California uses a radio-tag system called FasTrak for vehicles, and recently they sent me a new tag.

So naturally I had to open up the old tag, right?

This is the closest I was able to get to a non-blurry picture.
It's not super complicated inside: Two main big components, a battery (non-rechargable lithium!) and a switch. The antenna is built into the PCB itself.

It's officially called a T21 Internal Tag Fixed Battery Switchable

(some models don't have the switch)

part number R2916-219

This thing in the corner under the designation BP1 is an SFM-1640A by East Electronics, an SMD External-driveen Piezo Transducer.

This thing beeps when you change the number of occupants or (sometimes) when it gets scanned.

And the microcontroller is a TI MSP430F2121 Mixed Signal Microcontroller.

Datasheet says this runs at 1.8v to 3.6v, and ultra-low power consumption: 250 µA at 1mhz, 2.2v, 0.7 µA on standby, and 0.1µA when off (with RAM retention)

It's a 16-bit RISC architecture, 256 bytes of RAM, 4K +256B of flash memory

That battery is the Tadiran TL-4934 Lithium Thionyl Chloride.
Nominal capacity of 1 amp-hour.

if they're running this thing at 10µA on average this battery will last about 11 years

that's all I can tell without being able to photograph it better (I'm doing this while laying down) or do some continuity testing

so fun fact the spec for how these work is public:
https://web.archive.org/web/20131231125912/http://www.dot.ca.gov/hq/traffops/itsproj/Title_21/Report/Title%2021%20Specification.pdf

and as I guessed, they're RFID.
The reader transmits at them to turn on the tag so it can return an identification signal

it says these things wake up if you send them a 33 microsecond RF pulse at 915 Mhz, all ones in manchester encoding

so presumably with a SDR you could make this thing identify itself by sending the proper message at it.

other interesting notes from the pdf:
it runs at 300 kbps, which seems awfully fucking fast for a system that is only sending, like, 60-124 byte messages?

it also has an "agency code" in there. So the tag can be smart enough to not identify itself if similar tags are being used for some other purpose

each transponder has a 32bit code to identify it. which seems low, given that the US already has something like a third of a billion cars.
(This is a california-specific tag but they do mention specifically making this standardized so it can be used nationwide)

some more message structure info:
https://web.archive.org/web/20131231093328/http://www.dot.ca.gov/hq/traffops/itsproj/Title_21/Report/Title%2021%20Approved%20Record%20Types.pdf

oh god, the 32bits is structured, it's not just an ID to some internal Cal DOT database:
https://web.archive.org/web/20131231084853/http://www.dot.ca.gov/hq/traffops/itsproj/Title_21/Report/T21CATransponderIDVer17.pdf

the first 4 bits are your tag type, based on how many people are in the car. The next 18 bits are the facility number. the state of california has 75001 facilities, each of which can have up to 1024 tags

WHAT DO YOU MEAN THEY ASSIGNED A MILLION TRANSPONDER IDS TO A BRIDGE IN CANADA?

so the Golden Ears Bridge in Metro Vancouver, British Columbia had electronic tolls from 2009 to 2017, including transponders you could install on your car to auto-pay

this sounds like they cooperated with the california department of transportation and just used the FasTrak system for their toll system

ahh, here we go:
"Effective January 1, 2019, Caltrans adopted a new protocol known as 6C. The existing Title 21 protocol will continue to be used for seven years after that date, and then be discontinued. The seven-year overlap with the two protocols is intended to give the toll facility operators the necessary time to eliminate their existing inventory of the Title 21 transponders."

so I wonder if this means my transponder is actually 6C, not Title 21?

or if this one is title 21, and the new one is 6C?

the new protocol is designed to conform to ISO/IEC 18000-63 which'll let it interoperate with other states/nations implementations of automatic vehicle identification

links for info on the current implementation:
https://dot.ca.gov/programs/traffic-operations/electronic-toll

making them intercompatible is a good idea, imo.
the current situation is that the US has 15 separate standards, and two of those are for Michigan

a funding bill in 2012 required that they all be intercompatible by 2016, but that didn't happen. obviously.

the louisiana version is called "GeauxPass", which I assume is pronounced "GoPass" and not "GooPass". eww

Anyway these are just the RFID versions. Plenty of places are using versions based on license plate readers, either automatic or manual

oh wait, this one is definitely Title 21! it says as much on the PCB

so yeah, my new one must be 6C.

I did get two 6C tags, so I could disassemble one... but I think I've misplaced the spare

oh wow. The new one has no battery, and still supports the flex option where you can set it to 1, 2, or 3+ occupants

So here's the new version. It's got a little rotational thing instead of a switch. Much smaller, much thinner, less beepy.

Inside it's super simple. There's an antenna on the back, and a spring to make the wheel snap to the three numbers

Under the wheel, you can see the antenna is two parts, with a gap in the middle.

And there's two squares under the wheel.