On 18/04/2019 17:32, Harry Bloomfield wrote:
One idea I have is a magnet fixed to the edge of a pulley, rotating past
a coil powering an LED mounted on the tractors dash

Thats what Kirby vacuums use a magnet on the roller brush and it rotates
past the coil lighting an LED, they use it to show if the brush stops
rotating ie the belt has snapped.