Wow this thing is cheap. I shouldn't be surprised, it was!

First thing was to connect it up to an Arduino running the Servo Sweep example just to check it works. It does, but it's pretty noisy!

In trying to connect it to the gauge, I'd been hoping that I could attach the needle to it directly. It seems that it's too big for this though. So I'll have to keep the needle on it's original bearings and drive with a linkage. Which I'm quite happy about as they're beautiful construction. Destroying them would be a shame.

Offering up the servo and moving arms around suggested that the best place would be just on the side of the needle frame. Then a link to an arm on the needle shaft. Of course I need something to link the two - not having any rod lying around, a stray resistor was pressed into service.

A dab of superglue to hold it all together, then connected back up to the Arduino to check servo/needle movement.

Surprisingly, nothing bent or fell off! I had to adjust the linkage a bit to get the right ratios and range. I left it running for half an hour to check for security, and everything seems good.

Now then, I need to run this from the Pi.