Thursday, October 8, 2009

Eliminating Trapped Gas

I shoot CO2-powered airguns a fair bit for amusement and vermin control, but some guns get through more gas than others. Unfortunately, if you leave a CO2 cartridge in the gun for more than a week or so you risk getting it stuck against the seal - my possum carbine may sit there for a month without seeing action in the winter. Lubricants like molylube and silicone grease just muck things up, so I tried PTFE threadseal tape:

It works really well. As an added precaution, change the cartridge when there is a small amount of gas remaining to blow the spent cartridge cleanly off the seal.

The tape goes just once neatly over the cap on the end of the cartridge, back down to the shoulder, then wraps around a couple of times to stop the tape coming off in the guts of your airgun. Don't put on too much tape if loading into a tight-fitting or corroded cartridge holder or you might wedge it in.

Vik :v)

Saturday, September 5, 2009

Father's Day 2009

Well, locally it's Father's Day. I had a very nice breakfast cooked for me by my daughters, and was then presented with the following very appropriate card:

Note the garnish of bright orange G-clamps, and a dash of camouflage gaffer tape on the otherwise black tape card. MacGyver would have been proud. I certainly was.

Vik :v)

Thursday, August 13, 2009

The Spoondriver

Being in search of a way to build repraps, I pondered on ways to remove the need to solder circuits together, and an electric motor or heater driver seemed like low-hanging fruit. Go minimalist, I thought. "What would McGuyver do?" I asked myself. Then a hunger pang led to the thought of the awful cheap spoons in the kitchen, and an evil plan formed.

Well, evil if you're a spoon.

The simple solution does away with circuit boards and solder altogether. Three cheap components here to get from the electronics store: A TIP120 series transistor (121 or 122 is fine), a honking big diode (IN4007 or IN5404 are good) and a 1K resistor (any size, brown-black-red lines on one end). Get some small terminal strip while you're there. All the bits look completely different and have an obvious way round when it matters. If you're smart enough to figure out that the insulation needs to be stripped off the wiring before it goes into the terminal strip, you can build this driver.

Shove the leads in the terminal strip - not too far or you'll stop the wires coming in the other end. This might involve technical things like cutting the component wires shorter. In the pictures, the big red & black wires are +12 volts and ground. The yellow wire is the signal wire that connects to your Arduino, clone or Pinguino. The thin black and red wires connect to the load, in this case a small fan. It matters which way round those fan wires go.

It works. But it needs a heatsink to drive loads of a few amps. Bad news for spoons. Having torn off its head, I filed over the wound, bashed its neck flat, drilled a 3mm (1/8") hole in it and bolted it to the hot transistor. Bwahahaha.

The other stuff is an EasyDriverV3 stepper driver that needs eight wires soldered to it to make a stepper motor move. The cheap little switches will replace the opto sensors. That'll have to do for this round of reprap simplification.

So, spoons, don't mess with me!

Vik :v)

Thursday, July 2, 2009

Kettle with temperature readout

My wife needed a source of water for a speciality diet food - let's just call it "Optipuke" - that was at a temperature of 50 degrees C. Any more, the medical qualities of said food got lost. Any less and it tasted somewhat tepid and even worse than usual. Kettles seldom come with a temperature readout, so I purchased a US$8 probe thermometer rated at 150C from DealExtreme (I have an addiction). Fortunately our kettle fills through the spout, so the lid was free to play with. The only other thing I needed was a 10mm long piece of silicone tubing (rated to 300C) that just fitted over the probe.

I took the knob out of the lid, which left a hole that was conveniently just the right size for the thermometer. I then inserted the probe and slid the piece of silicone tubing firmly up to the top . This is essential to stop steam coming out of the lid hole, and stops the display rotating when you move the kettle. The probe I bought seems to be reasonably resistant to steam, and so my wife's dietary needs are now met.

As a bonus, when the diet is done I can dismantle it all and still have a functioning kitchen thermometer.

Monday, June 1, 2009

Electronic Stethoscope

Having broken the earhole end of my stethoscope, I started wondering how I could improve it. I've long fancied building an electronic stethoscope, but it has previously gone in the "too hard" basket. So I took the diaphragm end off it (the cold part) and set about finding an amplifier. Good old came up with the goods in the form of an "iSpy spy ear" for under 3 bucks.

The spy ear is a simple, 3 transistor amplifier with an ordinary little cylindrical microphone inside. I used heat-shrink tubing to attach the microphone to a short piece of plastic tubing from my hydroponics. It has an ID of 6mm and an OD of about 8mm, so it just happened to have the same OD as the microphone and take the end of the stethoscope.

Opening up the spy ear was easy; there's a single screw in the battery compartment. The casing needed to be carved out a bit with a Dremel, and then everything was pushed back together with a bit of hot Polymorph ("Shapelock") around the microphone to make sure it all stayed together. It's normally white but I happened to have some dyed black with candle soot. The wires attached to the microphone seemed rather weak - both broke - which is why I was keen on making the connection to the stethoscope diaphragm as sturdy as possible.

The end result: an electronic stethoscope. It's not wildly better than the ordinary kind, but it has a few distinct advantages. First off, when you knock the earphone lead it does not make unwanted noise like the tubes on the conventional type. Secondly, I can plug a powered speaker from the PC into it to help other members of the brigade listen to things as a group on training nights. Finally, if the surroundings are noisy like they tend to be in the back of an ambulance I just turn up the volume.

I'm going to get a set of in-ear earphones on a retractable lead and see how well it actually performs in the field. I know that commercial electroscopes are available, but not on my budget!

Vik :v)