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Topic: Create a wide curtain of light to detect entry (Read 177 times) previous topic - next topic

Hi all,
     I would like to know the best way to create a wide curtain of light maybe 1 metre x 1 metre and be able to count anything that passes through it e.g. A bolt traveling at 200 - 400 feet per second.

The bolt will typically be 5mm in diameter.

Any ideas?

I thought of put a lot of diodes but it could have gaps and miss it! So I hope there is a better solution

Thanks


jackrae

#2
Feb 06, 2015, 11:41 pm Last Edit: Feb 06, 2015, 11:43 pm by jackrae
Looking for a 5mm object anywhere in 1,000,000 square millimetres is a big ask
Microphone to listen for the shock-wave that will be created by something travelling at that speed

I could probably narrow the fields down to 300mm x300mm if that helps?

MarkT

How much ambient light is around?  If none/little you could look for dips in light
received rather than complete interruption - then one light sensor can cover the
range of angles from which its illuminated.  The smaller the object the smaller
the dip, so you might want several sensors spread out with separate illumination
"fans"

However then you're forced to deal in analog signals...
[ I won't respond to messages, use the forum please ]

polymorph

Crossbow bolt? What kind of bolt? How long? Could it be tumbling, or will it always go through straight?

Merely shining a light across and then looking for a dip in light is going to be very, very insensitive.

How about a video camera? It'll take something like a Due, Teensy 3.1, or Raspberry Pi to process video.
Steve Greenfield AE7HD
Drawing Schematics: tinyurl.com/23mo9pf - tinyurl.com/o97ysyx - tinyurl.com/q7uqnvn
Multitasking: forum.arduino.cc/index.php?topic=223286.0
gammon.com.au/blink - gammon.com.au/serial - gammon.com.au/interrupts

Magician

Quote
Merely shining a light across and then looking for a dip in light is going to be very, very insensitive.
True. This is why I suggest to shine a laser line, than when something get in the light path it reflects a laser beam. There would be a spike in intensity, not a dip. Than, to get good contrast between no object in the field and when it's right there, make non-reflective coating at another side surface (or point to sky), make object coating with high reflective index, and pump a power of the laser up. Using near IR laser 1W or so, I think it may works over great distance, may be good as missile interceptor  :) .

dave-in-nj

http://tinyurl.com/l25btb7




http://www.shootingchrony.com/products_SCMMCM.htm

Paul__B

OK, so the "Chrony" uses photo-sensors which look for a sharp dip in the light sensed using a "barrel" lens - similar in reverse to a laser with a "line" diffraction grating.  In the open, it is used without the diffuser which gives an even image across the "fan" so that there are no blind spots from seeing a part of a ceiling.

Note the criteria that it cannot be used inside with other than incandescent light (including spirit lamps!) as fluorescent or LED lighting is modulated.

This appears to be an established solution to the problem.  You need the right lens for the photo-detector.

An interesting alternative occurs to me for a rectangular opening of manageable proportion - such as 30 cm square.  By mounting two surface-silvered strip mirrors accurately parallel, a laser beam at a very slight angle could be made to "ricochet" between them about a hundred times and be collected by a detector at the opposite diagonal.

polymorph

That's good, Magician, I missed that you were saying that.
Steve Greenfield AE7HD
Drawing Schematics: tinyurl.com/23mo9pf - tinyurl.com/o97ysyx - tinyurl.com/q7uqnvn
Multitasking: forum.arduino.cc/index.php?topic=223286.0
gammon.com.au/blink - gammon.com.au/serial - gammon.com.au/interrupts

Marmotjr

Just an idea, but if the bolt is reflective (aluminum crossbow bolt?), could a fan/blade style laser emitter be placed on the top aiming down, and a series of detectors along each side.  When the bolt passes through the laser curtain, it would partially reflect the beam to one of the sensors on the side of the fixture.

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