Advice for flex decoder / pulse analyzer logging of simple wildlife tag pulse interval and level?

[Michael Lester]

Hello everyone,

I'm very new to RTL_433, and struggling quite a bit to be honest, but I have had some limited success in recording pulse output from wildlife tags in the 149-152 MHz range. As the pulse interval of these tags is very long in comparison to 433 devices, the RTL_433 pulse analyzer outputs just single pulses. It does provide pulse width information (nominally, 14 ms) and SNR of about 15dBm, but the crucial parameter that I can't get the pulse analyzer, or the flex decoder to produce is the pulse interval. 

The VHF tags in my project output a pulse interval that varies between 1500 ms and 4500 ms which is inversely proportional to the temperature of the animal - in this case a marmot (the tags are implanted). For example, a pulse interval of 1800 ms would indicate a warm, active marmot, whereas the same tag would slow down the pulse interval to around 4500 ms if the marmot was cold (either hibernating or dead).  

If successful, I'll be deploying the dataloggers in the mountains for several months,  so Ideally I would like to record a very lightweight log file which would hopefully be updated as follows:

Timestamp, pulse interval in ms, pulse amplitude

This may be possible with the flex decoder, but I've tried hundreds of permutations and I'm obviously doing something wrong...

Any help would be greatly appreciated.

 

[Christian Z.]

That's an interesting project! The default frame interval for rtl_433 (at 250k sample rate) is 250ms (four per second), so your reading would span really many frames.

The default pulse detector won't be up for that, in fact it needs different levels within each frame to differentiate pulse/gap.

Also the band is rather wide at 3 MHz, an RTL-SDR can barely do that (good cooling needed).

If you go with SDR I would use a custom pulse detector to properly read signals -- very slow averaging of the (base, noise) level to detect pulses.

Should be simple to code, you only want to read an amplitude at low rate.

But I guess a much cheaper OOK recevier would work better, if you can control the gain, which I haven't seen in those receivers. They usually raise the gain to give you noise as pulses.

Really you would want a receiver that only gives you the (absolute) amplitude seen over a wider band. Are there cheap commercial (analog) receivers that could be hacked, maybe?