Jeff, all,   Latest news from the front...   I wrote a program for the Arduino that allowed me to calibrate it for each of the six roads. This worked reasonably well, between the roads at one end, but when I turned it round, it was slightly out of position. Within a couple of half turns, the positions were well and truly wrong. Frustrated, I wrote another sketch, to do simple half, and eventually, full turns. I used the calibration bit of the program to work out the steps for a full turn