It’s been a while!

The last post I made here was July 2018… quite a lot has happened between then and now. Not only did Buttons sadly get involved in a car accident and need to have a leg amputated (don’t worry he’s fine and still himself!), but I also moved house and had a child.

Also there was a global pandemic.

Anyway enough of all that rubbish, the most important thing is that I’ve been working a little on Version 2 of CatTrack.

My requirements for version 2 were:

  1. Fewer components to solder
    • Soldering all of the matching components for the CC1125 took a while, not to mention the annoyance of a ground paddle on the CC1125.
  2. Ideally an integrated antenna
    • Again, my time to play around at home with hardware is now a lot more limited, so avoiding having to spend time matching into a chip antenna would be a bonus.
  3. Better battery life
    • As it stands, the battery life is around 4 months. I’m convinced this is much better than any GPS cat tracker product you can buy, but longer would be even better!
  4. An integrated buzzer
    • Several times when looking for the lost collar I found that I had an incredibly strong signal strength on the receiver, but could not see the collar anywhere. On one occasion it had been buried in the ground somehow!
    • Apple’s solution to this in their new AirTags is to use UWB to give you a fancy arrow to point to the device when you are close. My simpler solution is to include a buzzer such that you can easily use your ears (rather than an arrow!) to find it quickly when you are close.

I have already got the PCBs back and soldered the first iteration of version 2. There are a couple of little problems with it so I’m going to do another revision of the PCB to mop them up.

I’m now using the following components:

  • An Anaren A1101R08A RF module.
    • This module is fundamentally a TI CC1101 (similar to the CC1125 used on version 1), with all the matching components already soldered, plus an antenna integrated.
    • The sensitivity of the CC1101 is not as good as the CC1125 I used previously, but I am hopeful that the range will work out to be similar. The sensitivity is -112 dBm at 1200 baud, with a 14 dBm transmit power. I will do a test to confirm the actual radiated power from the integrated antenna.
  • A u-blox CAM-M8Q GPS module
    • This has the advantage that is supports multiple GNSS systems: GPS, GLONASS, BeiDou and Galileo. In reality I expect it to only work with GPS and GLONASS in my location, but that’s better than GPS alone anyway.
    • The GPS antenna is integrated into the GPS module as before, only this time it’s a chip antenna rather than a patch. This means that it requires nowhere near the height that the patch antenna of the previous module required, so things will fit in the enclosure a little more comfortably.
  • A Murata piezo sounder
    • This will allow me to locate the device when I’m close by. I’ve connected it to the PWM pin on the PIC, so I can play tunes on the piezo…. like the Peppa Pig theme!

There’s also a PIC Microcontroller, of course.

It was a bit of a minefield to satisfy the various antenna ground plane and keep-out requirements, but I think I’ve managed it. I’ve kept the size of version 2 to exactly the same as version 1. The size is a good compromise between satisfying the Anaren and u-blox antenna requirements, whilst still being fairly small.

I’ve redesigned everything from the ground up to favour battery life (again!), and I am confident that I will be able to get the standby current consumption down to under 1.0 μA. Depending on how often I wake the device to check for incoming transmissions, I reckon I’ll be able to get a battery life of around 7 months.

As is always the way, I did make a couple of mistakes on this first revision of the version 2 PCB:

  1. I had planned to drive the GPS module directly from the PIC GPIO, but the startup current required for the TCXO on the GPS module causes the voltage to sag and get stuck in an infinite loop, so I’ll control the power to the GPS module with a P-type FET instead. I was really trying to cut back the number of components I had to solder, but went a step too far here!
  2. I forgot to include battery voltage sense circuitry (more complex than you might imagine as I can’t afford to waste power in the resistors required for the sense circuitry potential divider). It’s really handy being able to remotely work out the battery life remaining, so I’ll definitely need to fix this.

Watch this space….!

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