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Hi,

I was chatting to a friend today and we considered the potential of converting a key-based central locking system in a car to one which can also use a remote, without paying to have a proper one installed.

The dash has a button to toggle the central locking; I thought if an IR receiver was linked to a chip that replaced that switch as the control (eg, rather than the switch triggering it, the chip chooses if the current should flow through) then with an IR transmitter you'd be able to toggle the locking without using the key.

As a security measure we thought that, because both the receiver and transmitter would be using a chip to control them, a PIN of sorts could be sent by the transmitter, to ensure only that transmitter could open the doors.

I know I could pick up all the material to do this for less than a hundred dollars - but would it work? Any thoughts - or even potential improvements?

pcmattman wrote:Hi,

I was chatting to a friend today and we considered the potential of converting a key-based central locking system in a car to one which can also use a remote, without paying to have a proper one installed.

The dash has a button to toggle the central locking; I thought if an IR receiver was linked to a chip that replaced that switch as the control (eg, rather than the switch triggering it, the chip chooses if the current should flow through) then with an IR transmitter you'd be able to toggle the locking without using the key.

As a security measure we thought that, because both the receiver and transmitter would be using a chip to control them, a PIN of sorts could be sent by the transmitter, to ensure only that transmitter could open the doors.

I know I could pick up all the material to do this for less than a hundred dollars - but would it work? Any thoughts - or even potential improvements?

As far as I know, recent transmitters that do this kind of thing (remote locks, car alarms etc) do some kind of challenge-response authorization. Simply sending a PIN (or password) is not secure - the transmission can be sniffed and played back. And you can never know who is eager to get their hands on your car... If you are thinking, "oh but noone knows the inner workings of my design", that is security by obscurity... which is not very secure.

Interesting thoughts - I guess you'd want to get the basic system working first before you added in any form of harder security.

The only downside to a challenge/response method is that it requires both sides to transmit and receive, rather than only one side transmitting and one side receiving. Not that that's too much of a problem, just uses a bit more power.

pcmattman wrote:Interesting thoughts - I guess you'd want to get the basic system working first before you added in any form of harder security.

You have a point. I meant of course, that once you have a basic system, you should make it a secure system.

pcmattman wrote: The only downside to a challenge/response method is that it requires both sides to transmit and receive, rather than only one side transmitting and one side receiving. Not that that's too much of a problem, just uses a bit more power.

Well, the central unit is fed from the car's battery (and when power isn't available, it switches over to its built-in battery), so this shouldn't be a problem. I don't know how much power the remote unit would require, though.

Also I would guess that insurance companies don't like home-built security devices.