How to unlock your car with just your cell phone. - Page 2 - Mercedes-Benz Forum

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post #11 of 16 (permalink) Old 02-24-2007, 02:45 PM
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It wasn't on a Mercedes. It was on a Mitsubishi Eclipse. Well I know he wasn't full of it because he had his PDA in one hand the stylus in the other. He had some kind of program on it that enabled him to do this. But no reason to argue about why or how he did or didn't do it.
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post #12 of 16 (permalink) Old 02-24-2007, 05:48 PM
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Originally Posted by n1philli
It wasn't on a Mercedes. It was on a Mitsubishi Eclipse. Well I know he wasn't full of it because he had his PDA in one hand the stylus in the other. He had some kind of program on it that enabled him to do this. But no reason to argue about why or how he did or didn't do it.
He was messing with you. A Mitsubishi uses an RF transmitter, not IR. The IR port on the PDA wouldn't do anything.
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post #13 of 16 (permalink) Old 02-25-2007, 01:48 AM
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He was messing with you. A Mitsubishi uses an RF transmitter, not IR. The IR port on the PDA wouldn't do anything.
This info explains a lot, thaks for clarifying (how many cars actually use IR or IR and RF like MB?). Odd that some people need to fool others like here. Fine if it was a joke but the guy should have let the other person know it was not real.
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post #14 of 16 (permalink) Old 03-05-2007, 08:45 PM
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It might be possible. I asked someone about this and got this as an answer:

The 400 mhz your key fob works at is the carrier. The modulated code is much lower like 10 kilohertz. The microphone is a coil near a magnet, which electrically can act like an antenna. It can't tell the difference between sound and electrical noise. Electrical noise is usually something you try to get rid of. If the signal is within the bandwidth of the cell , and can drive the speaker at the other end, the speakers voice coil can also also act as an antenna and might radiate enough energy to reproduce the modulated signal. It makes sense IF the signal strength going to the car is strong enough.

I haven't bothered trying this on my car since it uses a rollong code and the guy who sent me this might be full of hot air, but I thought I would pass it on anyway.

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post #15 of 16 (permalink) Old 03-05-2007, 11:37 PM
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It might be possible. I asked someone about this and got this as an answer:

The 400 mhz your key fob works at is the carrier. The modulated code is much lower like 10 kilohertz. The microphone is a coil near a magnet, which electrically can act like an antenna. It can't tell the difference between sound and electrical noise. Electrical noise is usually something you try to get rid of. If the signal is within the bandwidth of the cell , and can drive the speaker at the other end, the speakers voice coil can also also act as an antenna and might radiate enough energy to reproduce the modulated signal. It makes sense IF the signal strength going to the car is strong enough.

I haven't bothered trying this on my car since it uses a rollong code and the guy who sent me this might be full of hot air, but I thought I would pass it on anyway.
I don't believe in this theory. There is some correct theory behind the speculation but the attenuation at the process that is relevant, would be way too high. In addition it assumes unrealistically high non-linearity from the cell phone microphone and specifically the speaker circuitry.

You would have to have a "Chinese" cell phone that has not been type approved to have that lousy microphone design that it would first pick up the modulation from a 400 MHz signal at very low power from the car key (while e.g. a GSM phone would be transmitting at perhaps 25- 35 dB higher power, and close to its own microphone with strong AM which needs to be filtered). This mobile would produce all kinds of audio disturbance from any low and of course from any high power radio signals of similar frequencies and modulated at audio signal bandwidth.

Now if the microphone circuitry was intentionally modified to pick up audio range AM-modulation, this audio would get transmitted thought the cell phone connection and the audio signal would get produced by the speaker but the bandwidth of the audio signal would be strictly limited to less than 4 kHz over the phone line and even if the cell phone supported music etc. the signal bandwidth at the speaker would be limited to less than 20 kHz and would have very high linearity meaning no distortion and harmonics produced at high frequencies. There is simply not sufficient energy at the 400 MHz range that the car would pick up (this is 400 000 times higher frequency, anybody knowing how harmonics get attenuated would know the attenuation being almost infinite even on a highly non-linear circuit) even if the speaker was connected with wires to the car antenna, not to mention taking into account the path loss even if you were holding the phone at a few centimetres from the car antenna. The cellular regulations anyway generally allow a maximum of -36 dBm power (at a 30 kHz bandwidth) and a lot less than that for specific frequency bands.

I still consider the IR approach being the most realistic and even that would not work for MB with a rolling code.
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post #16 of 16 (permalink) Old 03-06-2007, 12:29 AM
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I think you are right about it being BS. Out here in North America cell phones have to be type accepted in order to be approved for sale and that means it must meet standards for stray radiation, frequency tracking and noise rejection. If it met those standards it could not be used the way it was described.

BTW:

Most of the high quality consumer electronics sold here including Motorola branded cell phones and the computer I am using right now are made in China. I would be willing to bet that a lot of Mercedes electronic gremlins wuld go away if they stopped depending on critical electronic modules made in former Eastern Block work collectives and started sourcing them from China like Toyota, Honda and even V.W. have done.

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