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We're using an LM393 Light sensor and a laser transmitter used for counting the fry that will enter into our machine. The machine counts properly when we use an object like a pen or a stick in front of the sensor.

Our problem is:
when water passes through the machine, it also counts the water and not only the fry.

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    \$\begingroup\$ Fry means "baby fish" or "cut up potato"? \$\endgroup\$ – The Photon Apr 1 '17 at 0:54
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    \$\begingroup\$ We don't have a problem on the coming of fry. The problem is just the water. \$\endgroup\$ – Junabelle Cajan Casio Apr 1 '17 at 1:31
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    \$\begingroup\$ Water in front of a Laser beam will tend to act as a jumble of lenses. I'd suggest you want to use a strong focused light beam rather than a small diameter Laser. \$\endgroup\$ – Jack Creasey Apr 1 '17 at 2:55
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    \$\begingroup\$ IN THE QUESTION: Your question needs MUCH more information to be complete, to help get a good answer and to not waste people's time. || Please provide a circuit diagram and diagram of layout and links to data for key components and key data in your text. Show beam width, light type (not just LASER) , distance between source and sensor, sensor type. An [LM393] (st.com/content/ccc/resource/technical/document/datasheet/group1/…) is usually a comparator - not a light sensor. ... \$\endgroup\$ – Russell McMahon Apr 1 '17 at 4:25
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    \$\begingroup\$ ... Do you have a sensor type spec link ... you can tell us about? Are you using hysteresis ? How much margin between operate /no operate. How much hysteresis? "Counts water"is meaningless! Tell us what actually happens. Always off, sometimes triggers. Regular random trigger ... ? You MUST explain what you want and what is happening well enough that people do not have to ask and ask and ask and ask and ask to get correct details. ... \$\endgroup\$ – Russell McMahon Apr 1 '17 at 4:25
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Instead of a narrow beam, illuminate the channel, below the waterline, with a flood of diffuse light (you can use a color filter to keep stray illumination from triggering the sensor). Then, focus with a lens so that blockage in mid-channel occludes the light at the sensor, causing the sensor reading to dip.

There are good red or IR light-emitting diodes that can replace the laser, and inexpensive acrylic Fresnel lenses that can do the focusing.

The key, here, is that rough water can randomize the light direction, and that loses laser beam intensity at the pickup; if the light is random and diffuse to begin with, that effect is less important.

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Your implementation is a typical example of how using too much accuracy to sense a low accuracy requirement can get you into trouble.

Lasers are the scalpels of the opto-business. However, you would not use a scalpel to carve a large ham. You simply could not get nice even slices without taking extraordinary measures to ensure accuracy of each cut.

Lasers are great for measuring really small things, like, refractive distortion in a turbulent moving transparent medium, or counting particles in a moving transparent medium (fish poop). Using a laser here means you have to filter out all those "details" from your signal. Otherwise you can not see the wood for the trees.

Proper geometry and a simple shadow detection system is really all you need.

Note the fish is channeled though a closed pipe with windows on the sides that cover narrow slots in the side of the channel.

enter image description here

Circuit connected to sensor should have sufficient hysteresis to switch when sufficient fish shadow falls on the sensor, and switch back at closer to no shadow at all.

Can you modify your current setup to use the existing laser. Possibly, if the noise introduced by the turbulence is random and at a significantly different wavelength from that of a fish shadow pulse, you may be able to filter that noise out. However, knowing how turbulence can roll around at low frequencies, that may be insurmountable without changing the dynamics of the water channel.

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    \$\begingroup\$ "Nemo". Snork. +1 \$\endgroup\$ – JRE Apr 1 '17 at 17:43
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Consider counting fish using OpenCV or other such computer vision. Here is a video where OpenCV is used to track a single fish. Although there is little information on how it is done, here another video's author claims matlab is used to count fish. I find this second video interesting as I suspect this is more representative of your situation.

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Use centrifuge on system input, so water and flies will be separated. Also you can use some kind of filter to pass the water, but not fly.

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  • \$\begingroup\$ fry = small fish .................. \$\endgroup\$ – Russell McMahon Apr 1 '17 at 4:26
  • \$\begingroup\$ Ok.. Stil centrifuge will work. Although i don't understand, the fish comes in in water or is thrown in the air? \$\endgroup\$ – Gregory Kornblum Apr 1 '17 at 4:28

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