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I am doing B.E. in Mechatronics Engineering. I am due to take a subject called 'Sensors and instrumentation'. So far I only have the syllabus for the course. I decided to do some reading in advance of the course starting.

The syllabus refers to "low pass sensors" and "band pass sensors".
When I web search for these terms I find many mentions of low pass filters and band pass filters, but nowhere on the Net are "low pass sensors" and "band pass sensors" mentioned.

Do the terms "low pass sensors" and "band pass sensors" have their own meanings and, if so, what are they?

I have been googling for the past five days for these two words but none of the results satisfactorily explained the meaning of 'low pass sensor' or 'band pass sensor'.

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  • $\begingroup$ Are looking for low pass and band pass filters? $\endgroup$
    – Mahendra Gunawardena
    Commented Jul 21, 2015 at 9:55
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    $\begingroup$ No @MahendraGunawardena I am searching for low pass sensors and low pass filters. But even google give me the results for low pass filters and band pass filters. $\endgroup$
    – user2338
    Commented Jul 21, 2015 at 11:41
  • $\begingroup$ Do you have any references or some type source? This the first time I have encountered low pass sensor terminology. $\endgroup$
    – Mahendra Gunawardena
    Commented Jul 21, 2015 at 12:01
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    $\begingroup$ Nope. I am doing B.E Mechatronics Engineering. I have a subject called Sensors and instrumentation. In that we have these topics.. They give us only the syllabus in our college. we have to refer them in internet. so I thought it would be apt to ask the experts here. $\endgroup$
    – user2338
    Commented Jul 21, 2015 at 12:56

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You are combining two concepts.

  1. 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. Even colour, temperature, dimensions, current/voltage/power ... .

  2. 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.

In the above context, a low-pass sensor is a sensor, intended to detect or measure a specific signal or condition, that accepts the lower frequency components of the signal but rejects input above a selected frequency. A band-pass sensor is one which processes only signals with frequencies within a given range of frequencies, and rejects signal higher or lower in frequency than the selected band of frequencies.

As examples:

A low pass sound sensor may be a microphone plus amplifier plus a filter set to exclude frequencies above say 4000 Hz. Such a sensor would respond to speech signals and the fundamental sounds of many musical instruments but would not be affected by high frequency noise signals.

A bandpass doppler radar may have the range of frequencies that it acts on limited to a range of Doppler frequencies corresponding to velocities from the speed of a person walking slowly up to about 100 mph. It would therefore tend to ignore signals from eg a door being very slowly closed (V << walking speed) or to signals from reflections from aircraft (V > 100 mph).

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