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Topic: Car proximity smart key (Read 7142 times) previous topic - next topic

theace

Hello,

Does anyone know the details of how the proximity smart key for car works? I'm talking about those systems where you have a wireless key for your car and when you approach the vehicle you press or touch a button on the door and the car will unlock.

It seems that the key is always transmitting a signal and the car is always listening, or vice versa.

I'm more interested in what chips are the keys using and how they achieve so much battery life giving the fact that they are always transmitting, or listening.

Thank you!

Riva

Don't PM me for help as I will ignore it.

theace

I've already checked the subject. I found some low power rf transmitters that have like 8-10mA current consumption when transmitting, which seems like a lot for a key that has the battery changed once a year or a couple of years. Maybe they sent the signal once a couple of seconds. Who knows? These are the details that I want to find out about.

Riva

Maybe the smart key just contains a passive RFID chip and the car transmits the signal when the button on the door handle is pressed.
Don't PM me for help as I will ignore it.

theace

No. It seems that something is transmitting all the time. For example, in my car, when I get near it, the interior lights open up. And this is without me pressing any button.

JesusGonzalez


Chances are that the world is a passive rfid, I have this kit and it works very well, it is interesting your project

http://www.dx.com/p/nfc-rfid-rc522-rf-ic-card-sensor-rfid-reader-module-w-s50-card-keychain-for-arduino-403052#.Vt7zNmOYMyo

Paulcet

I've already checked the subject. I found some low power rf transmitters that have like 8-10mA current consumption when transmitting, which seems like a lot for a key that has the battery changed once a year or a couple of years.
You're looking at it backward.  The RFID "transmitter" is in the car, not the key.  The transmitter that is in the key is a standard RF transmitter, which is only active when a button on the key (fob) is pressed.

gpsmikey

#7
Mar 08, 2016, 06:11 pm Last Edit: Mar 08, 2016, 06:15 pm by gpsmikey
I think there are two parts to the answer to your question - I would suspect that RFID is involved in the "proximity" portion of the question, but the "button press" to unlock involves a transmitter in the key and I would expect that it transmits a different code each time (they call it a rolling code).  That way, it makes it very difficult for someone else to capture what your key transmits and make a clone since the code it transmits is not valid the next time.  See this link for more information on how that works: Rolling code

I do know that with my new GMC truck, it has to "learn" about any keys that are valid and it identifies them.  You have to first have a key made at the dealership, then you have to go through a couple of steps to let the truck synchronize with the new key so it knows it is valid.  You have to have at least one recognized key to let it learn about the others.  If you lose all the keys, then you have to go through a process of getting a new key made and let the truck "learn" about the new one (which then invalidates all the "lost" keys so nobody else can use them).
mikey
-- you can't have too many gadgets or too much disk space !
old engineering saying: 1+1 = 3 for sufficiently large values of 1 or small values of 3

JohnLincoln

#8
Mar 08, 2016, 09:04 pm Last Edit: Mar 08, 2016, 09:14 pm by JohnLincoln
Think about it.

One half of the system has a tiny battery that can supply a few 10s of mA for a relatively short time.

The other half has a much larger battery, most likely to be rated at least 40 Amp hours, and is regularly recharged.

Which one is going to be asked to provide a moderate current, and which one is going to be asked to be passive/use negligible current?

polymorph

Passive RFID in the keyfob, and the car does not transmit continuously. It only has to check every few seconds with a short burst, then only transmit for a second if there is a tag nearby.
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gpsmikey

At least with mine, there are two parts.  There is the key (that apparently has some sort of RFID chip in it because there are NO contacts on the key - just a funny machined groove for the physical "key" part.  The fob with the buttons on it is physically separate from the key.  I can unlock/lock/start it with the fob (but not drive - only do a remote start to warm it up).  Using the physical key, I have to manually unlock the doors and then can use the key (without the fob) to start/drive.  I suspect the fob is the part that would be using the rolling code and the key has just a RFID chip in it although some of those also support rolling codes etc.).  At least this is what the 2015 GMC truck uses.  The key does not have a battery in it, but the fob does (I think it is the usual CR2032 3v battery).
mikey
-- you can't have too many gadgets or too much disk space !
old engineering saying: 1+1 = 3 for sufficiently large values of 1 or small values of 3

Boardburner2

Possibly the keyfob is an active rfid device.

It detects the signal from the car and uses the fob battery to transmit a short signal.

This can have much longer range than a passive rfid tag.

theace

Thanks for the answers. I didn't know about active rfid. I also found that there is an battery assisted passive rfid, although I don't find any diy guide. Everything related is only available as end product.

Boardburner2

Thanks for the answers. I didn't know about active rfid. I also found that there is an battery assisted passive rfid, although I don't find any diy guide. Everything related is only available as end product.

Sounds likely.

Car fobs are encrypted.
They can be hacked with something called a back channel attack,

Although not easy , once done any fob/car from that manufacturer is at risk.

Car manufacturers are mass market and probably do not want the risk of saying too much.

The means exists to do this but is not cheap/practical for small quantity.

doughboy

it has to be an active component, as the battery needs to be replaced in just a little over a year on a Lexus smart key.

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