SS421H - 

The SS421 Series Adjustable Hall-effect Under-speed Detectors are designed to monitor fan or motor performance. This temperature-compensated, Hall-effect IC contains a timing circuit and logic, that senses magnetic input frequency. The internal circuitry contains a timer so that one or two pulses at a slower repetition rate than the set point do not produce an unwanted output.
Features:

Wide Temperature Ranges for Design Flexibility

Temperature Compensated for Consistent Operation with Low-Cost Magnets

Bipolar Operation for Easy RPM Counting Using Ring Magnets

Built-In Timing Circuit Greatly Simplifies and Reduces the Cost of PC Board Design

Speed Trip Point Adjustable with External Resistor and Capacitor