I need a bite-throttle on my scuba-regulator to wirelessly control my foot-mounted thrusters, which are about 1.5meters distant from my transmitter. I need only "1 bit" (on or off) right now - not data (yet).

I purchased a passive-keyless-remote car alarm, put it in a waterproof bag, and observed that the key-fob properly receives the inductive wake-up signal when it sits on the ocean (salty) floor, and my pool (chlorine) bottom, with the alarm at 2m above. So - proof-of-concept success. PKE Car alarm in waterproof bag pool and sea testing

Photos of all the bits are here: https://forum.arduino.cc/t/scuba-diver-wireless-remote-help-needed-wiring-magnetic-induction-transceiver-components-3d1212-ads1220/905203/9

transmitting inductive 125khz antenna

Reading the specs and observing my oscilloscope, I see that the inductive antenna is being driven by a 125khz square-wave of 20v peak-to-peak. I've built a circuit that modulates it identically (just the 125khz, not the data), and the pickup coil (just the X axis in testing) receives this signal and can easily be seen on my scope for about 30 to 50cm distance - further than that, my scope cannot handle (signal vanishes into the noise at 20mv/div). The sensitivity of the PKE chip in the keyfob states 1mv ( http://ww1.microchip.com/downloads/en/DeviceDoc/PKERefDesignMnl.pdf )... which is where my question begins:-

I've bought the identical X,Y,Z pickup coil from inside the keyfob - and I'm stuck here: the reference design wants a 220pf capacity on the axes (probably something to do with LC parallel resonance or something - I'm lost in that jargon)... inductive 3-xis pickup coil

How do I build something that can tell me when the pickup coil "sees" the transmitter signal?

It's too weak and too fast to be "easily" amplified (not to mention - it ranges from +/-5v when adjacent the antenna, to +/- 1mv or less at 2 meters... and I somehow need to cater for all that range).

I assume there's a "trick" involved somehow? Probably some way to use the fact that this is a repeating signal to somehow "accumulate" successive pulses?


I'm happy to pay/tip if you don't help "for free" normally (assuming you answer right, or know-for-sure you can).

In anticipation of comments:

  1. This is not RF - portable RF does not go through sea - this is inductive.
  2. I want short 2m range and reliable operation - so acoustic is off the table.
  3. This is not impossible, despite the fact "nobody has done this before" (they have - e.g. inductive fitness monitors for swimmers, plus my PKE test POC).
  4. Yes, I know that orientation is important - I'm testing just with the X asis, and orienting my tests properly.

p.s. My POC thruster (this induction project is to get rid of that wire): Foot Scuba Thruster

  • 1
    \$\begingroup\$ 125kHz isn't very fast and 1mV is small but not ridiculously JFET amp? But I also do not know anything about antenna impedance matching. If antennas output current rather than voltage, maybe you need a transimpedance amp instead which are routinely used below 1uA (if that could be used as fair comparison against 1mV). LC resonance might be referring to letting the energy in the antenna build up via resonance over several cycles and make a larger output than it otherwise would. \$\endgroup\$
    – DKNguyen
    Oct 20 '21 at 15:19
  • \$\begingroup\$ The proposed scheme seem to be far from reliable if you ask me. I hope you have planned some manual on/off control on the thrusters (or just off at least). \$\endgroup\$
    – Eugene Sh.
    Oct 20 '21 at 15:24
  • 1
    \$\begingroup\$ How do I build something that can tell me when the pickup coil "sees" the transmitter signal? - precisely state what the transmitter signal is. Be accurate on frequency and modulation. \$\endgroup\$
    – Andy aka
    Oct 20 '21 at 15:28
  • \$\begingroup\$ I'm wondering if something like this will work for you: adafruit.com/product/… Its not 125kHz. Its 315MHz. I'm not sure how well the signal will pass thru water. You may also find some 27MHz ones like this. \$\endgroup\$ Oct 20 '21 at 15:41
  • \$\begingroup\$ There's something like this on ebay. ebay.com/itm/… \$\endgroup\$ Oct 20 '21 at 15:42

The specification on those 3-axis coils (amazing for ali*****!) goes something like this:

  • coil inductance 7.2 mH
  • coil resistance 200 ohms
  • coil self-resonant-frequency 330 kHz
  • coil Q 30

This coil looks like it would be used at a common frequency of 125 kHz, same as the transmitting coil's driver of 125 kHz.

To improve the pick-up coil's sensitivity, you should resonate it with a parallel capacitor, shifting it's resonant frequency down from default 330 kHz to 125 kHz. The detected voltage measured by the oscilloscope channel should greatly increase.
The problem is, your oscilloscope adds capacitance itself, and any cable that you attach adds more capacitance.


simulate this circuit – Schematic created using CircuitLab

I calculate that capacitance that you should add is somewhere between 193 picofarads up toward 220 picofarads. But that must include any capacitance your oscilloscope adds. When you detach the oscilloscope, resonance will shift up in frequency.
A X10 attenuator oscilloscope probe can help reduce the oscilloscope capacitance from influencing resonant frequency - use it...these add only about 11 picofarad capacitance, whereas a X1 oscilloscope probe adds much more capacitance - in the 100 picofarad ballpark. In the schematic shown above, the oscilloscope adds roughly 100 pf...to achieve resonance at 125 kHz, C3 should be reduced to 93 pf. When the oscilloscope is removed, C3 of 193 pf would achieve resonance. These values are approximate, and depend on the oscilloscope, cable lengths.

Since each coil is orthogonal to others, each should be tuned to resonance with a parallel capacitor. You would detect 125 kHz of each coil, producing a DC voltage from each resonant coil, then sum these voltages to "detect" a transmitted signal.
You should likely not try to add up 125 kHz signals from the three coils before detecting them, since each coil's phase is likely not aligned with other coils.

  • \$\begingroup\$ I purchased a pack of variable 120pf capacitors, and some AD620 boards. I'm hoping that I can twiddle the caps after the 'scope/AD620 is added/removed (or keep the AD620 in my final design), so that the circuit behaves the same when I'm done testing it. I still don't quite understand how the added C3 causes a DC voltage? I do not need to know - the fact it will is fine :-) thanks heaps for the help!! \$\endgroup\$ Oct 21 '21 at 5:28

You need to move beyond the oscilloscope, and begin to implement some actual receiver circuitry to reach the next proof-of-concept stage. The 125 kHz signal needs to be amplified, with either automatic gain control or a simple limiter, then envelope detected to pick up the code modulated onto the carrier. All this is built in to the PIC16F639 chip (a $3 item).

Don't reinvent the wheel. The application note for the PIC16F639 (found here) gives extensive design information, and references to other data sheets with more specifics. It has a schematic that shows how to couple the 3-axis coil to the 3-channel analog front end (AFE) of that chip.

If this is beyond your current skillset, you may not need to hire an engineer; a reasonably skilled technician can build the circuit using the schematic in the app note. Then you can do some meaningful testing regarding underwater range, interference, and reliability.

  • \$\begingroup\$ Can you suggest how I find a "skilled technician"? (I actually asked literally that question prior to this, but my questioned got killed as off-topic: electronics.stackexchange.com/questions/591222/… ). That PIC16F639 has a lot of stuff I do not want - like the wake-up circuitry - and I suspect that might actually prevent it from being of use... up to about now, I wasn't sure how much complexity existed in simply reading the signal off the inductor, but it's looking more and more complex!! \$\endgroup\$ Oct 22 '21 at 1:23
  • \$\begingroup\$ There are websites where you can seek out freelancers in EE. Since there's already a project posted here freelancer.com/projects/product-design/… I'm going to guess it's yours, otherwise you have competition! The LF receiver is the most challenging part, so it makes sense to leverage all the engineering that's gone into inductive Passive Keyless Entry systems. You'll find several chip makers in that space. Read all their briefs and app notes. \$\endgroup\$ Oct 22 '21 at 4:06
  • \$\begingroup\$ That's not my project, but I have posted many, on several sites. In every case, the end-result is that nobody can help. 90% are robot-bids anyhow (I always include a "trick CAPTCHA" - an easy question near the end that they must answer in their response), and 100% (including the robots) demonstrate either no willingness to do the work, or no ability to, in post-project preparation. After the last year, I can very confidently state that there are NO "reasonably skilled technicians" reachable online. Not by forum postings. Not by online job postings. Not by direct freelancer contact. \$\endgroup\$ Oct 22 '21 at 7:40

The suggestions above nudged the direction of my research, and I've ordered one of these:


I'm quite confident that there's no chance I would be able myself to match the 5m to 8m range of that pro-built kit, plus - bonus - it only has a 2 axis receiver, so I can orient that so as to minimise reception of unwanted transmissions coming in from the side instead of "above" (e.g. other divers with their own thrusters).


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