The PRS1 is a versatile, reliable and economical solution to most machine speed switching problems. It is not affected by overspeeding and will operate in either direction of rotation. The heart of the PRS1 is a solid-state CMOS logic circuit, that continuously measures the elapsed time between successive trailing edges (negative going) of an incoming pulse train or waveform. It compares this time with an adjustable set-point reference and determines if the period of the waveform is longer (underspeed) or shorter (overspeed) than the set-point reference. The internal relay is energized during "normal" operation and can be set to de-energize on either under or overspeed by a programming switch. The unit can be connected for simple over or underspeed detection or for latch/trip-off operation in a machine STOP/START circuit. The PRS1 features built-in hysteresis (differential between pick-up and dropout) of 5% of set-point speed. This prevents the output relay from chattering when operating at or near the set-pint speed. The set-point control is a 20-turn screwdriver adjustment accessible at the top of the unit with a 10:1 adjustment range. An LED indicates when the relay is energized. A built-in +12 VDC regulated power supply, furnished power for the internal circuit and for external sensor excitation. The plug-in module mates with a heavy duty, CSA approved base mounting socket with pressure clamp screw terminals that accept stripped wires without lugs. Closed back construction allows mounting directly on metal panel without an insulation barrier.