Reading Wireless Air Transmitter using Arduino

[-JD-]

I don't think it's likely that the alpha characters are used in the transmission. If they are, the computer ignores them as only the numeric is programmed into the DC when "pairing" to the transmitter.

That is currently the case (numeric-only filtering on the receiving computers) as far as I know, but if the prefixes are mapped into the transmission:

1. They could be filtered by some computer manufacturer with a new product, or even a software update.
2. The coding should be discovered for completeness of the data-stream documentation.

 

[rg422]

@rg422 what brand and alpha prefix were your transmitters?

Here is a photo of them:

One has a BM prefix, the other i don't know nothing is etched.

 

[TooCold]

Well, lots has gone on in this thread since I was fumbling around last year to figure out the modulation scheme. Pretty exciting! @MadUKDiver, brilliant job decoding the bit string. My interest (like the OP) is having a receiver to harvest pressure readings from all nearby tanks, on the surface and maybe underwater too. A standalone receiver/logger/display with alerts/alarms can have a more generous power budget, and so be more sensitive (greater range) and better noise immunity. If you were conducting a dive with students or new divers wouldn't you like a (reasonably cheap) way to keep tabs on everyone's air?

However, building xmit is interesting too - I hadn't thought of "hijacking" the signal to transmit other non-pressure data. That would be a reason for me to switch from bar to psi (cringe) to get the wider numerical scale.

Shouldn't need the DAC, just a port pin and bang out 38Khz square wave pulses into a suitable coil/ferrite antenna.

I haven't done radio in decades, but wouldn't a square wave signal into a resonant circuit be really noisy (so lose lots of power in spurious transmission)? If using a microcontroller, maybe better to output pwm on pin at a couple hundred kHz then low-pass filter to get an approximate sine wave.

I've bought a BD623 Motor Driver to buffer the output of the UNO. It has a bandwidth of over 100 kHz so hopefully up to the task
...
Edit: BD6211 Motor Driver looks more promising, it works down to 3v supply.

Using push-pull amplification sounds like a good idea, but how do you switch the direction of this kind of chip at 38 kHz - it has 2 separate input pins for fwd and rev PWM. Can it switch that fast? That's not the same as being able to drive the PWM inputs at up to 100 kHz. Like I said, I haven't done this kind of thing in decades.

Cheers

 

[TooCold]

I have been looking around and it looks like there is absolutely no project even attempting to do RF transmission at 38 kHz or even LF.

The transmitter and receiver circuits for this are not really radio. Radio transmits and receives the ELECTRIC field of the electromagnetic (EM) radiation using (usually wire) antennas. These scuba tank pressure reporting systems (and RFID tags, wireless charging, NFC on smartphones, tap-to-pay credit cards) transmit and receive the MAGNETIC field of the EM radiation using induction coils. The transmitting coil and the receiving coil make an air-gap (or water-gap) transformer. A varying electric current passing through the primary coil creates a magnetic field which propagates through the air (or water) to the other coil, where it induces a (weak) current in the secondary coil that varies just like the original. This is exactly what goes on inside a lot of audio amplifiers, so look for audio amplifier circuits that use an impedence matching transformer at the output before the speaker.

Here's an example from circuitstoday.com of a basic idea for the transmitter. I've crudely indicated what to remove, and added a capacitor across the coil to make a resonant circuit that would need to be tuned to 38 kHz. The "audio in" would just be a constant 38 kHz generated by the microcontroller and switched on and off to get the right timing between bursts. Of course this isn't a useable circuit - just shows the basic concept: amplify input pulse train, with output to a resonant LC ("tank") circuit. Looking for audio circuits and resonant circuits (of which there are scads) gets you closer than looking at RF.

So imagine removing the speaker and the output (secondary) coil of the transformer from the amplifier. The remaining coil will still radiate the magnetic field. If the other coil is set up across the room with its own amplifier and speaker, it can receive a (faint) copy of the magnetic field, amplify the signal, and send it to the speaker. We now have a magnetic induction transmitter and receiver pair for sending audio across the room.

[Edit: the above is a major simplification, but I realize now it's a also misleading when thinking about the receiver. Sorry. The incoming signal is a series of bursts of 38 kHz (about 10 cycles each burst I think), separated by either a long or short pause (encoding "1" or "0"). The reciever needs to recover the on-off of these bursts from the 38 kHz signal, which requires an AM style detector circuit. Searching online for "TRF receiver circuit" will get you lots of examples of the simplest (and oldest) design. TRF is "tuned radio frequency".

38 kHz sound is beyond human hearing, that is, ultrasonic. But it's in the same ballpark. So for our transmitter and receiver the electronics can be fundamentally the same kind of design; it just needs to be tweaked for this frequency, and can be simplified because it only has to handle one narrow band of frequencies aound 38 kHz. In fact many of the same components (transistors, op-amps) still work fine at that frequency. Of course the devil is in the details, so that's as far as I can take it without doing some serious relearning, but hopefully this makes some sense for those without much electronics background.

 

[TooCold]

@MadUKDiver - nice post working out parameters for the LC circuit. A 15 μH coil and 1.1 µF capacitor seems like the right ballpark (although I don't know why I think I should still have any intuitions about this stuff). But the inductive reactance seems way off.

Putting these into an online calculator [link] (Rod length 38mm; 6.35mm dia; Initial magnetic permeability 2000; Wire dia AWG-24; turns 17) gives an inductance of around 15 μH. Another calculator [link] states at 38 kHz an inductive reactance XL= 3.58 kΩ.

Given that 17 turns of 24 ga copper will have a DC resistance of some small fraction of an ohm, the resulting Q factor would be in the 10's of thousands, which I don't think is possible, is it?

I have an old broken pelagic transmitter somewhere I think I've given up on trying to fix, so I'll pull it apart and find out what it's output circuit components are. Or is this already known info?

 

[scubadada]

...I have an old broken pelagic transmitter somewhere I think I've given up on trying to fix, so I'll pull it apart and find out what it's output circuit components are. Or is this already known info?

If your transitter is an Oceanic, you can have it replaced with a refurbished or new transmitter for $120. I would imagine, before you take it all apart

 

[TooCold]

If your transitter is an Oceanic, you can have it replaced with a refurbished or new transmitter for $120. I would imagine, before you take it all apart

Great tip! I didn't know. But it's an aqualung, and already in pieces now anyway. It was damaged in what was obviously abuse (fell out of a vehicle cruising down the highway).

 

[MadUKDiver]

I have an old broken pelagic transmitter somewhere I think I've given up on trying to fix, so I'll pull it apart and find out what it's output circuit components are. Or is this already known info?

I've got a box of parts on my desk but not had time to put any circuits together or code some test firmware. So would be very interesting to know more about your transmitter circuit.

 

[TooCold]

Turns out it's pretty straightforward dismantling and reassembling the transmitter (except the pressure sensor assembly). It could easily be done without physical damage by anyone with a modicum of ordinary skill. Getting the contact alignment right is the only aspect that takes more care (5 little coil springs between the pressure sensor board and 2 leaf spring contacts to the battery compartment), but even that just requires paying attention to the index tabs: it's impossible to close it up if anything is misaligned.

The antenna is a long toroid ferrite core with 15 turns of 0.6 mm diameter copper wire. I haven't looked up what gauge that is. Length 24mm; O.D. 26.1 mm; I.D. 23.8 mm. I don't have an LCR meter but if I can dig up a capacitor with a value I can trust to be accurate (or
know if I can trust my cheap multimeter) then I can hook them up to my signal generator and scope and figure out the inductance from the observed resonance frequency.


There are only two chips and one transistor (and crystal and a bunch of resistors and capacitors):
- TI M430F133 (8 MHz mixed-signal MCU) link
- TI TLV2764 (single supply quad op-amp) link
- 335N N-channel MOSFET (no manufacturer's marking) link to one version
The caps with "?" values are small ceramic surface mount with no markings and refused to be measured in circuit by my multimeter. There doesn't appear to be any battery reverse polarity protection.

Here's a (partial) schematic. I believe the output side is complete. It's dead simple (and dirt cheap). The MCU doesn't have a DAC so pin 12 must be outputting square waves to the amp. MCU pin 7 (ADC ref voltage out) goes to somewhere in the input circuitry, which makes sense.


The op amps all seem to be part of the input side; its traces and those of the remaining components appear to go to the pads for the pressure sensor or to the MCU, some via resistors. The connections are either under the chips or obscured by the ground plane. The MCU pins to power the ADC are wired up. That, and the presence of the quad op-amp chip point to the pressure sensor being an analog device.

 

[TooCold]

Today I played around for a while with the PPS antenna coil and some spare components I had lying around. Among other things I found a precision 1 microF capacitor, and a precision 22 microH shielded coil. I also built two coils to experiment with. I built my xmit coil to get a 38 kHz resonance peak when used with my 1 microF cap (I simply adjusted the number of turns until the peak resonance freq was right).

Here's what I think I learned. Just remember I'm not a sparkie, I've forgotten most of what I used to know, and there may be a lot I don't know I don't know, if you know what I mean, so take with salt...

When the tank circuit on the PPS board (with antenna coil attached) is driven by a square wave signal, the resonance peak is at 41.1 kHz. At resonance, the square wave input is transformed (filtered) to a sine wave. However, this is very sensitive to both component selection and tuning.

When the coil is detached from the PPS board and connected across a 1 microF cap, the resonance peak is at 33.0 kHz.

Conclusions:
1. The PPS antenna coil inductance is about 23.2 microH.
2. Using a 1 microF cap, a 38 kHz resonance peak should require a 17.5 microH inductor.
3. The type of capacitor used in the tank is important, not just the value. Regular 1 microF bypass caps didn't work for me. Low leakage types required?
4. Driving the xmit tank with a sine wave is better, but square wave input is obviously good enough as long as the tank is tuned well enough to the carrier frequency.

I kept a log and took some pictures as I went - I've attached a doc with the details.