This is really cool and I like how the author describes how it works step by step. It makes it interesting even if you're not going to build one yourself.
I'm guessing that the data you extract from the RFID chip is some ID number that you need access to a database to do anything with? And that database is closed and only available for vets and similar? Either way, checking for the existence of a chip can probably be valuable enough sometimes.
And it can be integrated with a smart cat door or other local projects where you want to identify your pet.
> I'm guessing that the data you extract from the RFID chip is some ID number that you need access to a database to do anything with? And that database is closed and only available for vets and similar?
Mostly correct:
Many pet ID microchips also have a temperature sensor for some reason, which can be read out along with the fixed chip ID. But you're correct that the chip itself doesn't hold any owner information.
There are dozens of different microchip ID databases; some of them are linked to specific chip manufacturers, others are general-use. Generally speaking, the registries don't give out owner contact information; instead, they'll contact the owner themselves when notified that a lost animal has been found.
The AAHA has a tool that'll query most of the major databases at:
Awesome project with a great README. This makes me want to dust off my ancient analog electronics knowledge.
Does anyone know why the second stage op-amp cleaned up the signal so much? It appears to have removed a higher-frequency oscillation, leaving a very clean, nearly square-shaped wave.
The second op-amp is configured as a comparator. It drives its output to the positive rail when the input exceeds a threshold, and low otherwise. The higher-frequency oscillations aren't anywhere near the threshold, so they disappear.
This was a fascinating read if for no other reason than seeing how the filters were assembled. I have much more understanding of digital than analog, to say nothing of RF stuff.
I would love to see more stuff like this that is so understandable! Makes it seem much less like black magic...
Yes, it's a nifty technique. One detail that might not be immediately apparent is that it drives the LC circuit in a series-resonant configuration, where it looks like a low-impedance load that can accept a lot of current from the transistor half-bridge, but it receives the response in a parallel-resonant configuration, which is good for sensitivity when driving high-impedance loads like the 22K input resistance presented by the opamp circuit.
For this topology, best results will depend on low-impedance AC bypassing on the Vcc line, which he doesn't show on the schematic. I'm sure the Arduino's supply input has enough capacitance to take care of that, but if you adapt the circuit to other uses it's something to keep in mind.
Personally I would use a timer capture channel for input, that way you get automatic timestamp capture and you don't depend on gpio isr handlers firing on time in case there's some other concurrent interrupt.
Anyway I wanted to build something like this as soon as I chipped my kitty half a year ago, but this popped up just recently!