Do the remotes on modern cars operate by sound?

[Stephen Tashiro]

On the Sears forum, in a thread involving the risk of a repair shop locking your keys in your car, a poster suggests:


...in the future when ever you give your keys to a repair shop take the remote keyless fob off of the keys and put it in your pocket, if your keys get locked in the car you can open it again. Another tip is if there is a second remote at home have someone call your cell phone, hold your phone about 10 inches from the door and have the other person push the buttons from your house and the door should open just like they were there with you, it really works, try it yourself at home and you will see.


I don't have any car modern enough to have a remote control Do the remotes operate by sound and can you really open them over cell phone?

[mike holden]

Stephen,
Chrysler vehicles use radio to transmit to the car's receiver telling it to unlock the doors.
I believe that the "Mythbusters" TV show covered this, along with other methods such as using a tennis ball, to open door locks. None of the mythical methods worked.
Mike

[Dan Friedrichs]

No, that would not work - they work by RF (radio frequency), not at audible frequencies.

[Stephen Tashiro]

If found a YouTube of the Mythbusters test titled "MythBusters - Cell Phone Car Remote". The cell phone doesn't work but my understanding of the remarks by Mythbuster Grant are that he claims the remotes do work by high frequency sound.

Edit: Snopes makes a clearer statement that the remotes operate by RF www.snopes.com/autos/techno/keyless.asp

[Eric DeSilva]

You actually can unlock a lot of modern cars with a phone... You just have to call OnStar, or vehicle assistance brand is in your car... ;)

[ray hampton]

do the remote cause cancer when carry in your pants pocket ?
my remote got four functions, do that mean that it got 4 transmitters ?

[Brian Elfert]

The transmitters don't transmit in your pocket. They only transmit when you hit the button. I can't tell you for sure, but it is likely they have one transmitter and just send different coded messages for each function.

[Dan Friedrichs]

The cell phone doesn't work but my understanding of the remarks by Mythbuster Grant are that he claims the remotes do work by high frequency sound.

I haven't seen it, but I suspect he was probably just trying to simplify the explanation of how RF works. RF (radio frequency) is, in fact, much higher frequency than audible sound. But, so is light.

[Mike Henderson]

This is a weak memory, but I think I read somewhere that the hands-free systems (what I call "keyless") work in a challenge/response mode. The car and the fob share an encryption key. The car generates a random number and sends it through the encryption algorithm. It then transmits the random number. When the fob receives the random number, it passes it through it's encryption hardware and responds with the generated number. The car receives that number, compares it to the number it generated, and if it's equal, allows you to start the car, or open the door, or whatever else you can do.

Mike

[Bill Cunningham]

I think without a handshaking frequency key (digital or otherwise) you would probably have some enterprising thieves cruising parking lots with RF freq generators, popping locks all over..

[ray hampton]

The transmitters don't transmit in your pocket. They only transmit when you hit the button. I can't tell you for sure, but it is likely they have one transmitter and just send different coded messages for each function.

If you pack other keys in your pockets with your cars keys , then your transmitters can transmitter in your pocket [say that 3 times Quick]

[ray hampton]

I think without a handshaking frequency key (digital or otherwise) you would probably have some enterprising thieves cruising parking lots with RF freq generators, popping locks all over..


I recall reading or hearing this story about handshaking RF BEFORE

[Keith Outten]

You can open a PT Cruiser with two cell phones and a fob transmitter, Jackie and I have done it to open her car but don't ask why :)
.

[John Coloccia]

re: random numbers and fobs

I'm not a fob designer so this is just conjecture, but as an engineer if I were designing this system I wouldn't design a receiver into the fob. I would put a random number generator in the car and the fob, and synchronize them. I would use the output of the generator as the seed each additional number. Each push of the button generates a new code and in this way the car and fob are always linked and it is a unique code on each press.

Since people press the button out of range, you need a way to have some sort of tolerance or you will constantly get out of sync. You would simply just accept the next X number of codes in the sequence....next 1000 codes, say. So you would have to press the transmitter buttons 1000 times out of range until you became out of sync and had to go through some sort of sync procedure. Of course, once a code is received, everything is perfectly synced up again.

Anyhow, there are other ways of doing this too without necessarily needing to handshake. Just an FYI.

[Dan Hintz]

I'm not a fob designer so this is just conjecture...
John,

You're pretty close to reality on your conjecture. It's actually a pseudo-random number generator created via a linear shift register. Using the shift register allows you to recreate the pseudo-random number by knowing the shift register's tap points, some previous states, etc. If the two get out of sync (say, the keyfob isn't within range and it's pressed multiple times, or the battery is removed), it takes multiple presses to re-sync them (typical resync is one the order of <10 tries).

Of course, all of this happens automatically with the wireless fobs that don't require key presses.

[Curt Harms]

I have no idea how they work but I know that I can use two different fobs on an early 2000s Ford Ranger and they work first time every time.