Reading Wireless Air Transmitter using Arduino

[Cio]

Question.
If you're trying to produce a receiver, why not just reverse engineer a functional receiver?
A donor computer would provide you the next step, no?

This is what I am suggesting by looking into a transmitter, I did not think of opening up a computer which should do the same thing, good suggestion.

So we can expand our search to flooded computers too.

 

[Robbyg]

I tried to RX the transmissions while troubleshooting my Perdix AI xmitter. I was using an ICOM pro 756 II HAM radio and I saw nothing, even with the end of the coax up against the transmitter. It has a spectrum analyzer type function... nothing. The 38KHz frequency is at the very bottom of what it can receive.

The FCC database might have a report on the transmitter for testing. ELMO I think the database is called if I remember right.

I know the Pro II very well, I had a Pro III and later went with the 7600.
Due to craptastic solar cycle I have not been on the air too much. After reading your post I fired up the 7600 and put the beam on 40M and picked up nothing on 38Khz. I really did not expect to see anything on the bandscope since the Spectrum Analyzer is much more sensitive but it was worth giving it another look. 73s..

 

[OTF]

The issue may be that the transmitter only transmits a short pulse of data periodically to conserve battery. Probably a little tougher to detect.

 

[Robbyg]

The issue may be that the transmitter only transmits a short pulse of data periodically to conserve battery. Probably a little tougher to detect.

Don't know, but I kind of doubt it since I am also using a waterfall display and do not even see a blip. Also I tried setting different triggers for the signal and nothing happened.
I may be missing something very fundamental but the lack of a detailed FCC filing has me wondering what they are doing.

 

[Doctor Rig]

Love what your doing!

I’m enough of a geek to think how nice it would be (out of the water) to get a tank pressure reading without pulling out my computer.

Keep us informed on your progress! Thanks!

 

[Robbyg]

Ok guys I found the signal.
I should have stuck with my original idea that the Antenna was the problem and that maybe this signal is mostly magnetic in nature. It turns out the signal is almost totally magnetic and has so small of an electric field that the Spectrum Analyzer cannot detect it. I therefore made a Loop antenna and bingo the signal jumped right up. I am going to link some video of the audio of this signal. I pressurized my first stage and then created a small leak by loosening an LP hose. This way you can see the PSI changing with each pulse.

If you have a good audio program on your PC you can see the data in the signal and probably start looking at how it relates to the PSI reading. It seems to be Freq Shift Keying that is happening (FSK) type signal.

Here is the video link
https://hamradioamps.com/downloads/ShearwaterAI.mov

I will try to upload some direct Spectrum analyzer screen shots between today and tomorrow.

 

[njmorrell]

Some years ago I was having a problem with my Suunto Vytec and transmitter so decide to investigate. I made a simple resonant circuit using a ferrite rod antenna from a wireless clock (one that picks up the LF signal with time information) with a two transistor amplifier. It worked very well and I could see the signal very clearly on my oscilloscope. It turned out the transmitter was working fine but not the dive computer, so I sold the transmitter and chucked the computer. Since then I have not owned another transmitter and the project has been in my junk box ever since.

The circuit notes are a bit messy. I also attach a photo of what I made.

The ferrite rod has a nice high inductance (2.3mH) so not such a big capacitor needed (0.018uF). The calculated frequency is 24.73kHz but my note indicates 27.3kHz which was probably the observed frequency. I just added capacitors until I got a strong resonance.

From what I can remember the transmitter just sent short signal pulses with varying periods depending on tank pressure. The lower the tank pressure the more frequent the pulses. I never got any further.

Hope this helps.

Nick.

 

[Robbyg]

Nice work.
BTW if the file I linked is not playing, just right click it and save it to your drive.
I don't know how many people accessed it but my hosting service has now throttled the speed down so low that my browser times out before it starts playing.

Here is a smaller MP3 version that you can run through Audacity or another Audio analyzer.
https://hamradioamps.com/downloads/ShearwaterAI.mp3

 

[njmorrell]

Hi, I listened to the audio. I notice the period of the pulses did not change so I would guess the pressure is encoded in the signal together with the identity of the transmitter. I agree it will probably be FSK modulation.

The transmitter I tested with, was an early one with no identity, it would just pair with any computer (Suunto) and I definitely remember the period of pulses changing with pressure.

Looking back I am thinking that if you don't know the approximate frequency to make a suitable tank circuit a simpler design might be to make a high gain amp followed by a low pass filter say around 50kHz. Maybe two or 3 stages with low pass filters between amplifiers. This is probably how the receive circuit is set up inside the dive computer because there is no room for a big inductance.

Rgds Nick.

 

[Robbyg]

I wonder how those Suunto transmitters dealt with multiple divers transmitting close to one another? When you look at the Pelagic signal structure their is a first section of the pulse that remains the same and I think its safe to assume that is the serial number and then the last 1/4 of the pulse have a lot of shifting happening. I assume that is the last tones for the least significant digits of the Air Pressure.