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Question about IR detection — Parallax Forums

Question about IR detection

lardomlardom Posts: 1,659
edited 2012-12-22 21:36 in Propeller 1
I'm tweaking a IR distance detection object to see how things change when I modify values. I'm trying to see how steady I can make the readings between the IR emitter and the reciever. I understand the need for a remote control to emit a frequency such as 38kHz to filter out ambient noise. It is transmitting commands. I don't understand why 38kHz is used in distance detection.

Comments

  • ElectricAyeElectricAye Posts: 4,561
    edited 2012-12-22 10:33
    lardom wrote: »
    ...I don't understand why 38kHz is used in distance detection.

    As I understand it, by using a carrier (in this case 38 kHz), you can filter out other sources of light that fluctuate at non-38kHz frequencies, for example the changing light of the sun, or the flicker of other light sources such as fluorescent lights or even maybe the 60 Hz fluctuation of an incandescent bulb. The electronics allows you to make a band-pass filter that passes only 38kHz signals, so that signal is the only signal the device will "see".

    I don't think there's anything magical about 38 kHz that senses distance, it's just a way of detecting the particular IR light that is emitted as opposed to detecting other forms of light in the room.

    EDIT: I corrected my mistake when I said a "notch filter" when I really meant band-pass filter.
  • John AbshierJohn Abshier Posts: 1,116
    edited 2012-12-22 11:11
    The IR detector is tuned to detect a certain frequency. 38kHz is one of the more common frequencies. I would recommend reading chapers 7 and 8 of Robotics with the BOE Bot
    http://www.parallax.com/Portals/0/Downloads/docs/books/edu/Roboticsv3_0.pdf

    John Abshier
  • lardomlardom Posts: 1,659
    edited 2012-12-22 11:14
    @ElectricAye, "...notch filter" is something new to me. If I try to design a wall following robot maybe a 'notch filter' will allow me to place two IR sensors side by side and keep it parallel to a wall.
    @John Abshier, Thanks for the link. I will follow up.
  • ElectricAyeElectricAye Posts: 4,561
    edited 2012-12-22 11:47
    lardom wrote: »
    @ElectricAye, "...notch filter" is something new to me. If I try to design a wall following robot maybe a 'notch filter' will allow me to place two IR sensors side by side and keep it parallel to a wall.
    ...

    Larry,
    actually, when I was speaking about a notch filter, I should have called it a band-pass filter, but in any case I was talking about what goes on inside the IR detector module. I didn't mean it was something you needed to do yourself externally to that module. John Abshier's link probably goes into better detail. To use 2 IR sensors to keep something running parallel, you probably want to separate them rather than keep them side by side, and then alternate their emissions, running one, then the other in rapid succession so they won't interfere with each other and yet you'll still be getting good readings very quickly.
  • lardomlardom Posts: 1,659
    edited 2012-12-22 21:36
    @ElectricAye, alternating sensors is a great idea. My batteries will thank you. Your reply forced me to think about emitter/reciever placement more thoroughly. I saw that I really do only need one sensor facing the wall and one facing forward for object avoidance.
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