I’ve pretty much ruled out any kind of design that requires a continuous connection via cellular modem.
The advantage of a continuous cellular connection is that I would be able to remotely send a query to the collar (either via SMS or a data connection) and get it to report its location back to me, from anywhere in the world.
Sounds really cool, but unfortunately you could only ever get a battery life measured in days out of such a design, at least without designing a trailer full of batteries for Buttons to tow along behind him. This is why the pawtrack collar (discussed here) only lasts for 2-3 days. I want months, not days!
In order to get months rather than days, I need the collar to spend the vast majority of its time in a really deep-sleep mode. Assuming I manage to design-in a 200 mAh battery, then in order to get a 90 day battery life I’d need to make sure that I consume an average of no more than 93 µA (that’s 93 millionths of an Amp!). That’s over 100 times lower power than a typical LED!
I reckon that the way to achieve this ultra-low power consumption is going to be to design a tracker which works in a similar way to devices like tile. This uses Bluetooth Low Energy (BLE) to allow you to communicate with the tile device from your phone. Because of the nature of the BLE protocol, the coin cell battery in a tile can last for a year.
I could certainly design a tracker to use BLE, just like the tile, but the problem with BLE is that the range is pretty limited. You’re never going to get over 100 feet range, even across a completely empty field! Not ideal for a cat tracker.
All I need to do is design a device that sacrifices some of the 1 year battery life of the tile to give us some extra range, hopefully up to around a mile radius from wherever I’m stood. Unless something’s gone really wrong then that hopefully means I’ll be able to find Buttons whilst sat in my lounge.
I reckon the way to do this is with a narrow-band RF design. I think I could have the collar sit in a really low power receive mode, with the GPS module powered off. I would then send a narrow-band RF message to the collar from a base unit, instructing the collar to wake up and turn on the GPS receiver. As soon as it gets a GPS lock it’ll transmit the position back to the base unit.
A range of 1 mile and a battery life of 3 months would be perfect for me. Obviously the battery life would decrease each time you instructed the collar to wake up and transmit back its position, but given that it’s primarily a device to find your cat when it’s lost, I reckon this is fine.
So, onto looking for some suitable components to try and achieve this..!
I wonder if a coin cell would work?
Hi Bryant! It depends on what the current consumption of the GPS module and transmitter ends up as. Bluetooth Low Energy transmitters use a coin cell, but my transmit power will be considerably higher than BLE. The high internal resistance of coin cells could render then unsuitable. I’ll be investigating it as an option though!