Archive | January, 2017

Christmas Lights Obelisk/Pyramid

9 Jan

Barbara wanted some pretty Christmas lights. It was a bit late for Christmas (i.e. it was January). So?


We started with some 100×25 Oregon, ripped in down to 50×25. Here I’ve chopped four 1600mm lengths to form the sides of the pyramid/obelisk. We found a nice looking christmas tree on Google Images then used a graphics package to measure the angle – turns out that 80°  from horizontal was the angle we thought looked good.


Some simple trigonometry gave us the size of the base (close enough to 500mm), to get the angle we wanted. Some more 50×25 oregon and various offcuts from the mitre saw gave us a base.


We braced it up for strength, since I planned on placing one or two concrete blocks onto the base to weight it down. (We get strong winds around our place).


Some 7mm holes and M6 bolts attached the ‘legs’ to the base. Forgot to allow for the 25mm offset from the ends of the pieces of wood, so our angle won’t be quite correct.


A very sophisticated (not) mechanism fastened the top together. One zip-tie.


Here’s the result so far, with a human for size comparison.


Chicken wire! Messy stuff to work with, but a compressed air stapler made it easy to pin it down. We only put wire over three sides, leaving the fourth side open to allow placing concrete blocks (for weight) and christmas tree lights (for pretty).


We put it up on an earth bank overnight, to check it wouldn’t blow down.


Barbara fitted the lights through the chicken mesh. Took quite a while. Here we ran a quick test in a darkened room. Looking good!

Come nightfall, it was very pretty.


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.