Wireless Systems Design

Locked Your Keys In The Car? Get Out Your Cell Phone

There has been a rumor going around in one of those dreadful e-mails that your friends and co-workers feel compelled to forward to you all the time. If you lock your keys in your car and you have a remote keyless entry system, you can get outside help to open the car if you have your cell phone with you. Just call someone that has a duplicate key fob that will open your car. Then, hold you cell phone near the door lock and have the person with the key fob call you back. The person with the key fob should then put the key near their phone and push the unlock button. The door should open.

I was skeptical, to say the least, about this rumor, and was about to dismiss it as one more Internet hoax. But I thought I better try it out first. Well, low and behold, it works. I tried it with both GSM and cdma cell phones, and it reliably opens (and relocks) the car.

I have been racking my brain for days about how this works. Two or more different wireless technologies are involved. I even used a 2.4 GHz cordless phone to make the call. The remote keyless entry usually uses on-off-keying (OOK), a form of ASK, to modulate a 315 MHz carrier with the correct serial digital code. Somehow the calling phone picks up this information and transmits it to the receiving cell phone, which then magically retransmits it to the door lock receiver in the car? Yeah, right.

All I can think of is that the digital code from the key fob modulates the sending phone and the receiving phone subsequently picks it up. But how does it transmit the code to the lock receiver in the car? Does the 315 MHz signal ride on the cell phone carrier some how? I thought I was a pretty good wireless guy, but this one baffles me.

If you have any thoughts to share on this, drop me a line at [email protected] I’ll be sure to pass the word on to the others here next month.

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