How do PKE keyless remotes work (inductive data transmission) and how would I "hack" one to make a transceiver?

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Joined Mar 31, 2013
I'm wanting an undersea short-range remote (wireless button for dive thrusters) and induction seems perfect, so I bought a passive keyless car alarm to see how they work. I do understand that the car transmits an inductive signal, which the remote picks up (and then the remote does it's own 433mhz thing, which I don't care about).
Luckily, the remote has a blue LED which blinks each time the car sends the signal (one time per second approx). I've tested that it keeps blinking even with my thruster between them running (so I know the motor+ESC is not ruining the signal).

There are 2 key fobs - they have a button on each to "activate" - whichever one I press causes that FOB's LED to blink - so we know that the car is modulating the inductive signal in some way that only "wakes up" whichever remote it is talking to.

The remotes have these receiving coils in them:-

And the car has 2 of these transmitter things (coil antennas I expect):

The bits I cannot work out, is how the car is modulating a signal, and how the coil is picking it up: I put an oscilloscope over one of the transmitter outputs, which reveals some square (coil disconnected) or triangle (coil connected) wave forms, which *seem* to show each wave individually modulated (my cheap and buggy DSO203 oscilloscope isn't really trustworthy though, so I'm not sure). I was under the impression that inductive transmitters sent bursts of signal as some specific frequency, not bursts of all different frequencies ?

I built an arduino circuit to try and log the signal timing coming out of the transmitter, which just has me even more confused (my plan was to try and record then play back the signal to try make the LED blink), and it's all over the place (no obvious easy point to start working from)... maybe it's some weird kind of multi-phase signal? (no FFT on my DSO)

And the receiver has me totally stumped - I can't measure anything at all on the coils (at any orientation) - I presume some kind of amplifier is needed? What should I be using for that, and how do I deal with (what I expect will be) a huge differing signal strength at (say) 1cm (or 1 inch) versus 2m (or 6ft) ?

My goal is something simple I can build myself with arduino or similar MCU's and supporting hardware - so I don't really need to understand what already doing on above - just what I would need to pump into the transmitter coil to cause something to happen that I can pick up at the receiver coil? - I only need "1 bit" (on/off) for my remote control.

Clues/comments/thoughts/suggestions anyone? [FYI - I prefer inductive - I don't want sonar and no sensible radio works under sea as far as I know]