Interfacing fish finder take 2

Please post a schematic of how you have connected it, and what you propose to build. At least one tuned circuit is required to build up the voltage for the transducer.

Here is the schematic I came up with.
1- What I still don't know is what will happen when I remove all the components and circuitry after C3 and C4 (below the two diagonals). I don't know if they help to achieve the echo level at the signal tap point. It would be very nice if I could cut everything just there? There is no DC in that part of the circuit (I think).
2- I could probably add capacitor and stuff around the two comparators? It work like this on my breadboard but is probably far from optimal. Coupling cap? Filter?

I doubt there would be a problem to cut those traces. The signal from the two antiparallel diodes will be more or less a square wave with maximum amplitude about +/- 0.7V, so I doubt you need two comparators.

For weak return signals from long range echos, you will probably need a stage of amplification before the comparator, and it would surely be a good idea to use the existing second tuned circuit as input to that amplifier, for selectivity and noise reduction.

I did try to amplified the signal with the first half of the opamp but it did not work and I don’t understand why. I used a basic non-inverting amplifier.
Tomorrow I will send scope data showing why two comparator was better. But not by much… 1 gain stage + a comparator would be better on the long run.

Please post a link to the part labeled "MN6002". That is not a standard designation.

The only thing that comes to my mind is an MCP6002 op amp, which would be a very poor choice for anything in a circuit like this.

Yes it is a mcp6002. Single rail low voltage thus easy to connect to my 3.3v rail and output a high voltage that can be directly read by the esp32.

The non inverting amp I tried was not amplifying. Vin = Vout on the scope. Echo p-p in = echo p-p out.

Here is the output from the first comparator: we can see some ripples in the "square" output. This tend to confuse the pulseIn function and return false measurements.

On the second comparator output, the riples are not present anymore and there is a clear low signal between the trigger and the returning echo. Even for short distance. Here about 9 cm. With shorter distance, the signal combine and there is no more low phase between the pulse and first returning echo.

And finally, this last picture show the Tap signal (yellow) and how it is at pin 2 of the mcp6002 (blue). Ideally, this is the signal I would like to amplify (the returning echo is only 600 mV).

The op amp may be oscillating, probably because it is not properly bypassed. Use 100 nF directly across the power pins.

However, the MCP6002 is a terrible choice for this circuit, because it is so slow. The gain bandwidth product is 1 MHz, so the best you can do as an amplifier is gain of 5 at 200 kHz.

For the comparator, use a genuine comparator chip like the LM393, and for an op amp, choose one with a gain bandwidth product of 10-20 MHz.

Why not just use the amplifier already built in to the fish finder?

jremington:
Why not just use the amplifier already built in to the fish finder?

The fish finder use a NPN transistor and don’t really boost the signal. It just drive the second inductor. There is no real amplification going on in the fishfinder post treatment. I think they rely more on the chip capacity to output a good RSSI signal. So like you suggest, filtering the noise out and producing a good signal enveloppe. This is more sophisticated and allow for signal analysis to find fishes or plants.

I am looking for simplicity and don’t know much about OpAmp as you can see. This is why I rather not include a second inductor and only use one dual opamp. The LM393 can work on 3.3v single rail and you say it is a good comparator and it’s not expensive (all very interesting point). Would it also be suitable (better than the MCP6002) for boosting the signal? A gain of 5 (if I can make it work!) is probably more than enough at least up to 20-30meters.

When you talk about osccilation, is it related to the comparator ripples or to my non-working opamp amplifier?

You will discover that "driving the second inductor" is a very significant, frequency selective amplification step, and is a critical part of the design.

I get back to my scope.
The transistor + inductor amplified and clean up the signal. I see a 1 volt increase. It's a gain of ±2. Channel 1, input. Channel 2 inductor out.

Can you guide me to achieve a gain of 2 or 3 on the MCP6002? I use those configuration without success.

I need to select a IF tuner inductor for the circuit and the inductor on the actual unit are not identified.
I dig on internet and find the transducer test characteristics (Q factor, resistance and test frequency). See picture.
With the equation Q = (2PiL*F) / R and the available test infos, I calculate "L" to be 16 mH.

There is a lot of variable inductor on the net and most are in uH.
1- does 16 mH look weird for a Q of 48 @ 200 khz?
2- Is there other thing than henry value that I should look for?

thank you

The LM1812 may help you a lot. I thought the transducer of Lucky fish can be use on it. You can find some info on paper "Down-Under Depth Sounder". The circuit works quite similar to Lucky fisher but they use op-amp and counter to read the water depth.

hey have you ever try with JSN-SR04T or AJ-SR04M and use the fish finder transducer? can you help me? Im currently using arduino UNO. it will be very helpful if you can help me with underwater depth sensor with arduino.

The JSN-SR04T don't work underwater and it is not the same frequency compare to the lucky fishfinder so we cannot switch the transducers.
If your project can work with a open case lucky finder, I can help you make it work by guiding you cutting specific trace on the pcb and then tapping the signal to an arduino. It work very well and I was able to improve the short distance sensitivity from 0.9m to less (value somewhere in previous posts) since I d'ont care about fish finding or echo shape. Unfortunately a open gut lucky finder don't suit my need. So this is still a standby project in my case. I ordered some transducers but have'nt find time to test them yet.
For this to work, you need a Mosfet like this :FQP30N06L or hack the onboard triggering transistor array. Both work and the latter might be easier.

C.

okay I'll try your advice. while im trying to find out where I can get these mosfet cause it is super hard to find electrical component in my area, I want to know if we can make the arduino release 200Khz PWM signal, can we make it work? and if it need 190v of 200khz signal is there is any way we can increase the voltage to the minimum working voltage? and use op amp for the returning echo? thank you for your reply it was very helpful. im sorry for my bad english.

1- I build me test system on ESP32. It will probably work on Arduino.
2- You don't really need the mosfet. You can hack the onboard electronics.
3- what you do need and that I forgot is a opamp and the components to reproduce the schematic in post #48 but be aware that the setup work but Jremington recommend other more suitable opamps than MCP6002. You may have more choice with Arduino since it work on 5 volts instead of 3.3v like my esp32.

hey i just want to give some update. I've tried to use AJ-SR04M as the driver for fish finder transducer and its work with pretty decent result


I know it have diffirent frequency but it still work. I think if we can make 200KHz with just same voltage level as AJ-SR04M it will work fine. I've tried the fish finder (not Aj-SR04M) too in deep ocean and it have about 40 m depth reading limit.

next week ill try with more accurate measurement. even tho that AJ-SR04M has 6.5 m limit and since sound move about 4.411 faster in the water so the limit also increase to about 20..m or so. and that enough for my project. I'll keep up next week.

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Many products require a Liquid Crystal Display (LCD) Interfaced to a
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Today, I found this great manual, which implies the picture on the display can be quite rich.

Some More Reading

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