Battery Charger (6v Sealed Leadacid)

Those who have been following Workshopshed on Twitter may be aware that I've a project to repair a childs toy train. This was purchased non-working from a charity shop and is electrically driven. I'll post up a separate article on diagnosing and repairing that once it's complete.



When I removed the 6v battery I checked it with a multimeter it had a feeble 1.72v unloaded. I quizzed a few people on twitter and Andrew and Alan both suggested charging it and see how it reacted, in particular checking that it did not draw excessive current. Alan also suggested that I needed a constant voltage supply to charge it.

However I did not have a suitable charger, Alan suggested they were quite simple so I googled for circuits and found Myra Van Inwegen's article on a 6v sealed lead acid charger. This uses the L200 regulator which has an option for current limiting which is ideal for a charger.

I thought this chip sounded familar. I had a root in my box of bits and turned out an assembled Maplin kit for the L200. I'm not sure what I planned to do with this but I must have been keen because I bought two. This kit needs some form of supply and as Myra mentions this needs to be approx 2v higher than the charging voltage. In it's current form you'd want to use a regulated supply but you could also add some smoothing capacitors on the input side and use an unregulated supply. All I had was a 12v one that I've been using for my Arduino Uno experiments.



I lookedup the specification of the Uno to see what kind of power socket I needed to get for my supply, which turns out to be a 2.1mm centre positive connector. I purchased one of these and a 0.56Ω 10W resistor for the current sensing from the local Maplin. I also found an old PC heatsink (perhaps 486 or Pentium) and cut it into two, one part for the regulator and one for the current sensing resistor. I used some old biro tubes to make standoffs for the pcb and bolted that into the box. The heatsink was tapped so I could mount that away from the edge of the case with some M4 bolts.



I added an LED and on off switch to the box and wired up some big crock clips on the output. These conveniently came with an inline fuse as they were from a solar trickle charger.



When I first powered it up (unloaded) I adjusted the output voltage to 7.22v (I was aiming for 7.2v but it's not a very sensitive trimpot). I then hooked it up to the battery and checked the voltage across the sense resistor to make sure it was not drawing too much current. This was very low so I was happy.

I left it running through the day, monitoring the voltage on the battery and checking it was not getting too hot.

The resulting graph of voltage against time is roughly what we'd expect for a lead acid battery.



It's a 4.5AH battery and seems to be taking charging current of 0.1A so that would suggest 45hrs to charge. After 12hrs it's upto 6.13v and I hope to keep charging till get to 7.2v so it could be some time longer before I get a fully charged battery. I also have concerns that the battery is not holding it's charge properly as when I monitor the voltage it does tend to reduce even though the multimeter should have quite high internal resistance.

Double Height Letters and a hidden hive of tech in Westminster

On Monday night I got to attend the London Arduino Meetup, this group is a mixed bunch of beginners and enthusiasts explaining and learning about using the arduino in their projects. The session was based at the Westminster Hub, a series of rooms on the first floor of New Zealand House just off Trafalgar Square. To get to the session I passed by a series of groups discussing a wide range of topics, it reminded me a lot of the Google Campus (but with people who work late).

After an intro by host Marc and a round robin of what we were all upto we got into the talks and demos. We had a good selection, Christian and Liam explained how to connect our arduinos upto the internet of things and Danny showcased a robot kit he's been working on. Mark from the "Future of Manufacturing" group explained about the Luma Project he's been working on with another London Arduino Meetup Regular Debbie Davies. Their plan is to build a giant spaceship with lights for Burning Man 2013.

Here's a more detailed review by Matt Prentice.

Bring along your projects for the "Jam" session I was told. So late last week, I hacked together a demo of my rotary table project. I wired up a new potentiometer to replace a broken 10 turn one and pieced together a couple of examples to generate double height characters on the LCD display.



Source Code for Double Height Letters

After the talks the 4 speakers setup their kit on different tables and quickly had big groups around them. I was a little hesitant but Marc encourage me to demo my project so I setup at a table near the front. In true maker faire style "If you make it they will come" and shortly I had a few people quizzing about the project. I also had some valuable feed back from Matt who asked if I'd thought of turning the problem on it's head and driving the table with a motor rather than measuring the angle, he also raised the tricky question of "just how accurate does it need to be". I have actually looked at the motor approach and although not totally dismissed it, it does raise issues of size vs power for the motors needed. His second question has got me wondering, I'm going to re-read Ivan Law's book on gear cutting and have also quizzed the chaps on the Model Engineer forum.

References:

LCD "Bigfont" Numbers on the Arduino Forum
Hifiduinos LCD Custom Fonts, Large Numbers
Tom Torfs binary constant Macros

3D printing a pattern for sand casting

In Model Engineer No 4329 July 2008 there was a set of plans by Jan Ridders for building a coffee cup stirling engine. I liked the model but had some ideas for adapting it to make it a little more interesting also also a bit more like a steam engine design. I also started collecting bits to make the model, mostly thick aluminium sheet in the form of old frying pans.



The main changes for my design are the flywheel and the support for the wheel, otherwise the changes are cosmetic.

Back in 2011, I contracted "Roboraith" on Fiverr to build me a 2 part pattern in CAD for the flywheel from my sketches. After a few iterations we came up with something workable.



Retrospectively it would have been better to have a 3D printing expert advise on the design but at the time I did not know anyone with a printer. I found Richard Gain online and he volunteered to look at my files. I'd designed the pattern with pins and holes to fit together when in the molding box. However, when Richard looked at printing it there was an issue. The half with the pins is problematic in that there would be no support for the wheel once the pins were printed. We did conclude that printing two of the female parts and adding metal pins after the fact would work just as well. Richard printed me a couple in ABS as I was a bit worried that the PLA would not have good resilience in an outdoor environment.



I need to add the pins and check it works ok with the sand but I'm confident that it will make a good casting.

The next thing to convert to CAD is the support frame. This should be a lot easier to build as it's a single piece and is a straight extrusion of the 2D shape. I should be able to do that myself using Autodesk 123D which is a free 3D design application.

iMakr - The Opening of a 3D Printing Store in London

Today I joined 300 other makers, designers and tech enthusiasts at the opening of iMakr. In fact the opening has proved so popular that it has been spread over 2 days so that more people can attend.

The store describes itself as a "Total 3D Experience" and stocks a range of both 3D printers and 3D scanners as well as a selection of other accessories and software. IMakr plan to host events and demonstrations to allow you to learn and understand about this technology.

We had a bit of a slow start and quite a crowd built up on the pavement outside. I met a few interesting people in the crowd including printer builders, new startups and the people from Printcraft who link up to the big 3D printing services APIs to get your models printed.



Whilst we were waiting canopes were served on the pavement which was a little sureal. Eventually Sylvain Preumont arrived and gave us an introduction to some of the special guests such as Adrian Bowyer, the inventor of the RepRap and Sam Cervantes the CEO and founder of Solidoodle unfortunately I missed most of what he was saying over the noise of the crowd.



The entrance hall is also a showroom and demo space surrounding the stairs that lead to the lower floor. Above the stairs is a wonderful piece of 3D printed sculpture.



Around the entrance hall were demos of several machines and a stall from iMaterialise explaining their services.




Also next to the entrance is a spendid brick version of the IMakr logo which formed a backdrop to some of the speakers.



On the lower floor were yet more examples of different printers along with some 3D scanners and people who make things in 3D. Down here I also bumped into Richard Gain a puzzle maker and 3D printer owner who had printed me a flywheel pattern for casting just a few weeks before and not surprisingly we chatted about casting, puzzle making and 3D printing.

Scattered around the walls of the shop were a series of boxes containing examples of 3D printed items and artwork.



There were many examples of 3D printers and several of them were being demonstrated. Some of the people attending were overwelmed by the selection but this is kind of the point of the store which is to help people decide which printer to buy or if they want to get a service to do their printing instead.



The store is quite large but with 300 people it did get a bit crowded at times.



Along with the printers were a few scanners and there was quite a queue of people waiting to have their heads 3D scanned. One interesting item I'd not seen before was a 3D mouse which was coupled to the Anarkik3D software. This was particularly interesting as it had resistance feedback so you could feel the materials you were interacting with on the screen.



A very interesting place and a most enjoyable evening, a big thanks to everyone who had a part in the opening and the people I met.

The iMakr store will open from Thursday and can be found at 79 Clerkenwell road EC1R 5AR, London. Demos and other events will be running from next month.

A sand bag

"A sandbag? Are you expecting flood?" I was asked.

"No it's for metal forming" I replied.

When I'd been making the candle snuffer, I'd found it difficult to get the cone shape on the end and using the anvil covered the piece in scratches which then took effort to remove. Looking on google I found a few references to using a sand-bag or shot-bag to form curved metal which I thought should help with that kind of work and other projects.

Hotrod - Basic Techniques
Custom Classic Trucks - Sandbag Demo



I also found a couple of articles on making one yourself which inspired me to track down some materials and make my own bag.

EHow - Make a Sandbag for Metalwork
Makestuff with your hands - Making a leather sandbag for metal forming

Being in London I'm lucky to have a variety of shops within walking distance of my office. A web search showed that Borovick Fabrics on Berwick Street had leather for sale. I had a look at their big pieces and then spotted a "scraps" tub by the till which had a couple of pieces that were plenty for my needs and also at a very good price. They also had some thread but I needed to pop around to Kleins around the corner to get some leather needles.

Just a quick thought on leather needles.... If they are sharp enough to go through leather, then they will also easily go through your fingers!

I marked up the leather with some chalk in the shape of a large lozenge. I figured that having choice of a straight edge or rounded edge would be useful but it also got me a larger bag from the piece of leather. I also used some scraps to make a finger protector as after stitching for an hour my fingers got a bit sore.



After cutting it out, I glued the edges together with PVA and then stiched around to the end leaving a small hole for the sand. As you can see from the picture below this was perhaps a little small as I had terrible difficulty in turning it the right way around.





Filling with sand was delayed as I needed to dry the sand out to run through the funnel. Luckily it was a warm weekend and I left some in a tray for the afternoon. It takes more sand than you'd expect so best to dry more than you think you'd need. I glued and stitched up the hole, it seemed to be leaking sand slightly so I double stitched it. Hopefully it won't explode in use.



I don't have any imediate projects than require the bag but I'll let you know when and what it get's used for.



The total cost came in at about £7, I already had the sand but you can buy a 15Kg bag from Argos for £2.99 (prices correct at time of press). Note that many of the commercial bags you will find on the net don't come with content so you will still need the sand or shot.

If you are not up for making your own bag then it is possible to buy them. One supplier I found who does these is Stakesys, I've never bought anything from them but they come recommended by some of the people on the Mig Welding forum.

*** Stop Press ***

Since making this, I've had some issues with the sand escaping from the bag. I've reglued some of the edges but suspect this might be a continual problem. My options are to swap the sand for something larger, use a liner (perhaps a plastic bag) or just live with the issue.

*** Stop Press ***

Candle Snuffer

Another little mini project. Some of our candles can be a bit tempremental when blown out with wax flying everywhere and I burn my fingers snuffing them out "manually". So I went old school and made a little candle snuffer.

The raw materials are some thin copper tubing left over from the welded flea project and some copper pipe left by our builders, for those who are regular readers, yes they did leave a lot of stuff. I cut the pipe into a semicircle and after several attempts formed a cone using a couple of pairs of pliers.



I moved my hearth onto the main bench so I'd have room to work and clamped up the parts for silver soldering. I mixed up some flux and pasted it onto the joint. The soldering job was done with a butane torch and was straight forward and uneventful.



It needed a bit of a clean up and polish.



To give the handle a bit of bulk and thermal insulation (probably not really necessary) I wrapped it with some yellow Sugru.



All that was needed then was to test it out. The first pass was not quite perfect, I suspected air was getting under the rim. So I took it back to the workshop to flatten out the bottom a with a file. After that it was ready for use.

Mini Project - Making a Trivet

After the bathroom was renovated we had some marble border tiles left over and due to an accident with a photo I also had a broken picture frame. I decided to put these together and create a small potstand or trivet.

The frame was in 2 parts so I used the smaller parts to make a square frame by cutting new mitres using a junior hacksaw then gluing them together.



Then part of the hardboard back produced a platform for the tiles to sit on.



I repeated the mitring with the main frame taking note of a tip I'd seen on a dovetail cutting video. This was to ensure that the edges that will be visible look good. If the backside does not look so great this should not be an issue.



I glued up the frame and grouted my tiles into place. The corners wern't quite perfect so I used a little interior filler in those and touched them up with a permanant marker the match the colour of the frame.