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ping))) for double feed detection — Parallax Forums

ping))) for double feed detection

mjtrac@gmail.commjtrac@gmail.com Posts: 3
edited 2012-03-07 11:53 in Accessories
I'm wondering if there is any way to split the emitter and sensor in the ping so that it can be used for double-feed detection? I'd want to determine how much attenuation there was in the signal after it had passed through a gap that might contain one or more sheets of paper.

Or could you use two by disabling the sensor on one and the emitter on the other?

Comments

  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2012-02-29 09:15
    mjtrac,

    Welcome to the forum!

    You can disable either transducer in the Ping))) by covering it, but that will not help you with your app. The reason is that the Ping))) responds digitally to a pulse's time-of-arrival. There is no indication of signal strength.

    -Phil
  • mjtrac@gmail.commjtrac@gmail.com Posts: 3
    edited 2012-02-29 09:49
    Thanks. Drat. The double feed sensors I see tend to run in the $500 range.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2012-02-29 10:29
    Why not try an LED with a light-to-frequency sensor? Each sheet of paper between them will add to the attenuation. By modulating the LED at a rate slower than the output frequency of the sensor, you can remove the influence of ambient lighting. If you'd rather go the analog route, TAOS also offers light-to-voltage converters. Either of these would be much preferable to a phototransistor, BTW.

    -Phil
  • mjtrac@gmail.commjtrac@gmail.com Posts: 3
    edited 2012-02-29 10:41
    Thanks again. At $3, definitely worth a try.
  • davejamesdavejames Posts: 4,047
    edited 2012-03-07 11:04
    Phil Pilgrim (PhiPi) wrote: »
    By modulating the LED at a rate slower than the output frequency of the sensor, you can remove the influence of ambient lighting.

    Mr. Pilgrim,

    Can you elaborate on this? I'm not "gettin' it".

    Thanks much.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2012-03-07 11:49
    By integrating a reading with the LED on, then another with it off, you can subtract the latter from the former to eliminate the effect of ambient lighting. But the LED's on/off frequency should be very low w.r.t. the frequency output from the sensor, in order to accumulate accurate data.

    -Phil
  • davejamesdavejames Posts: 4,047
    edited 2012-03-07 11:53
    ...ah! A "null & difference" approach.

    Got it - thanks!
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