Laser break detection for high speed passing

So, I'm working on this project where a laser and a photoresistor are connected to an Arduino. The goal is when a 2inch board swings through, it breaks the laser beam, which triggers an LED to light up. The challenge is that the board is going to be traveling at 30-50mph.

Is a photoresistor able to detect the break in that quick of a moment? Does anyone have a better idea on what to use that will work better in detecting the 2inch board passing through?

Thanks!

Comments

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  • At 50mph, the 2in board will obscure the beam for ~2ms. Plenty of time to register the break.
  • Seairth wrote:
    At 50mph, the 2in board will obscure the beam for ~2ms. Plenty of time to register the break.
    Not with the OP's photoresistor, though. Their rise and fall times are on the order of tens of milliseconds.

    Liveforliving, get yourself a phototransistor, and use it to detect the broken beam.

    -Phil
    “Perfection is achieved not when there is nothing more to add, but when there is nothing left to take away. -Antoine de Saint-Exupery
  • The challenge is that the board is going to be traveling at 30-50mph.
    What are you trying to measure ? How thick is the board ?
    If you just want a presence/absence detection then a phototransistor may do, but keep in mind it detects ambient light too.
    If you want to measure the velocity with any precision, a phototransistor may not be enough.

    You can get photo-logic receivers, in various forms. Those are a little more advanced than phototransistor, with a amplifier and schmitt / square wave out.

    Examples would be
    http://www.digikey.com/product-search/en/sensors-transducers/optical-sensors-photo-detectors-logic-output/

    If the board is thin, and on controlled path you could also look at
    http://www.digikey.com/product-search/en/sensors-transducers/optical-sensors-photointerrupters-slot-type-logic-output/

  • Thanks for the feedback. All we are trying to do is detect the absence of laser in order to trigger a light that alerts us that the object has officially crossed the area.

    In this case, it is objects flying/swinging across that will have approx. 2x2 of area to break the laser.

    I'll look into phototransistors if they are better than the photoresistors for the purposes of detecting a difference within milliseconds. We can always put the photoresistor or phototransistor in a small tube to block out ambient light.
  • Phototransistors are not only much faster but a better spectral match with a red laser. Photocells' peak sensitivity is with green-yellow light.
    "When you make a thing, a thing that is new, it is so complicated making it that it is bound to be ugly. But those that make it after you, they don’t have to worry about making it. And they can make it pretty, and so everybody can like it when others make it after you."

    - Pablo Picasso
  • https://www.parallax.com/product/604-00085 is dirt cheap, quite sensitive and very, very fast. I was, as a first try, trying to characterize the transmissibility of the R, G and B of a WS2812B through various solutions (pool water at various pH levels using phenol red as the reagent). The problem I ran into was that the TSL237 is so fast reacting that it was picking up the PWM of the 2812 thus complicating the intensity measurement (the 237 reports light intensity as a varying frequency). I scoped the output of various flash lamps and so forth with the 237 and it was very fast and temporally accurate. I was even lucky enough to catch a nearly lightning bolt at night - the multiple return strokes clearly showed on the scope! Very cool little device. Just though I'd present my experience....

    -Mike
  • pmrobert wrote: »
    https://www.parallax.com/product/604-00085 is dirt cheap, quite sensitive and very, very fast.

    Interesting, I would not have thought of a Frequency Output device as fast, but if you measure every period, and run it at moderate light levels the frequency is comfortably sub-ms.

    Another Sensor that has good range / speed trade off, are the Code-Learning IR receivers.

    These have wider band (20 ~ 60kHz) than fixed-frequency RX, and will decode each IR modulation cycle (as opposed to bursts).
    eg TSMP77000 TSMP6000 (and there is a separate-diode model too)
    Less Range at 5m, but I think more widely useful than narrow band iR-Rx.
  • Yes, jmg, perhaps I should not have used the term "very, very fast" :-)... Not even close. But plenty fast enough for the OP's application I think.
  • If you use a laser as the light source then you can pretty much use any kind of sensor. There will be so much power that even a simple photodiode feeding into a resistor will probably give sufficient voltage response. However, by far the simplest solution is to get a logic level photo sensor like the QSE159. They're cheap ($1), fast (<10us) and ready to interface to Arduino port pins.
    Detect and measure
  • I would recommend defocusing the laser somewhat. That way alignment won't be as critical, and tiny movements of the laser or detector will be less of a problem.

    -Phil
    “Perfection is achieved not when there is nothing more to add, but when there is nothing left to take away. -Antoine de Saint-Exupery
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