Martin Underwood wrote:
> I've just read something which sounds far too good to be true. Is there any
> truth in it?
>
> Apparently if you accidentally lock your keys in the car, and the keys have
> a remote-control to unlock the door, it is possible to use a mobile phone
> and a second set of keys to unlock the door. You use your mobile to call
> someone else on their mobile who has your spare set. While the mobile call
> is connected, if you hold your mobile close to the car, and the other person
> presses the unlock button close to their mobile, your car will unlock,
> letting you get at the keys that you locked inside.
>
> Sounds a right load of bollocks: it suggests that the remote in some way
> communicates with the other person's mobile, then the signal passes to your
> phone, and from there to the car's locking mechanism.
>
> Does it rely on the remotes being the old sort that emit an ultrasonic
> signal rather than modern sort which use an RF signal? Can mobiles transmit
> an ultrasonic signal or are they restricted to normal 300-3000 Hz telephone
> voice signals.
>
>

It's technically not possible for the RF (~433 MHz)
signal to be passed within the traffic channels of the
mobile phone. Even if the data were somehow being
demodulated to a voice-band signal within the phone,
it's hard to see how the distant mobile could use it
to re-modulate the original VHF frequency!

Nevertheless, I've seen this "demonstrated" on a TV
programme, but the vehicle was only just outside the
building from which the call was made. I wonder if it
*may* be possible for the key signal to be re-radiated
by the phone within its local field. It's easily
demonstrated that a keyfob's range can be boosted (by
tens of metres) using the human body as an aerial.
Could the phone be providing an "active" aerial that
does something similar over a longer range?