Difficulty Level = 8 [What's this?]

I built a wireless robotics platform from a cheap R/C car, an Arduino with XBee shield, small microswitch sensors, and a Processing program running on a remote computer to control the vehicle. The vehicle is completely controlled by the code running on the remote computer which allows very rapid prototyping of the code to tell the vehicle what to do and how to react to the sensor events received from the vehicle. I’m hoping this is a good way to teach my 9-year old son about programming.

Wireless computer-controlled robotics platform built on cheap RC vehicle, Arduino microcontroller, and XBee radios

Before I get into details, here’s an overview of the features:

• All logic controlling the vehicle is performed in a Processing program running on remote computer. The Arduino program listens for commands from the remote computer.
• Bi-directional wireless communication over XBee radios with (theoretical) 1-mile range. I’ve accomplished 1/4 mile range with these radios.
• Sensor events are transmitted from the vehicle to the controlling computer. This vehicle has 3 microswitches – two on front bumper and one at the rear.
• Original circuitry of vehicle replaced with dual H-Bridge circuit to control drive motor and turn motor. Drive motor is controlled with variable speed.
• Power: Vehicle motors powered by 4 AA batteries. Arduino with XBee shield powered by 9V battery mounted at front of vehicle.
• Simple communications protocol: 2 byte commands from controller to vehicle, one byte sensor readings from vehicle to controller.

The Hardware

There’s nothing special about the configuration of the XBee radios. They are running the AT firmware (“transparent mode”) which allows them to simply exchange serial data. The Libelium XBee shield on top of the Arduino makes it easy to read/write serial data from Arduino code.

Arduino and XBee shield on top of the vehicle

Inside the vehicle is a simple circuit board with an L293 quadruple half-H driver to drive the motors. The drive motor and turn motor are connected. I had to rip out the original circuit board (but I saved it!).

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Difficulty Level = 5 [What's this?]

I decided to explore the more advanced features of XBee radios by building a remote temperature sensor. You can get quite a bit of control over an XBee radio without a microcontroller at all. You can configure the radio to send sensor readings at particular intervals when it detects changes on certain input pins. For the details on configuring XBee radios, see the documentation at Digi International.

For this project, I configured the radio at the sensor end to read the analog input of pin 19 every 4 seconds and to send a sensor reading packet. Both the sender and receiver radios should be running the API firmware. Input pin 19 on my sensor radio is configured (parameter D1) with value ’2′ which means that it will read analog input, and the IO sampling rate (parameter IR) is set to ’1000′ which sends a sample every 4096ms.

An LM34 temperature sensor outputs a variable voltage depending on the temperature. The mapping is extremely simple: 10mV for every Fahrenheit degree. So, at 72 degrees F, the output is 720mV.

Why did I choose pin 19? I started with pin 20, but I burned it out. The pins can only handle an analog input of up to 1.2V, and I think I may have sent too much into the pin. How? Well, let’s say it involved holding a cold Pepsi can on the circuit to cool off the temperature sensor, and I shorted out a connection with the can. Oops. I’m lucky I didn’t burn out the entire XBee chip.

Remote temperature sensor

Here is the circuit for the remote sensor:

Schematic for remote sensor circuit

For the receiving side, I used an Arduino with an XBee shield and a two digit LED display:

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