Scuba-Diver wireless remote: help needed wiring magnetic induction transceiver components (3D1212 + ADS1220?)

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I'm building foot-mounted thrusters which are controlled wirelessly from a bite/tongue-switch on the regulator/snorkel, as well as a thumb switch on a glove.

I prefer not to use audio/sonar to minimise interference and disturbance, RF of course will not go through seawater, and the range (max 2 meters) is perfect for a magnetic solution.

I'm stuck with 2 parts:
a) what to send the transmitting antenna (how many volts, amps, and what signal shape), and
b) how to read the tiny signals in the 3D pickup coil.

I'm using this pickup: 3D1212-722J

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And this transmitting antenna:

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And I bought this PKE car alarm which contains a complete working system of the above:
https://www.amazon.com.au/EASYGUARD-Passive-Keyless-Starter-EC003-1/dp/B07QH46PL4

(The key fobs contain the 3D1212-722J pickup).

I have verified that the setup should work (the blue LED on the key fob "blinks" one time per 2 seconds when it is in range of the antenna, and stops blinking when out of range. I can see signals in the antenna on my oscilloscope, and read (tiny) signals from my isolated (not in the fob) 3D1212 coil when it is near the antenna. The keyfob LED blinks fine even when my thruster motor is between the antenna and fob and running at any speed - so interference from the brushless motor is not a problem).

I do not need the "security" of the chips in the PKE alarm - I only need to send a few bits of data a few times per second (one bit for control, and a few extra bits so 2 divers near each other don't drive the wrong thrusters).

I think the components are 125khz

My first question relates to the 3D1212-722J pickup coil - it's an inductive coil (3 of them, in X, Y, and Z orientation). I've bought an ADS1220 SPI ADC that has a PGA of upto 128 and upto 2000 samples per second.

My guess is that the output form the coil will be AC, very low volts, and probably 125khz - how would I connect that to my ADC ? I'm guessing I need to add some volts to it, so the signal never goes negative, but not sure how to do that while keeping my ability to use the 128 PGA (which I'm guessing I'll need to adjust on-the-fly, since the signal difference at 2meters .vs. 2cm will be vast). I'm lost about how to deal with my 2ksps sample rate and the 125khz signal (I'm guessing I repeat the signal for some time, and something at the coil-side somehow accumulates it?)

I'm happy to pay $ money for help - I've tried posting on a few paid analogue-electronics sites, but I think inductive remote control is so obscure that nobody has been able to help me so far.

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You will need to filter and amplify the signal. At 125kHz you can AC couple the signal with a capacitor to some biased voltage. Then you want a combination bandpass filter and amplifier. I don't recall the "typical" configuration but a quick google should help.
Detection:
I don't think analog detection is appropriate or practical for this approach. I suggest you use On Off code (similar to that used by remote home thermometers).
Create a digital stream maybe at a 1 or 10 ms rate, create a lead in pattern then send your data then a stop pattern. Similar to RS232. The other end would only have to detect on/off and not an analog signal.

I couple of lifetimes ago I worked for a company that was working on magnetic "H Field" communications. From what I've heard it didn't workout. I do remember the "best" coil configuration was a large 20" or so round coil of ??? turns. The frequency was about 100kHz. I don't recall if the coil was tuned to resonate at some frequency.

You should know because seawater is conductive, currents will flow in the water and the resulting field will reduce the intended field. Something like a shield. This is even though seawater is not magnetic.
You might just purchase copper wire on a large(er) diameter spool and test it out.

I don't know if this is any better but have you considered IR (like remote controls)? It would be real easy to test. All you need is a TV remote in a plastic bag (taped seal) and a IR sensor connected to an Arduino. I don't know how IR does in seawater, I'm hoping it will bounce it around so you don't need line of site.

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Blue light goes the furthest, then green, then red. IR, likely not at all.
Paul

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Stay tuned - I'm going to pack the car alarm and fob in some plastic bags and chuck it in the sea to be sure... back in an hour...

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Interim update - works in my freshwater pool - now off to the beach...

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Success! My PKE keyfob blinks the blue LED (indicating it decoded the inductive transmitter signal) from the bottom of the ocean with the car alarm floating in a bag about 6 foot above.

It also worked with the alarm floating about 10 foot away, with the keyfob about 1 foot below the surface.

It works at least 8 foot away in my freshwater pool as well (same tests - on-the-bottom, and a foot below surface)

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If I get time, I'll upload the gopro footage - trying not to get wiped out by waves while filming and measuring was pretty intense.

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My ADC has a programmable gain of upto 128 - the bit I cannot work out is how to AC couple something I'm going to amplify to "some biased voltage".

For example - lets assume the coil has 0.1 volts peak-to-peak. If I pick a 0.1v bias, I get a 0.0 to 0.2v range, which I can then amplify 16x to give me 0.0 to 3.2v input to my ADC. All good.

however - if the diver moves their hand near the thruster, the coil is going to be picking up exponentially more volts - for example - 2 volts peak-to-peak. So now, I need to dial-down may gain to just 2, but my bias voltage now needs to be dialed 10x ....

I'm pretty good with signalling - so long as I can work out how to receive anything, I can handle the comms no prob.

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Great job! It is refreshing to see someone on the Arduino forum actually test things, rather than struggle with theory.
Paul

1 Like
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I was thinking something like this:

image
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Then, assuming the communications are digital, a comparator after the opamp.

If the input signal has such a large voltage range you may want to use just a comparator or look into an AGC circuit.

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