For Kirby Legend and Heritage vacuums that HAVE a belt sensor on the power head (green light).
The sensor is the green light that goes on while the vacuum is running to reflect that the brush roller is spinning. The light goes out when the vacuum is off or the belt breaks.
Replacement vacuum cleaner roller with sensor magnet for the Kirby:
Heritage and Heritage II
Legend and Legend II
Replacement Kirby part 152584 vacuum brush roller designed to fit Kirby.