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Avoiding electrocution while testing 240v circuits

3 Jan


I needed to measure the output voltages on a bunch of unmarked transformers I’d accumulated. (No, I’m not a hoarder. I can stop any time I want to. Perhaps.)

I’ve done this before, with just a bunch of alligator clips, a mains cord, and a multi-meter. It’s very simple, but it’s *dangerous*. Reaching over live 240v wires to take measurements isn’t a good idea. Worse is that a bunch of wires in mid-air have a tendency to move around, generally shorting something wires together.


I recently made up a number of little test boards to hold test circuits in place without having to worry about wires getting loose. Here, for example, is a test with an arduino (on a solderless breadboard), an L298 motor driver, and a worm geared motor. The boards are 85mm long, with two rows of M4 holes 10mm apart. The rows are 75mm apart. They’re mounted to a laser cut piece of 6mm mdf, with two long rows of M4 tapped holes. (MDF taps quite well with a tap in a cordless drill).

I really don’t know, yet, whether these ‘boards mounted on boards’ are a good idea or not. However, they’re cheap and worth trying. In particular, they hold wires securely for testing which was exactly what I wanted for testing my transformers.


Here’s a test victim hooked up to a test setup. I grabbed a spare chunk of 12mm MDF (I didn’t even bother cutting it square). I marked a grid of cross marks at 10mm x 25mm spacing using the laser cutter, then drilled and tapped just the few holes I wanted to hold things down. Chocolate blocks, hot melt glued down, gave me a secure but adjustable fixing for wires.

Note that I used a female EAN mains plug rather than a standard power cord to supply power. I physically unplugged it every time I changed the circuit, which I probably wouldn’t have done if it was a plug into a wall socket.

This particular transformer made me glad I’d gone to the trouble, as it was very easy to power it off when I plugged it in and it started to hiss and smoke. Looks like I picked up a 110V primary transformer somewhere along the line and it *really* didn’t like running on 240V.


Even when I ended up using crocodile clips, they were much more secure when clipped into the immobile terminal blocks.

A quick test of a crude opto-coupled triac circuit felt a lot safer when wired up like this as well. I wouldn’t trust a solderless breadboard at 240V.


Laser cut numbered pegs for workmen

7 Nov


We had a tradesman coming out to do some “stump munching” of some of our (large) supply of stumps. We only want a few done, and some of them are in the deep grass. So, laser cutter to the rescue.


Here’s the laser cutter chopping out some mdf numbers – about 80mm wide x 140mm high. Mdf is 3mm thick, but I should have used 6mm.


For a quick job, they came out ok. The bright yellow plastic would be really good for this but it’s PVC. Cutting pvc on the laser will rot your lungs from the hydrochloric acid vapour. Worse, it damages the optics of the laser!


A very quick spray paint with flourescent pink from a rattle can.


Some cheap bits of wood. Once again, a box of kindling from the supermarket.


Bits of wood pointed on the mitre saw, and pink numbers stapled on with the air nailer.


These did the job. You can see them from some distance. Next time I’ll use thicker material as the staples pulled right through on a couple when I hammered the stake into the ground.



Quick laser-cut carpentry clamp storage

30 Oct


There’s a saying in carpentry “You can never have too many clamps.” This is true until you need to store them. I’ve got a bunch of these cheap “F” clamps which make a messy pile. I knocked this holder together out of 3mm mdf. The slots are 5mm wide and go back 70mm.



As usual, I used the very handy “BoxMaker” extension for Inkscape, though I drew the slots in DesignSpark Mechanical. Note that the top layer (with slots) is doubled up with an extra layer to make it stronger. It might have been OK like that but I later added some simple braces. After that it was plenty strong enough.



Here it is attached to the wall. (It’s actually attached with a French Cleat so I can move it around if I want to).


And here it is fully populated. I screwed another piece of wood to the bottom of the back, a bit thicker than the cleat at the top, so the top now slopes towards the back by a few degrees.

Very quick. Very cheap.



Laser-cut dogbone wire storage

29 Oct


Some convenient storage for small amounts of wire.



The problem. I’m a bit of a hoarder, I admit. This is a tangle of twisted pairs of wire, each about 2.5m long. It’s messy, which means it’s in the shed, rather than the craftroom where I’m likely to use it.


I tried cutting a spool out on the laser cutter. This is a 120mm diameter spool, with 3x tabbed plates in the centre. It fitted together very well. Once I squeezed it in the vice, it held together with no problem, even when not glued. However, it’s far bigger than I need.


I made some smaller spools. These are just 80mm across. Again, they fitted together well, and they are a more convenient size, but still too much work.

About then, I realised I really didn’t need anything so complicated (though I’ll use the spools for some things, I’m sure). Instead, I went with a very simple, and therefore quick to cut, ‘dogbone’ shape. These are 80mm long, 40mm wide, necking down to 20mm. All cut from 3mm mdf. I can cut 6 of these in the time it takes to cut one of the small spools.


I designed a 3d printed holder that the end will just fit into, located by the shape and the 4mm hole/pin. There’s a hex shaped handle to fit in a cordless drill. A clip fits over the side to keep it in place, though the fit was actually so firm that I wound some without bothering with the clip. One of the nice things about this was that I could use the .dxf file from cutting the dogbones to define the shape. The actual size of the mdf part was smaller all around by 0.1mm (half the 0.2mm kerf of the laser) than the design specified. That proved just enough margin to get a good fit.


Here’s the 3d printed holder with the laser-cut dogbone in place.


Mounted in a drill chuck, with wire being wound on. I wound with very little tension which makes for a messy coil, but I hadn’t bothered rounding the edges of the mdf so I didn’t want too much force kinking the wire. I did design the centre of the dogbone to be just big enough for a wrapping of 50mm/2″ masking tape, but didn’t bother. If I want to use this for valuable wire, I’ll just 3d print some barrel pieces to make it round.


This is the “after” shot. I’m *much* more likely to use up this wire on projects than when it was a tangled mess. I cut 42 dogbones out of a $1 piece of mdf, though I could have got 60 out with a more careful packing.

Compact ATX power supply to lab supply conversion

7 Oct


I wanted a simple 12V + 5V supply of power available on my (already very crowded) desk. There were already two other supplies in the room (one of them a fancy fully ajustable lab supply), but they were … metres away. I wanted something right beside the keyboard for powering Arduino experiments, etc.

I took a standard PC power supply, cut off the plugs that go to the motherboard, and wired a small (100mm x 75mm x 30mm) box onto the end. The actual supply is tucked a few feet away.

This is my original 12V/5V conversion. It’s ugly but it’s been in use for nearly 20 years. At the time I didn’t know what types of (cheap) connector would be most useful. I fitted a push to open, release to clamp fitting off a stereo, a ‘chocolate block’ screw terminal block, some RCA phono connectors, and a solder tab that alligator clips attach to easily. In actual fact, I’ve found the RCA connectors the most useful. They’re quick, easily salvaged from old electronics, and I haven’t melted any yet, even running 5+ amps through them for hot wire cutters.

Here’s another compact conversion – just a simple terminal block with GND, +3.3V, +5V, +12V, and GND again.


And another one where I unsoldered everything except three of the 12V lines and three of the ground lines, plaited for convenience.


Here’s the victim – a bog standard supply. I’ve cut off the mother board plugs and removed the zip ties. Note the messy tangle of wires.


I used the laser cutter to make up a couple of “cable combs” to tidy up the tangle of wires. I split the wires into two sets, as they were different lengths.


Here you can see a comb in use, taking a tangle of 12 wires and keeping them nicely parallel as I zip tied them. A side effect was that the resulting cable was much straighter, without the curves that were in the original wires.


This is the unit that will sit on my desk. A simple laser cut tabbed box (yay for Inkscape and the Tabbed Box Extension). There’s a chunk of angle iron (20x20x3) glued in for weight, and beefy resistor to put a load on the 5V rail. (I believe most modern supplies no longer require this). Two rows of RCA connectors – 3.3V, 5V, 12V. Since I long ago standardised on red for 5V and yellow for 12V (same as the power supply wires), I don’t need to label them. There’s a socket (black) for the 5V standby which stays on when the supply is turned off, a switch which turns on the supply, a power led, and another press connector stolen from a stereo. (They come red and black, I painted one tab yellow).


As I mentioned, some of the wires were shorter, so I decided to add in another box with just 5V and 12V and some salvaged connectors. This box was made with thicker 6mm mdf to give it some weight. The black objects were 3d printed covers to disguise the salvaged state of the connectors.


The printed covers did make things look tidier. However, they were a BAD IDEA. It took way more effort than they were worth, and the one fault I found when I tested the system was in one of those covered connectors – now potted with hot glue and no longer accessible.


I rebuilt the “secondary” box with a new design. I made another 100x75x30 tabbed box in 6mm mdf, but this time with a cutout area and four 3mm holes. I tapped the holes, M4, with a tap in a cordless drill. A 3mm mounting plate goes over the top, this time with proper panel connectors, and screws down. This was much better than the first attempt, and I’ll use the technique again.


Here’s the result. It all took more time than I’d expected, particularly the actual soldering of the wires. With so many wires fitting in a small space, I had to do some messy joins. Lots of incentive for me to get on top of laser cut/etched printed circuit boards.

However, I’ve got a useful tool that may get me doing more Arduino/robotics projects. I also learned a lot about using Inkscape, and found some techniques for making enclosures that I will definitely use again.

Tidying up cables for the laser

26 Apr


I’ve had the new laser cutter hooked up via various extension cables and plug boxes. Every time I turned the shed lights on, I had to listen to the sound of the laser’s exhaust fan and air pump, as it was too much trouble to hunt down the appropriate cables and unplug them when I wasn’t using the laser.

I decided to make it easier on myself by using a switched plug box – but then I needed to remember which switch turned on which device.

With a cad diagram sorted out, I grabbed a random chunk of white painted hardboard (I have no idea what it came off – probably some bit of demolished furniture). I covered the surface with masking tape.


Next step – burn all the mounting holes. There was some metal shelving near the laser, with holes at 50mm spacing, so I made holes for attaching the board using those. Also holes for zip ties, and for the plug board (which I’d hot glued small feet to).


Now, lightly engrave the text, along with some indicating lines. The sockets on the plug board weren’t evenly spaced, hence the odd angles.


The result, fresh our of the laser.


I peeled off the masking tape over the letters. It came off easily, but I should have been more careful to press the remaining tape back down. I’d wiped the board down with a damp rag before I applied the masking tape (it was very dusty) and the heat of the laser may have bubbled the remaining moisture.


Now an application of black spray paint.


After an hour, I peeled the remaining masking tape off. There’s a few blotches, especially near the bottom, but it’s certainly readable. The letters are about 50mm (2″) high.


The final result. A bit rough, but quite satisfactory for a first try, and it should do the job.


A (cheap) wood rack

9 Mar

I had a horrible mess in my wood storage area, so I made up this simple rack to tidy it up.

wood rack 023

This pattern of wood rack (with storage bins on one side and sloping sheet goods storage on the other) is well exampled on the net. In particular Steve Ramsey’s nicely documented build make a rolling lumber cart ,  various designs on LumberJocks, and this nice English/French site rangement-pour-panneaux-et-retailles. However, most of the examples I found used sheets of 3/4″ (20mm) plywood which is very expensive in New Zealand. Besides, a lot of my wood was scavenged and free, and I didn’t want to spend good money on storage for it, so this rack is made from some thin 9mm ply and a lot of scraps. Another difference is that this isn’t a mobile cart – castors won’t work well on a dirt floor.

The area was very messy. (The pink and blue cupboards used to belong to my twins when they were much younger).

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The first step was to make some temporary covered storage using some pallets, a tarp, some eye-screws, and a few bungee cords.

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I decided to make the storage bins 400mm x 400mm, which is wider than the capacity of my (small) tablesaw.  That meant clamping a temporary worksurface to the saw top to make the cuts.

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The only flat surface for assembly, anywhere near the shed, was the bed of my trailer. It worked quite well. The lines on the plywood mark some treated fence palings to raise it off the ground.

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Some free offcuts of mdf supplied spacers. I clamped four together to ensure each set of spacers were the same length.

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It turned out that four sheets was a trifle too much depth for this 200mm (8″) mitre saw, and I had to tidy up one corner on each batch.

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Once I had my spacers and the 400×400 squares of plywood, I assembled a set of bins. Rather a messy operation and a crude looking result.

wood rack 011

Now I could start assembling the structure in-situ.

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The shelves for the centre were, again, thin 9mm ply. That meant I couldn’t screw into the side of them so I cut small rails for them to sit on. One of my recent pickups was some 2.4m long strips of 20mm plywood, but only 20mm wide. These worked very well for rails. In the photo below, I am setting up to cut a 10 degree bevel on the edge of one of these 20×20 strips. It’s the sort of cut which isn’t really feasible without a tablesaw.

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I fastened the rails to the shelf before I attached them to the sides. This worked well.

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For the first (lowest) shelf, I was able to put some plastic buckets and some 4x2s down to support the shelf while I fixed it in place.

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Working my way up, I soon had the other shelves in place. (One of my daughters helped with this bit which made it easier as the two ends are 2.4m apart and the area was so messy that moving around wasn’t easy).

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Once I started filling the rack, I discovered quite a lot of nice bits of wood that I’d forgotten I had. The final result didn’t really pack quite as much in as I’d hoped, but it did make it far easier to see what I had. It also got rid of a major problem of wood lower down the pile developing a curve.

Large sheets are stored on one side, while thin but long strips, metal rods, etc go in the shelves.

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With planks, dowels, and narrower offcuts of mdf, going in the bins on the other side.

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