Cheap RFID interface

I don't think a passive device can be involved in a rolling code situation.

Garage doors (and cars) do it by following a pattern (a rule of some sort). By pressing the remote a few times, the receiver can learn the rule. My understanding is that the receiver is smart enough to skip missing codes as well - which would be needed if you push the remote button out of the "hearing" of the receiver. I know one that I looked at would still work 16 codes away from the last accepted one - so you could press the remote 15 times while away from the receiver and still have it work. If you went beyond 16, you'd need to resync the receiver.

There's a description of one implementation of this kind of system in this document (see page 7 (23)):
Rolling code receiver documentation
 
The Parallax system is about as secure as you can get - unless someone steals your RFID or otherwise gets its ID number. If they can program another RFID unit with the same ID, then they can do whatever you can. RFIDs come from the factory precoded, but it may be possible to build a fake, programmable one.

There are no rolling codes involved here - the RFID always returns the same number.
 
I am not worried about security really, anything can be broken, I mentioned those links in case it helps us figure out how the speedpass RFID works. I have an active transmitter in my car, and a small passive one on my keychain, I would love to use those.
 
After a little research, I decided that this is pretty darn cool. Mainly because it's so cheap.

Because it always transmits the same code, someone with one of these sensor boards could 'snoop' your code in an elevator or anywhere else where she could get her handbag within 4" of your pocket, but that's pretty extreme, and I believe there is no other practical way to steal the code.

Because it transmits at such a low frequency (125 kHz) the signal from your actually using it could radiate for quite a distance (possibly hundreds of feet) but someone would need a 2000 foot long quarter wave antenna to pick it up (another project for BSR). And if you should notice somebody walking around your neighborhood with a 2000 foot antenna, you could simply keep the reader in a metal mailbox or other shielded container. You could unlock your front door by pretending to check the mail.

As soon as they are off backorder, I will order a few, and take Smee's suggestion of adding a powered RS485 interface to it.

BTW, E, I read those posts on the SpeedPass, and they were interesting. Here is my understanding of SpeedPass compared to Parallax:

Though both the Parallax board and the SpeedPass are excited by similar frequencies (SpeedPass is 134 kHz), the Parallax transponder responds by modulating a reflected AM signal, and the SpeedPass transponder responds with a separate FM FSK signal. Also, although the SpeedPass encryption algorithm has been reverse-engineered, it still is not published. So for now, we are relegated to using SpeedPass only at the pump.
 
cool,I am not too worried about the security breach considering all the work you have to go through. If someone wants your SpeedPass / car that badly, there are easier ways. Pretty interesting how that tiny SpeedPass can do so much tho.
 
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