You are combining two concepts.  

**A sensor** is any device that responds to a desired signal in some manner to produce an output that is used to detect the presence of the desired signal or to allow some desired attribute of the signal to be measured - such as its amplitude, frequency, rate of change, relative phase, etc.

The terms **low-pass** and **band-pass** relate to the processing of a sensed signal so that only a portion of the signal is used to provide the sensor output. It may seem counter intuitive to ignore part of a signal, but this is useful when the signal may be accompanied noise or by other interfering or unwanted signals. 

More very soon