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.
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)...
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:
- This is not RF - portable RF does not go through sea - this is inductive.
- I want short 2m range and reliable operation - so acoustic is off the table.
- 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).
- Yes, I know that orientation is important - I'm testing just with the X asis, and orienting my tests properly.