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Infrared Distance Sensor

  1. May 15, 2009 #1
    Dear Experts,

    I bought a pair of infrared LEDs (transmitter + receiver).

    I read that infrared can be used as distance sensor.

    I have seen projects using it (eg. Sharp GP2D120 sensor) for robotics and toy cars. Just to clarify, I am not using the Sharp sensor but a normal infrared LED pair bought from an electronics parts store.

    I tested it and measured with volt meter and it seems that different materials or objects reflect different intensity of infrared to the receiver.

    I am wondering how can this be used as Distance sensor when there is no consistent voltage when sensing different materials.

    Are there Infrared sensors that can measure distance accurately for different materials like wood, metal, plastic, concrete surfaces or materials?

    Has anyone tried the Sharp sensor before? There are a few models of it.

    At the moment, based on my testing , it fail to see how infrared led can measure distance accurately.

    So, could you please help clarify my doubts.

    Thank you very much.

    Best regards
  2. jcsd
  3. May 15, 2009 #2


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    I think you've missunderstood how these things work. Sensors like these generally operate by modulating the light (or sound, the principle is the same for acoustic sensors) using an on-board oscillator and then down-mixing the reflected signal; i.e the measurement is based on time, not intensity.

    You should be able to find a datasheet if you google a bit.
  4. May 15, 2009 #3


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    If you mount the IR LED and IR Receiver on the front of your toy car, so that they can't see each other, (put a barrier between them), then when there is something to reflect off, the receiver will get some IR signal.

    To avoid just any IR signal giving a false reading, you could modulate the transmitting IR LED with a 1000 Hz square wave to turn it on and off rapidly.
    You then detect this on the receive end with a NE567 tone decoder or similar device.

    Just the presence of this signal would tell your car that it is close to something and it is time to put the brakes on or turn away.

    I doubt if you could really measure distance with such a device because the time interval would be so short. Infra Red light travels at the speed of light, of course. I make it about 30cm per nanosecond.
  5. May 15, 2009 #4


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    I had a quick look at this particular sensor, and I am pretty sure it does NOT use time as I suggested above; I think it just uses triangulation (back to basics). I.e. it has a reciever with a number of "pixels" in it distributed over some distance from the transmitted. This also explains the short range (it is limited by the width of the sensor).
    By sensing the posisiton of the maxium you the angles and then the distance.
    If you look in the datasheet is seems to be quite accurate, and there is not much difference between grey and reflective surfaces.

    (It does use a modulated signal, but that is probably just part of a PLL to get a better S/N ratio).
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