WARNING !
This thread is not '51 related. The target MCU may appear bigger than it is.

So I have a radio antenna sitting on top of my radio mast and there is a stepper motor assembly with gearbox to rotate that antenna. That stepper is driven by a MCU which communicates with a box on my table. When I turn a knob in this box, the antenna rotates and the LCD display shows the new antenna direction.

However - that LCD is so boring...

So I took a dive into my "junk" box and found a Magnesyn compass indicator. This has been sitting in a Mustang... The thing has three coils in 120 degree angles and a permanent magnet in the center. The needle can freely rotate and the diameter of the instrument is about 7 cm. Definitely cooler than the LCD - especially with the green cockpit light turned on..

Originally this was driven with 24 volts @ 400hZ and there was the sensor in parallel with the indicator. The internal connection is such that the coils are in series so there is four wires coming out of the thing.

I have already set up a PCB that has Atmel AT90S1200 and some logic for the 12 position switch that is used to set the direction. Four of the port pins are used to drive the instrument. The output from the MCU @ 5vols is more than enough to rotate the thing. It already started moving when I measured the resistances of the coils....

My intention is to imitate the original drive signal with four square wave outputs. It is my assumption that the phase differences determine the needle position. Note: Originally this was driven by AC.

I will post the pictures and the program here - maybe there is someone who likes ancient Atmels and WW2 instruments.