In this instructable, i will show you how to build your very own Remote Controlled Central Locking system.

There are a number of different central locking/alarm systems you can buy at your local automotive spare parts retailer, but by using a fancy one for a week, i quickly determined that there were too many problems with the module, making it not very convenient. Having to unlock the car uniquely with the remote was one of the major problems i had with it as i wanted to be able be unlock the car using the key as well as with the remote without any hassle.

This instructable will cover the design procedure only and not the installation process as each vehicle is different.

Step 1: Circuit Design Considerations

The operation of the circuit is pretty simple; out of many others, there are 2 wires connected to the car's BCM(Body Control Module), namely LOCKWIRE and UNLOCKWIRE . When the lockwire is shorted to ground very quickly, the 4 doors will lock. Similarly, the unlockwire shorted to ground will unlock the car. Therefore the circuit just has to connect these 2 wires to ground within a short delay when the corresponding button is pressed.

By probing the wires of an existing alarm system, it was found that the lock and unlock delay was 250ms. Therefore, i used 250ms as the time that LOCKWIRE and UNLOCKWIRE had to be shorted to ground.

The second design element of the circuit is the indicator feedback which should indicate whether is car has received the LOCK or UNLOCK signal. I wanted to flash the indicators once when the LOCK signal is received and twice when the UNLOCK signal is received.

Step 2: Circuit Operation

The circuit diagram and arduino code is pretty much self explanatory. Upon receiving the 5v signal from D0 (Button A pressed to LOCK the car) , the arduino sends 5v to pin D3 of the arduino to short LOCKWIRE to ground for 250ms via a N-Channel MOSFET. The arduino then sends 5v to pin D6 for 500ms to trigger a relay via a N-Channel MOSFET (This flashes the indicator lights for 500ms). N-channel IRFZ44N MOSFET was chosen over certain transistor due to the design considerations that were to be applied in the automotive environment where the circuit would be located (Temperature limits in this case). A flyback diode is used to suppress the voltage peaks of up to 40V which occurs when supply to the relay coil is cut off.

A 7809 Voltage regulator is used to provide 9V to the arduino and 5v is fed to the receiver from the arduino board.

Step 3: Testing the Circuit

Using the circuit diagram, a breadboard prototype is easily setup to check if everything is working as expected.

Here, i used some LEDs to simulate the indicator lights flashing as well as the LOCK and UNLOCK operations.

Step 4: Building the Circuit on Perfboard

To build the circuit, only simple basic tools were required. Soldering iron, solder wire, some stranded wires cut to length and a pair of helping (metal) hands were all that i used.