The SS300-24 Electronic Speed Switch is a 24 volt single set point speed switch with SPDT relay dry contact output, which uses a magnetic pickup input signal. The trip point is set by a potentiometer.
A LED indicates when the signal source is present. A second LED turns on when the trip point is reached. An optional time delay is on board to delay operation of the relay for 2-6 seconds after the set point has been reached.

This model has several applications: It can be used for overspeed shutdown, which will shut down the engine if RPM exceeds the pre-set limit. It can be used for speed sensitive pull-in/drop-out, which will engage or disengage PTO's, 4-wheel drives or other switch points according to the speed of the engine being monitored.
The unit can also select between two pressure switch set points according to speed of the engine or transmission and thus give maximum protection to the equipment while at operating RPM and eliminate nuisance alarms when at idle.
 
Murphy's SS300-24 electronic speed switch.
SS300-24: Single Set Point Speed Switch W/ SPDT Relay Dry Contact Output (20700167)
Magnetic pickup
2 Hz differential
24 VDC (18-30 VDC) power requirements
Minimum input voltage signal: 4.5 Vrms
Maximum input voltage signal: 50 Vrms