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Well. I felt like it was time to make something really lowtech. I decided to make a voltmeter
but I didn't want to use bought electronic components.
It wanted to make it as simple as possible and buildable with the technology of 150 years ago.
The first thing to decide was the type of the voltmeter. A moving iron voltmeter wouldn't have been
very complicated, but it needs quite a powerful coil. Additionally it wouldn't be very accurate.
The typical analogue voltmeters are of the moving coil type, but these are really compilcate to make.
You need a very thin wire for the coil, tiny springs and a magnet with a matching form.
After some thinking, I decided to modify the moving coil voltmeter type. Springs are not really
feasible if you want it to keep simple. So I had to replace it with something equvalent. Rubber? No.
I would have needed a very thin rubber band. The solution: Gravity! It is anywhere (at least on earth),
costs nothing and ist quite constant.
Another problem is the tiny coil of the moving coil instrument. I decided to place the coil
aside and make the magnet moving. Now we have a ... well ... moving magnet instrument.
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In the front and the side view you can see the coil. It is the coil of an unlucky 12V relay. You could wind one yourself if you lilke, but you need lots of patience. I removed all ferromagnetical parts from it, including the core. The magnet would attract it and stick to it. The instrument's needle is a simple drinking straw. On the lower end I hot-glued a small but strong Magnet to it. If current flows throught the coil, the coil repels the magnet and moves the needle. The needle axle is made of a sewing needle. Two small metal sheets (you can see only one in the pictures because the second one is behind the cardboard) keep the axel in the center. They were cut out of an old metal can. The bearings are small plastic pieces cut out of a plastic cup. I think metal would have a lower friction, but it must be amagnetic because the magnet is very near. Aluminum from an aluminum can should work, but I didn't have it handy. The coil must repel the magnet. If it attracts it, the magnet will accelerate towards the coil, because the magnetic field gets stronger as the magnet gets closer and "clack" the magnet sticks to the coil.
As you can see, the the instrument is highly nonlinear and has a voltage range from 0 to 12V. From 0 to 1V you can even read millivolts, but the resolution drops as the voltage incrases. Zeroeing the instrument is easy, simply tilt it ;-).
PS: Please keep in mind, that this type of instrument wont work in space as it needs gravity.