Bluetooth conversion project- Ballistic Chronograph !! ideas please

[Tracker]

Hi Folks,
I have a Caldwell Ballistic Chronograph which measures projectile (bullet) speed quite accurately.
https://www.caldwellshooting.com/range-gear/chronographs-and-wind-meters/ballistic-precision-chronograph/720001.html

The product has a a very useful accompanying phone app which logs the speeds, provides average, median, max, min etc.

However!!!! the app gets its data via the Mic pin on the phones 3.5mm jack.
The Chronograph is supplied with 20 Ft of audio cable (the 4 pole type). 
The mic line is modulated with a 3khz data stream (400mv p-p) which communicates readings to the app.
Data stream is one way only, from Chrono to the app.
It works ok but is clunky. And modern phones (like new Apples) don't have headset jacks anymore.

Caldwell have come out with a BT version of the Chronograph  (and updated the app to accommodate BT) but its quite expensive and my Chrono works perfectly well.
When I enable BT coms in the App, it reports 'Caldwell Chrono not found, is Caldwell Chrono Paired?

Ideally the solution should send the 'audio' over Bluetooth and be recognized by the phone app , sounds simple enough but the devil is in the detail right!

I was think along the lines of an Arduino with Bluetooth module (built into the Chronograph) , but I don't know enough about the BT protocol to know if this is a workable.

Open to any ideas you guys may have?

issues expect to encounter...
I don't know how Caldwell implemented the BT upgrade, do they still send 3khz data down the audio Mic channel (simplest way I guess) or did they go all fancy and decide to digitize the information over BT... if they did then I'm a bit screwed.
I don't know what BT device name the app is expecting to find, but given the error code mentioned above I am hopeful it's as simple as 'Caldwell Chrono'.

anyway, looking for suggestions or any advice.

Thanks
Tracker
 
   

[Ian.M]

The timing data would have been digital to start with, so Caldwell's technically simplest implementation would be to feed the bluetooth module with the data *before* its modulated on an audio carrier for direct connection, so you are S.O.L. unless you have the skills to decompile the app. 

As Bluetooth offers bidirectional connectivity, the app may (and probably does) handshake with the chrono so you'd need to reverse engineer that well enough to emulate it on the Arduino, which is likely require hands-on testing with the genuine BT enabled chrono, so again unless you have a *LOT* of the old chronos to convert, (e.g. for a gun club) you are S.O.L. again as the sane decision once you have a new one is to sell off the old one, not jump down the reverse engineering rabbit-hole.

[Tracker]

Thanks Ian, you have probably summed it up nicely!
I feared it would be tricky.

I guess the alternative is to ignore BT and look to make a wireless 'TX dongle to Rx dongle' connection to carry the modulated audio.
But I'm not convinced this is going to be any better then 20ft of cable... the appeal of the project was the BT aspect so I may just stick with what I have.

[Tracker]

Hi Erik,
As per your PM request...

Phototransistor is BPY-62
Focused with two plastic lenses as per photo
IC is TL064I opamp
Plastic housing has anti-reflective molding ridges at the mouth.

I have also attached a photo of the main PCB with processor.

The phototransistors work with ambient light but are focused on a white diffuser about 12 inches away.
As the bullet dimms the light the sensor detects the drop.... not unlike the way exo-planets are detected using the transient method 
But many folk complain that the dips are not detected in low light or indoors, so Caudwell added a pair of LED bars to provide extra light at the diffuser.

Hard to replicate this, considering the unit is only about 150 Dollars in US.

[Haenk]

BT Pairing might need a (*the*) device ID and a (*the*) PIN. So a BT sniffer and some software are probably required. It might be possible to decompile the app though; with a little bit of look the required pairing data could be there in plain sight.

Suggestion: https://github.com/greatscottgadgets/ubertooth/