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underwater fish detector

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I would like to create an underwater fish detector - completely operating underwater (personal project)
The enclosure will be a waterproof hollow cylinder (possible material to use are: acrylic, Polycarbonate or PVC), a PCB will be sealed inside that enclosure containing the sensor (the sensor of choice should not be waterproof because it will be inside a waterproof enclosure).
The fish can pass through that hollow and will be detected while passing. the inner diameter of the enclosure will be in the centimeter range to allow one fish to pass at a time.
I am not dealing with precise measurements or long range (no need for those expensive high precision sensor)
I cannot use big or expensive sensors due to budget related reasons and size constrain
max operating voltage of the PCB is 12V (preferably 5V or 3.3V)
Here's a link to picture that illustrate the idea for better understanding
I am using 2 sensors so i can make sure i detect that it passes through
optional receivers can be implemented if the sensor i use is just a transmitter and not transducer

Here comes the challenging part, the sensor choice, i will list some i thought about and i hope i can get some ideas from you that can help me.
- Ultrasonic sensor (i saw some people successfully did it but nothing is documented with low cost sensors):
Blind distance of ultrasonic sensor for small measurements.
I don't know if the acoustic wave can pass through acrylic hit the fish and be detected
- Optical sensor (such as VL53L1 or any other choice)
Require clear water
water can scatter the light but i am using it for short distance so i don't know if it will work or not
require transparent enclosure
- ESP32 camera (image recognition algorithm)
Require complex algorithm
Require clear water

I would like to hear if you have any other idea or if i can improve one of the suggested ideas to successfully achieve the project.
Thank you very much.

David Hess:
I think optical will be the easiest.

I wonder if an LC tank circuit with the inductor wrapped around the fish tube would work.

I'd think optical is the way to go as well, but rather than a distance sensor, you could probably just have some IR illumination and then a photodiode or two - just looking for something opaque to pass through the beam.  You'd want to keep the emitted light out of the visible band so you don't effect their behavior in using it.  You could also use several emitters or detectors to measure movement/avoid murky water causing issues/discern multiple fish from one/etc.  Provided you can seal a hole in the enclosure, you could also potentially get the optical parts in direct contact with the water and then just seal the electronics side.

Ultrasonic would be possible, but I doubt practical in this situation.  To get a good measure of what's in the water, you'd probably want the transducer to interface directly with the water, not through a separate enclosure.  There may still be meaningful data can can be had from a dry sensor, but the difference between the reflection of the enclosure and the reflection of the fish in the water outside of it will be subtle, and it will probably be a good chunk of effort to identify that signal.

Do you mean detecting the electromagnetic field change when the fish pass through the tube?
I was really thinking about it, but i don't know how to make it and how to measure the change in that E-field. Do you have any idea or any video showcasing such approach?

Thanks for your answer.
Do you mean i need to use multiple light transmitter on one side and multiple receiver on the other side?
Do you think IR can pass through water?


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