underwater fish detector

Thread Starter

fedimakni

Joined Feb 27, 2024
3
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.
Note:
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
https://drive.google.com/file/d/13LIXOK5Zvt1R61uHieQIGLCPWyVfH69t/view?usp=drive_link
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):
Challenges:
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)
Challenges:
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)
Challenges:
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.
 

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k1ng 1337

Joined Sep 11, 2020
960
Cool project. I'd go for a flow sensor or some variant. As a fish passes through the tube, there will be a pressure change in accordance to Pascals law, with the caveat of the tube being open on both ends.

If the MCU is constantly monitoring the baseline pressure of say, a membrane installed in a bypass tube (not accessible to the fish), the presence of a fish will be registered as an abrupt change in pressure at the sensor.

1.png
 

Thread Starter

fedimakni

Joined Feb 27, 2024
3
k1ng 1337
Thank you for your response, i like your concept. but do you think wave can affect those measurement or not?
I have 2 other questions:
-If the main channel is moving, could that make a noticeable change?
-Do you have an idea what kind of membrane i need to use? will a normal pressure sensor do the job in this case?
Thank you again.
 

k1ng 1337

Joined Sep 11, 2020
960
k1ng 1337
Thank you for your response, i like your concept. but do you think wave can affect those measurement or not?
I have 2 other questions:
-If the main channel is moving, could that make a noticeable change?
-Do you have an idea what kind of membrane i need to use? will a normal pressure sensor do the job in this case?
Thank you again.
I suppose that depends on many factors specific to your setup but yes, I think it will work. Here is some stuff to think about:

  1. Conservation of Mass: This law states that the mass of an isolated system remains constant over time, implying that the rate of mass flow into a system equals the rate of mass flow out of it. In the context of the fish counter project, conservation of mass ensures that the volume of water entering the tube equals the volume exiting, allowing accurate fish counting based on changes in water pressure.
  2. Bernoulli's Principle: Bernoulli's principle describes the relationship between the velocity and pressure of a fluid in motion. It states that in a streamline flow of an ideal fluid, an increase in the fluid's velocity is accompanied by a decrease in pressure, and vice versa. In the fish counter project, Bernoulli's principle explains how changes in water pressure resulting from fish movement within the tube trigger the flow meter.
  3. Newton's Laws of Motion: Newton's laws govern the motion of objects and their interactions with forces. Newton's third law, in particular, states that for every action, there is an equal and opposite reaction. In the fish counter project, this law explains the forces exerted by fish as they move through the tube, causing changes in water pressure that are detected by the flow meter.
  4. Pascal's Principle: Pascal's principle states that a change in pressure applied to an enclosed fluid is transmitted undiminished to all portions of the fluid and to the walls of its container. In the fish counter project, Pascal's principle ensures that pressure changes caused by fish movement within the tube are effectively transmitted throughout the fluid, allowing for accurate detection and counting of fish by the flow meter system.

The two statements in bold are the cornerstone of my idea. These observations assert there will be a force experienced by the flow meter regardless of the size and velocity of the fish. The exception of course is this is a system open at both ends. It comes down to designing a section of the tube to accompany a flow meter and interfacing with a MCU.

As for the flow meter itself, that's hard to say without more details. You mentioned the pipe being in the cm range but how big exactly? Is this in an aquarium or mounted under a trolling boat? More details will get you better answers from everyone. In any case, take a look at this diagram:

Untitled.png

If you put this tube in the ocean or let it sit in an aquarium with or without a pump running, a baseline pressure differential will develop once the tube reaches thermal equilibrium with the environment.

I'm fairly certain that if you the make the tube slightly bigger than the fish, the wave front and wake generated by the fish will be of sufficient amplitude and duration to trigger a digital flow meter installed in the bypass tube. If needed, you can make the bypass tube smaller in diameter which results in a greater change in pressure at the constriction (Bernoulli's Principle). This might not work for minnow size fish though as I imagine their displacement is very small.

Here is an interesting flow sensor but I don't think this one is waterproof. Check out the datasheet, notice how there is a turbine in there. This is what I was referring to as a membrane.

114991172 Seeed Technology Co., Ltd | Sensors, Transducers | DigiKey
 

Janis59

Joined Aug 21, 2017
1,849
Bit sceptic about doing by own the things where all the humankind had invested the brightest minds and result is available for coins.
I have experience with two such echolotes. One I bought second hand for my sailboat, it cost me about 50 Eur and is straight-minded beep and back type the machine in the brick size able to get down to 700 meters deep. I see there well the ground existence, I see there bigger fish clouds and it depth, and practically nothing more.
Another device is rather modern chineese floating egg-sized playmate using the frequency sweep principle (chirped signal), thus providing info also about how hard or mild is the ground surface, how large are the fishes in the fish cloud, and many more useful layers of data. That is battery hanged gadget just be attached to board, and all the info you get via WiFi onto the phone. It cost me about 100 Eur cause it was then very new prduct in the markets - now it cost about 3x more, this "Deeper Pro". I dont believe You shall create anything better even by means of 1000 Eur.
 
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