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 doors.
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 it.
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 fine.
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 next.
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 place.
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!