How I Hack a RC Toy Car to Control It From an Android Phone. Part 2: Connecting to Arduino

This blog post the second part of hacking an RC Toy car to control it from an Android phone. In the previous post, I tore down RC Car’s transmitter to see how the circuit works and to discover the connection points.

After dismantling the transmitter, I create a simple schematics to visualize the wiring.

Reverse-engineer the RC toy car circuit

Looking at the diagram, the easiest way to connect and control the transmitter from the Arduino board is by creating a cable jumper to bypass the buttons.

The cable jumpers will use NPN transistors as switches and the transistor themselves are controlled by Arduino’s digital pins.

There are 4 buttons on the RC transmitter. Each to control forward, reversed, and turning left and right. So we will need 4 jumpers and transistors, and 4 separate digital pins on Arduino to control the transistors.

The simplified schematics for the connection with Arduino is something like this.

This is how Arduino controls the RC transmitter :

1. Whenever Arduino receives a command from the Android app, it sets the Arduino digital pin as HIGH.
2. The pin then activate the Base terminal of the NPN transistor
3. Since the transistor base is activated, current flows from Emitter to Collector and circuit is closed. This emulates button pressed event.
4. The signal is then transmitted via RF and the actuator or motor on the toy car is activated.

The minimum components that you need for the hardware are as follows:

1. NPN2222 transistors: 4 pieces
2. 7 Ohm resistors: 4 pieces
3. Some cables

You can of course add some bells and whistles if you want to.

Next we are working on the Arduino code.

Other than controlling the RC transmitter, Arduino also functions to establish Bluetooth connection to Android smartphone using an HC05 Bluetooth module and translate the command from Android phone into activation signal for the digital pins.

The Arduino code is available on Github.

Once the RC transmitter is connected to the Arduino board like shown in the schematic above and code is uploaded, we are done with the hardware part. The last piece of the components is the Android app that will act as the user interface for the whole project.