The present invention relates to a detector 10 for monitoring rotation, in front of which passes a target 19 wherein it is required that the passage frequency thereof be monitored with respect to a normal passage frequency, comprising a sensor unit 11 connected to a microcontroller 20 which delivers a binary output signal 15. Detector 10 includes means 30 for conversing with the operator, connected to the microcontroller 20 in order to set the detector 10 to a working mode or learning mode, which learning mode enables the microcontroller 20 to measure the normal passage frequency and enables an operator to select an operating range from a plurality of predefined ranges, through the conversational means 30. The microcontroller 20 calculates from the selected operating range, a triggering frequency and a resetting frequency.