This article was first published in 2009.
Nearly all cars of the last 15 years have remote central locking as standard.
You press the button on the key and the locks slam. Sometimes you’re notified of
what has happened by a beep or flash of the indicators, but in other cars
there’s no indication at all. The latter can be a bit of a pain – until you test
a handle, you’re never quite sure that the car is locked.
What this little project does is beep when the doors
lock. There’s no indication when the doors are unlocked (none is usually needed), and the modification will work with all systems – including those like the one
shown here that use a single button on the key to both lock and unlock the
The basic idea is to wire an electronic buzzer in parallel with the door lock
solenoid or motor. These motors (or solenoids) are fed current with one polarity
when unlocking the doors and the opposite polarity when locking the doors. If
you put the buzzer the right way across the wiring, it will sound only when the
doors are being unlocked.
To make the sound you’ll need a 12V electronic buzzer. This should be fairly loud as in
use it will be buried inside the door, muffling it. You might also need a diode,
but we’ll come back to that later.
Removing the Door Trim
The first step in the install is to remove the door trim. We chose the
driver’s door but any door with electric locking can be picked.
Be careful when removing the trim. In addition to clips, there are often a
few screws, usually hidden. In this car (a Toyota Prius), there was a screw
hidden behind a flap inside the door handle...
...and another inside the armrest. Remove these screws before gently prying the
door trim loose.
The inners of the door cavity are usually protected by a plastic liner, held
in place with adhesive black mastic. Gently pull this loose, starting in the
area where the door lock motor is likely to be.
Finding the Right Wires
In the case of this car, the locking button is connected to the door lock
mechanism by Bowden cables. By following these we were able to find the door
lock motor itself.
Here is the lock motor, almost completely hidden inside the door. But there’s
no problem – we don’t want to access the motor itself, just the wires leading to
And here are those wires. The bundle is wrapped in loom tape but...
...this is easily sliced open to reveal a bunch of wires. But which ones are
the right ones to connect the buzzer to?
The answer is easier than you’d first think. Just carefully bare a small
portion of each of the wires, making
sure that they cannot touch each other or the metalwork of the car. Then try connecting the
buzzer to the different wires until it sounds briefly when the doors are locked.
Yep, that easy.
If you are using an electronic buzzer, you won’t do any harm if
you connect it across the wrong wires first.
Once you have found the right wires for the buzzer, try unlocking and locking
the car a few times. In some cases you might find that the mechanism is now a
bit erratic. If this is the case, this little critter – a diode – is likely to
be able to solve the problem. (Small diodes are available from any electronic
store – a 1N4004 is fine.) Wire it into one wire going to the buzzer (try each
way around until the buzzer still works) and then check that all is again
Insulate the previously bared wires that aren’t required and then solder a pair of wires to
the correct leads. Thoroughly insulate these connections with tape.
In the case of the Prius, the diode was needed and it was connected up
The new cable was wrapped in tape and then the system again tested.
The buzzer was mounted inside the door using double-sided tape – out of view
in this pic but that’s what the fingers are doing! Be careful when picking the
mounting position to ensure the window can still move up and down without being obstructed.
Again to avoid fouling the window mechanism, cable-tie the new wiring into
After that it’s just a case of reassembling the door trim and you’re
finished. Now, no more wondering if you really did lock the car!