here goes nothing.

T + 16 minutes

Horn sounds. Drive immediately over to the other side of the arena, so we have our backs to Disconstructor. They follow and we move backwars to ram them, miss them and we end up side by side, trying to turn around each other. We break off and move out halfway down the arena to get enough room to build up for another ram on Disconstructor. But something’s wrong; we’re not moving fast enough, and soon enough we were stopped completely. Frantically shake the controller and transmitter but we’re not going anywhere. Disconstructor move in for the kill and our message board is wiped out instantly (8mm polycarbonate). They circle around and take another swipe at the front of the robot, knocking the crusher jaws back into the frame of the robot, and smack a final hit on the right side of the robot. Cease is called.
The time from us being immobilised to being hit by Disconstructor was very small, so I think the damage they dealt out was fair.

T + 18 minutes

Kim Davies bulldozes Cayenne out of the arena. One of the judges (I think) asks us if we can repair it, which could have meant a discretionary place was on the cards. But from the look of things we’re not going to be running again this week.

A lot later…

Quickly surveying the damage, it doesn’t look like any of the internals have been badly damaged. Two of the rear spikes have been ripped off, the linear actuator’s clevis has been sheared off and one of the chassis beams has been bent, putting the right front axle out of alignment.

We didn’t notice for quite a while that one of Disconstructor’s teeth was still stuck in our armour (highlighted on the picture to the right). Disconstructor said the teeth were such a hard grade of steel that they couldn’t angle grind them, so I considered it quite an achievement that we’d managed to break one of them off. It took a lot of persuasion with a 4lb hammer to remove it.

We had been extremely lucky with the speed controllers. One of our interfaces, which are mounted inside the speed controller enclosure, had been hit and the interface microprocessor was cut clean in half. I’m used to getting segmentation faults in my programs, but not quite this severe. Another inch and they would have cut the speed controller in two. The interface comprises less than ten pounds worth of components, so we had a lucky escape there.

Reasons for the failure…

As far as I could tell both halves of the robot were working fine until we lost power. This means we probably had a ‘common mode failure’ - a fault in the design of each system rather than a random component failure. A gradual reduction in power would be what you’d expect from a flat battery, but all the batteries were still pretty much fully charged - we measured them at about 12.7 volts each after we’d got them home. The most likely culprit I can think of at the moment is the darlington driver circuit which holds the speed controllers’ ignition relays on. The circuit is identical to that used in the weapon interface, which we knew was overheating. On previous tests (when we’d been running for a good five minutes) we were running without the top armour on, so the ventilation would have been much better. This is just theory at the moment, we haven’t had time to retest everything.

Reflection (19th September 2007)

I never did find out what was wrong with that incarnation of Cayenne. Two of the speed controllers were found to be broken; the other two are still in Cayenne to this day and the broken ones were replaced with a RoboteQ controller. I could have done better given an extra day to tune up and debug, but that is always the case. No further series of Robot Wars were filmed, so I never got a chance to have another go. Still, I’m very glad I got to stand in the famous booth with a working robot, if only for a short while. It’s very tempting to give up the night before a fight, when nothing seems to be working and you’re covered in cuts and grazes from rapid, haphazard metalwork.

Disconstructor unfortunately drove into the pit in the first televised round. It was a good robot which could have gone a lot further with better luck, but it would have been nice to get knocked out by someone better recognised.

Cayenne was dismantled after the fight and rebuilt with a completely new chassis as Jalapeno. It went on to fight at Magna and held its own quite well in a one-on-one fight with Tornado at Preston.

LED Message board hacking

2003-04-23T15:33:00.000-07:00

LED message boards look nice, and are much more eye-catching than a LCD or TFT screen. However, proper message boards are expensive - about £150 - so you’d need a good reason to justify buying one.

Maplin occasionally have ‘toy’ message boards on sale much cheaper - around £25. However, these can only hold a short message and this has to be programmed in via a small built-in keyboard. They are not quite so well built as the proper message boards, and have slightly smaller displays, but apart from that they only differ in terms of control electronics.

Under the hood

Maplin’s message board is called a ‘GM Light’. This produces no useful results in Google, other than lots of pages for GM Light Trucks and a pistol of some sort. However, pulling the device apart reveals some more interesting information. There are eight ICs on board. There are five of one sort, which can be easily identified by the part number printed on them. The relevant code is 74HC574, which identifies it as a latch. Two more are 74HC164, a shift register. The remaining chip is oddly packaged, looking like another piece of PCB with a resin blob in the middle where the chip presumably is. There are no markings on it, so presumably it is an ASIC, and its use will remain a mystery.

By tracing some of the tracks on the board, it’s possible to figure out how the board should work.

How it works

The board is divided horizontally into 4 blocks of 16 columns and one block of 8 columns. One of the latch chips drives each of these blocks. One latch chip drives a whole row in each block. The latch chips all share the same data bus, so the latch clock lines must be triggered individually to load different data into each latch.

The shift registers are joined together so that the highest bit output of one shift register is the data input for the second. This effectively daisy-chains both chips, enabling a bit pattern to propagate through them.

From this information I could figure out one way in which the board could work. This may not be the way the ASIC does it, but that doesn’t matter.

By loading a single pulse into the shift register at a time (i.e. 1 every 16 clock cycles), all the outputs of the shift registers will turn on sequentially. Each output from the shift registers is connected to a transistor which, when on, conducts the cathode of each LED to ground. Consequently, one column in each block is activated on each clock cycle. The inital pulse to the shift registers must be regenerated every sixteen cycles.

The latch chips control the rows, so data loaded into them will control the pattern of LEDs seen in the currently activated column. By loading new data into each latch for each new column, a pattern can be displayed. Note that there is no ‘output enable’ (or at least if there is, we didn’t use it) but the latches can be loaded so quickly that any glitch is invisible.

Rewiring

Obviously we can’t write to the latches or shift registers ourselves while the ASIC is trying to do the same, so the ASIC has to go. A more patient person might use a solder pump or wick to unsolder the ASIC from the motherboard, but I went for the brute force and ignorance approach. I jammed a knife under one end of the ASIC and ran a soldering iron down each side of the chip repeatedly until it came away (it’s effectively surface mounted). Unfortunately, some tracks on the board are routed through pads for the ASIC, and my approach ripped a few tracks up, so I had to patch the PCB back together again. The yellow wire on this photo is one of the patch wires.

Once the ASIC was gone, the remaining rather fiddly job was to attach wires to the appropriate chip pins - 7 to the latch data lines, one to each latch’s clock line, one to the first shift register’s data line, and one to the shift registers’ clock.

It’s then a relatively simple job to hook up all the lines to a PIC16F871. I put all the lines through a 1k resistor to avoid damage due to contention - although all the lines should be inputs to the message board PCB, there might be something else on there driving a line that I hadn’t noticed.

Programming

The next step was programming the PIC16F871 to drive the appropriate lines. The code to multiplex the display has to run quickly to maintain a reasonable image, so I allocated a ’screen buffer’ to hold the current pattern for the LEDs. With one byte for each of the five 16-column blocks, this is 80 bytes - a lot for a PIC16F871 which has very limited on-board RAM. This leaves only 16 bytes free in the main page. Actually, the display only uses 7 bits of each byte, and the last half of the fifth byte is invisible (off the edge of the display) but it makes the software much simpler to consider it as an 80*8 bit array. In fact, the eight columns which are invisible are useful - they can be used as an offscreen buffer to download the next character into.

Once the code to display a multiplexed pattern is done, the next stage is to make the pattern change. The two important jobs are to scroll the display (simple) and to insert new characters into the pattern (difficult). Scrolling just involves running through the buffer and copying each location to the location further down. This happens much more slowly than the multiplexing code.

To insert new characters, we need to know what the next character should be, and also what bitmap makes up this character. The font is held in the PIC16F871’s program memory. Program memory is much more plentiful on the 16F871 than RAM, but requires some memory-mapped IO in order to retrieve it. There is a slight time penalty, but this isn’t so important as a new character only needs to be fetched infrequently, with respect to the speed that multiplexing occurs.

Getting all the timing and character downloading right took a long time, but eventually I had a scrolling message on the display. This was the majority of the work for version 1.0.

Version 1.0

Version 1.0 of the message board had a message hard-coded into program memory. I loaded it with an insulting message, and attached the whole board to the back of my combat robot, Cayenne

Unfortunately, I don’t have any pictures of Cayenne with the message board attached. A couple of days later, I took Cayenne to the qualifying sessions for Robot Wars 7. There is never enough time to do what you want at a robot combat event, and taking photos is a fairly low priority. There are, however, several pictures of Cayenne with the bits of the message board that were swept out of the arena after the fight.

Version 1.1

Once my enthusiasm returned, I thought of another use for a message board.

Using several ‘talkers’ (chat rooms) on a laptop with a small display poses a problem - how to keep track of all the messages arriving from several sources at once, especially when you’re working on something that requires the full display. Sure, you can make each terminal beep when it receives a message, but you still don’t know which window it came from, or if it’s important enough to switch away from your main application.

So, why not get all the messages from your talkers displayed on a scrolling LED display? To do this, I added serial communication to the original display, so it would display any text sent to the serial port of a PC. The PIC16F871 has a serial port on board, but it only works on TTL voltage levels. Help comes in the shape of the wonderful MAX232 chip, which generates the +/- 12V required by RS232 from a 5V source.

Serial communications

Cunningly, I initially wired it up in TX-TX and RX-RX configuration. Luckily, I used my £10 laptop for testing and the MAX232 chip didn’t explode, so after a bit of debugging I swapped it for the more conventional TX-RX, RX-TX configuration and connected it to my server. Now, my server could talk to the message board.

I implemented a 16-byte rolling buffer on the 16F871 - the only memory available on the chip - and got it to send XON/XOFF bytes to stop the server sending too much data at once. There were a lot of problems with the buffer - mostly due to my own incompetency, assigning two variables to the same address, and not preserving variables in the serial recieve interrupt routine. Debugging was tricky as Microchip’s MPLAB can’t simulate the serial port. However, after a few irritating nights, the message board worked as planned.

From the talker to the display

In order to use talkers as normal, I use the unix tee command to split off output. For example,

telnet example.org 8080 | tee -a example.log

Then a separate process runs tail -n 0 -f example.log > /dev/ttyS0 - having set up ttyS0 for 300 baud communication with xon/xoff flow control. Why 300 baud? That’s the speed my robots’ radio control system runs at, and I copied the code directly from the receiver. 300 baud is still faster than the message board can cope with.

This displays everything coming through the talker on the message board. Of course, it would be better to filter this. I currently have a sed script to remove ANSI codes, and this could be expanded to check for interesting messages (new message posted, user login etc)