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Avoiding electrocution while testing 240v circuits

3 Jan

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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.

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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.

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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.

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

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Laser cut numbered pegs for workmen

7 Nov

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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.

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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.

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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!

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

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

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

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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

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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.

 

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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.

 

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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).

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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.

 

 

Compact ATX power supply to lab supply conversion

7 Oct

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

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

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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.

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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.

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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.

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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).

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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.

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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.

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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.

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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.

Cheap laser engraved plant markers

5 Oct

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We recently planted a whole heap of berry plants (Cranberries, Chilean Guava, Orangeberry, Loganberry, Boysenberry, Blueberry, Blackberry, Black Currant, Red Currant, Gooseberry, Pomegranate, and Strawberry).  They came with stickers, but we wanted something more permanent.

 

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This is the berry patch (and the neighbour’s house)  it’s hard to tell but there’s about 50 plants in there, excluding the 100 stems of raspberry.

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I bought a cheap ($7) box of kindling from the supermarket. These boxes came with various sizes of wood, but the one I picked had a lot of 300 x 50 x 10 (about 12″ x 2″ x 3/8″) pieces, at least at the top. I probably got at least 20 good pieces out of the box, and the rest was good kindling anyway. I imagine this is all cheap pine (roughsawn).

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The result was not bad, certainly good enough for a quick and dirty marker. My first experiment was the one at the the top (Orangeberry). This was RASTER engraved and ended up about etched about 1mm deep. It was clear and readable, apart from the smoke damage at the top. The downside was that it took ages to burn – nearly 20 minutes just for one marker.

Inkscape, once again, came to the rescue. There’s a very handy extension under Extensions – Render – Hershey Text, which takes text and writes it in VECTOR format, using the sort of fonts that were used by pen plotters in days gone by. It’s very readable, and far, far, faster. The slowest label above took under 30 seconds to draw.

Note: Tucked in the documentation was a suggestion to run Path – Simplify on the text produced. This only takes a moment, and it does smooth out the text and make it look nicer.

 

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These were the engraving parameters I used on my 80W CO2 laser. Basically 85%, but reduced slightly on the corners. However, to make the text more readable, I mis-focussed the laser. I set the focus distance with a 12mm thick block sitting above the target. This made the lines quite a bit wider. Interestingly, it also changed the cross section to a much more V shape, rather than the usual |___| shape.

 

 

 

 

 

 

 

 

Laser cut boxes to tidy kitchen bench

2 Oct

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A quick project, most of it done the night before a party.

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My wife complained about the mess on the kitchen bench. Lots of little bits of “stuff”, of many sizes. I thought about making a partitioned tray, but decided we might do better with an array of small boxes, allowing flexibility to use the space efficiently, and to swap boxes around if one section outgrew its container. Also, as anyone who’s had to get pointy things (e.g. drawing pins) out of a fixed compartment will attest, it’s much easier to lift up a box and empty it out.

 

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We weren’t too sure what would be a good base size for the boxes, so I ran off a few samples of different sizes. We tried a 40mm x 40mm, and 50mm x 50mm, but quickly found that 60mm x 60mm worked well. We settled on 30mm high. Since these were just test boxes, I didn’t bother flattening the top edge, as that was the slowest step. These are all made from (cheap) 3mm mdf.

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All of these boxes were designed with the wonderful (free) “Tabbed Box Maker” extension for the (free) Inkscape program. http://www.inkscapeforum.com/viewtopic.php?t=18315

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The interface is very simple – just put in your dimensions and instantly get all the tabs worked out. It’s *much* slower doing it by hand, as all the tabs have to be a bit wider than the slots, to account for the laser’s kerf (cutting width).

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The laser cuts quickly, but do enough cuts and it adds up. For 5 of the 60×60 boxes and 5 of the 120×60 boxes, the cutting took 17 minutes. I really need to realign the laser as I’m sure I should be getting better speeds. Here are the 50 pieces required.

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And here is the remainder after I cut them out. This is a 600×400 piece of mdf, worth about $1. Alert viewers may notice a problem with the shapes. I carefully flattened one edge of each side, so the the top of the box would be flat, not crenelated. However, in a moment of dumb, I flattened one edge of the base (far left) as well. Had to recut those.

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Here are a few of the boxes. Default settings in tabbed box maker gave me parts that fitted together easily but tightly, and could be squeezed into firm position. They held together quite well, but I added a bit of PVA wood glue just for certainty.

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The result looked very promising. I burned a second set of 5 x 60×60 and 5 x 120×60, and one long 300×60 for scissors and a letter opener. Much tidier.

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I wanted to store pens and markers as well so I made a double height box (60x60x60) and some dividers.

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When you get the measurements and calculations right, laser cut parts just slide together in a very nice way.

 

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Here’s the final (so far) result. It does spread things out more than the original piles, but at least you can find things. When we decide it’s finished growing, I’ll make a laser cut tray that just fits around the whole set.

Tidying up cables for the laser

26 Apr

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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.

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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).

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

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The result, fresh our of the laser.

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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.

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Now an application of black spray paint.

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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.

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

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