Electronic Switches (I) P. Ltd; offers 4 Digits,  Zero Speed Switch counter with 1/2inch seven segment LED display. Units operates  on 220 Volts AC supply and accepts input from sensors.  

 

Two set points, one for Upper speed limit & other for Lower speed limit. Relay is ON only if RPM is in between set points & OFF if RPM crosses lower or upper speed limit.

This counter is used in all sort of industries for monitoring motor speed.