An easy-to-use inductive proximity shaft speed monitoring device with an adjustable 0 - 30 second start-up delay, all housed in a 30 mm diameter moulded body. The self-contained unit has a single set point, which signals when the shaft speed has dropped by 20% of normal running speed. It detects dangerous slow down and underspeed conditions on elevators, conveyors, air locks, mixers, fans, grinders and other machines