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Use a PIR Sensor to detect a movement of a paper

Discussion in 'Sensors and Actuators' started by smskit2000, Jul 2, 2015.

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  1. smskit2000


    Jul 2, 2015
    I need to detect, when a paper come out from a laser printer and count no of printed papers.For that I hope to use a PIR sensor to detect the movement of the paper from the printer. Is it posible to detect the movement of a paper using a PIR sensor. Please be kind enough to help me.
  2. hevans1944

    hevans1944 Hop - AC8NS

    Jun 21, 2012
    Maybe, but probably not.

    A PIR or Passive Infra-Red sensor is based on sensing the difference in infrared radiation from a warmer object against a cooler background (or vice versa, sensing a cooler object against a warmer background) using a pyroelectric sensor (or detector). The pyroelectric sensor responds only to changes in temperature. It is insensitive to the steady background infrared radiation we are constantly immersed in unless that radiation is "chopped" with a mechanical object that alternately exposes and covers the detector.

    A PIR motion sensor typically uses two pyroelectric sensors, electrically connected differentially, so their outputs subtract. A set of Fresnel lenses focuses an image from two slightly different points of view onto the two sensors. If an object that is warmer (or colder) than the background is present in both views but is stationary, there is no differential output from the pair of sensors. However, if an object moves across the field of view, one sensor will "see" a different level of radiation than the other sensor and their difference will produce an output signal while the object is moving.

    The problem is that paper, coming out of a laser printer after passing over the hot toner-fuser roller, may not be warm enough, compared to the background temperature, to produce an acceptable signal from the PIR sensor. The emissivity of paper, even with toner on it, is very low so there is not much difference in radiation between the paper and the background. There is also the question of response time to distinguish two sheets of paper in rapid succession. Most PIR sensors incorporate a delay mechanism before producing an output, simply to avoid producing "false alarms." Depending on this "dead time," it may not be possible to distinguish between two sheets of paper.

    It's certainly worth a try to see if it works. PIR motion sensors are inexpensive, so rig one up and see what you get. You might get lucky!

    A better sensor, IMHO, would be an infrared LED paired with a photo diode (or photo-transistor) that the paper is forced to pass between, interrupting the infrared beam with each sheet. You can also modulate the IR beam from the LED emitter with a high-frequency square wave, and then demodulate the optical detector signal to reject ambient light if that is a problem in a brightly lit office environment. There are also commercial detector systems what will do for you, but you need to pay attention to how fast they will respond.
  3. AnalogKid


    Jun 10, 2015
    Thinking about office-sized printers, an opto-interrupter in the exit path would be relatively easy to add. But for desktop models, thermal might be the only way to go since the pages just fall out. I think they are plenty warm enough to be seen as warmer than the catch tray. But after a few pages, the printed stack will get closer and closer to the paper's exit temperature. Interesting approach, but probably dependent on a printer's exact thermal characteristics.

    Arouse1973 likes this.
  4. Gryd3


    Jun 25, 2014
    A small micro-switch could also be used as long as the lever was easily activated.
    I don't think a opto-interrupter is too large even for a desktop model though.
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