Cell phone door unlock trick just WORKED!!!

[brianl703]

Originally Posted By: javacontour
But do they not allow any retransmission, or have a limit. Standing less than 10 feet away from the car may retransmit enough to reach the receiver in the car.


What I'm thinking is that if the signal output from the keyfob causes the cell phone's transmitter to output a signal like that of the keyfob...why wouldn't that output cause a feedback loop?

Repeaters operate on different input/output frequencies for this very reason (unless they are store/forward repeaters which store the incoming message and then repeat it after a time delay).


Quote:
They don't have to re-transmit at full power, just enough to reach the receiver in the car.


The spurious output (which is an output that is outside of the transmitter's specified frequency range, which it must be if it's actually retransmitting the keyfob's ~300MHz signal) of the cell phone's transmitter does not need to be at full power to cause problems...even if it doesn't reach the cellular tower.

Keyfobs are part 15 devices limited to a certain output power. Anything which would retransmit the output of a keyfob and make it stronger is a violation of part 15 rules AND can result in harmful interference.

 

[javacontour]

I was thinking more along the lines of radiation in the intermediate part of the system.

Somehow either a harmonic, or a mixed signal of the cell phone and the key fob is received by the other phone, and the key fob signal comes out of the mixer on the receiving phone side and that signal is amplified enough to make it to the receiver in the car.

A lot of ifs, certainly.

I wonder if a 1800MHz GSM phone could carry enough of a 315MHz key fob signal if mixed in to have that same 1800MHz taken back out at the other end and getting the 315MHz signal out of the intermediate portion of the receiving cell phone to reach the door lock receiver? (I think domestics use 315MHz. 5 times that is 1575MHz and 6x that is 1890MHz. How close are GSM Phones to 1800MHz?

 

[javacontour]

I also believe the keyfobs are Amplitude Shift Keying for domestic cars. While Japanese cars use Frequency Shift Keying.

My head hurts. I haven't played with RF in years, probably back to 1992.

I would see weird things as a signal officer where moving an antenna 10 feet in another compass direction would give a usable signal, yet nothing to indicate anything wrong with the line of site.

I've seen "back lobe" shots where we didn't want the "bad guys" to pick up our signal, so we pointed our antennas 180 degrees away from where we wanted the signal to go, etc.

 

[sciphi]

It's a GM car. You can likely re-transmit the remote signal with a rusty clothes hanger and a rain dance.

Nothing against GM, I drive a Buick myself, just that hearing you could use the cell phone trick to unlock a GM car would not shock me, given how GM's vehicle computers are very liberal when accepting signals. There's some truth to the saw about how a GM car will run poorly longer than most cars will run period.

 

[brianl703]

Originally Posted By: javacontour
How close are GSM Phones to 1800MHz?


I believe they can operate on a number of different channels. Also GSM phones are TDMA, and they pulse the transmitter on and off--it's not left on continuously as you would expect for a normal FM transmitter. (That's why they do such a fine job of causing interference to any audio devices around).

 

[brianl703]

Originally Posted By: sciphi
There's some truth to the saw about how a GM car will run poorly longer than most cars will run period.


My 1988 Ford Mustang will run with just about any of the sensors unplugged, including the MAF. About the only sensor it absolutely needs to run is the position sensor in the distributor.

 

[OVERKILL]

Originally Posted By: brianl703
Originally Posted By: sciphi
There's some truth to the saw about how a GM car will run poorly longer than most cars will run period.


My 1988 Ford Mustang will run with just about any of the sensors unplugged, including the MAF. About the only sensor it absolutely needs to run is the position sensor in the distributor.


Yessir, EEC-IV was/is a blessing

 

[crinkles]

I've woken up to find the boot unlocked and popped open, and I swear I didn't touch it, and I usually store my keys next to my mobile on the bed table...

Happended once at home and once at work

 

[oilyriser]

If I want to tease someone and their car keys are nearby, I hold up the remote, press the unlock button, and ask the person how far away their car is?