So, next project!

I'm not certain if this will do what I need but with any luck it just might. Ah, yes, the task? Well I have microphones. Unfortunately they need to be amplified before being suitable for use with most consumer-grade gear. They don't need anything beefy for this though - what they need is a pre-amp to bring them up to the right level.

The idea being that one could go out to interview someone with a couple of microphones and some cables. These could then be wired to a phone or MP3 player for recording. The gear to conduct the interview would fit easily in something as small as a handbag.

When recording multiple people one tends to find that they speak at different volumes. Not a problem in person (usually...) but to the audio gear they'll sound vastly different. The basic answer to this is going to be keeping the two sound channels separate. A 3.5mm stereo jack will be used for both input and output. Traditionally one would use 1/4" (6.35mm) audio jacks or even XLR plugs (to match the microphones). No dice for this job though as what they're going into is a L├Ąckerol tin I got from Scandinavia. There's room for a 9 volt battery and a bit more. With any luck I'll have the space to get the whole thing to fit!

Pretty small, as you can see. Here's it without most of the parts:

It's going to be tight!

So what's going in side?
Well the plan is for a slightly modified version of this project.

Well I don't know about you but some of those symbols aren't ones I initially recognise (I think they're ANSI symbols) so I did my best to redraw the diagram into something I'd stand a better chance of understanding at a glance:

Hopefully I didn't balls that up in some way. What you can hopefully see is that there's only one potentiometer in the circuit diagram (although there will also be a trimmer replacing that circled 100K resistor). There's two on the tin though...
Yeah... I need to get the circuit to fit twice! Stereo, y'see?

Also not pictured are the two 3.5mm audio jacks that are part of the design. Worst case scenario I could instead wire those to actual wires that hang out of the case but ideally I want it all to be internal. It's not all that many parts, thankfully. The ICs are LM741s that are fairly tiny, a few capacitors and resistors, and the rather small trimmers (potentiometers that aren't expected to change after being set).

So, things that need to be done:

  • Missing parts: 2012 me ordered the parts and missed out the 47K resistors I need. He also ordered linear potentiometers rather than logarithmic ones.
  • Wire. I could really do with a colour other than white!
  • Insulation. I'm not sure if it's required but it might be a good idea to lay down some plastic inside to keep the parts isolated from the case. On the plus side the casing should help filter out RF interference!
  • Knobs. It might be fun to add some cool knobs to the volume control pots.

Disclaimer: I'm still fairly new to this sort of thing and I've only just managed to solder something that wasn't a complete pig's ear. With that in mind let's dive in to what I've been working on in my spare time.

Many years ago I got my grubby meathooks on a book Games Workshop used to publish "How to Make Wargames Terrain". Not the flashy 2004 version mind you - the 1996 version. In all honesty I found it fairly worthless and uninspiring, especially in a time before the kind of tutorials we have access online today. There were a few nuggets of wisdom in there though such as the existence of this tool:

(From the original book)

Now I don't know about you but I'm not good at spending medium amounts of cash on myself. I'll save up for something expensive and buy it, I'll buy the odd small treat for up to about £7.50 but anything above that just feels a bit too much for just me. Well hot wire cutters fire into that price range (plus the cost of 4.5V batteries if one goes for the cheaper battery operated ones).

These days I'm older and I've got a lot of odds and sods lying around. One of these things is an ancient computer power supply. We're talking 140W.

It's on its own though so there's no way to turn it on. The kettle lead is plugged in but without a case button wired into it there's no on switch. Well that is at least easy to fix:

The black leads are all ground. There's only one green wire and grounding it immediately turns on the PSU. Sorted!

Right, then what's next? Well we've got lots of connectors that have both red and yellow wires. The red ones are 5V rails while the yellows are 12V. Oh and before we go on these are DC. The PSU's job is to convert AC input to DC output. 12V DC is, for the most part, safe. Probably unwise to handle it with wet hands, admittedly, but under normal conditions skin resistance prevents 12V being enough of a gradient to be dangerous.

The next component is NiChrome wire. A few metres can be had on eBay for bugger all. I bought the smallest amount I could - a metre of 0.5mm stuff - for £1.50 including postage. Man, making stuff was so much more expensive before eBay.

Ideally the wire for the hot wire cutter needs to be kept under tension. It's fairly stiff stuff but as we all remember from physics classes: metal that heats up expands. Hmm.

So I needed a device that could:

  • Keep the wire under tension even as it expands
  • Have plenty of safe handholds
  • Not melt when if exposed to a little stray heat
  • Take up as little space as possible

Lots of people make these using PVC pipe but I haven't got any that's the right size so that idea fell at the first hurdle. Wood would work though, assuming it was reasonably tough. I took a look in a nearby shed and found an extremely dead deckchair. The canvas had rotted off and it was riddle with woodworm and rot. Luckily some parts of it were still dry and reasonably tough.

A bit of sawing, drilling, and wiring later and I had a pterodactyl!

The rubber band keeps the wire under tension and the rest of the design allows it to be collapsed when not in use. In theory I could also mount a longer wire if I felt like it. Attaching one wire to a 12V rail and the other to a ground rail provided me with something that happily chews through even thick insulation foam:

It actually runs a little hotter than I'd like and so I'm considering testing it on the 5V rail to see whether that makes any difference when cutting simple polystyrene. Its current heat level is fine for thicker foam but it goes through white expanded polystyrene like it's not even there - not so good if one doesn't have a steady hand. A switch between the two might be a good addition if that works.