Windows Phone, RC cars and you...


Today's Hardware Friday project comes to us from Joel Ivory Johnson who shows us all how we can marry up some of our favorite toys, Window Phone and our RC car collection (you have one of those right? Well if not, this post might have you reconsidering that...)

Controlling an RC car with a Windows Phone


Earlier in the 2013 year while working with the Emerging Experiences group at Razorfish I had the opportunity to do some work with the Netduino, Arduino, and several other technologies that seem to play a significant role in the Maker Movement. Among other things I had converted an RC car to be controlled over Bluetooth with a Windows Phone. A few months ago someone saw a video of the car and asked me how I did it. The comments section on a video wasn't large enough to contain what I thought to be a sufficient response, so I'm writing this article instead.

Recognizing that there's a spectrum of backgrounds and experience levels that one may have when approaching this I've decided to start over from square one and make some changes along the way. The original RC to Bluetooth conversion was done with a Netduino Plus 2. This time around I will be using the Arduino because it is more ubiquitous and generally less expensive. For this revision of the car I'm staying away from the other improvements that could be added for the sake of simplicity. I'm also keeping my focus narrow and staying on the path to get the car up and running. Right now the goal is to cover what is minimally sufficient to get the car running with an understanding of what is going on. In later writings I plan to make improvements on the UI of the control application and add more capabilities to the car.


To understand this article you'll need to have an understanding of basic digital electronics and programming. You don't need to know or remember the difference between CMOS and TTL gates to understand whats being written here. I'm staying as basic as I can. You will want to have familiarity with one of the C based languages (C, C++, C#) to understand what is being written within this article. I'm using Windows Phone 8 for this first revision of the article but I may return to add Android and iOS in later revisions.

  • What is the Arduino
    • How is one Arduino Different from Another?
    • Input/Output Voltages
    • Processor Clock Rates
    • USB Ports
    • Digital Port
    • Analog Port
    • PWM Port
    • Communications Port
    • What peripherals are Available for the Arduino
  • Getting Started
    • Software Installation
    • Creating a New Program
    • Input and Output with the Serial Port
    • Servos
    • Controlling the Drive Motor
    • Bluetooth Communication


Don't Let the Car Get Away!

Since we are approaching a point at which the car is almost prepared to be sat down on the ground to be driven there's something I want to address. You must be prepared to respond to a loss of a connection. If you don't the car could end up continuing to do the last thing that it was doing until it gets a connection again. If the last thing it was doing was going ahead full throttle chances are you won't be able to reestablish a connection without sprinting after the car. There are a couple of solutions that come to mind to handle this. On many of the Bluetooth receivers that I've seen there is an additional line that connects to a led that will change its blinking sequence to indicate whether or not the Bluetooth receiver has a connection. It's possible to connect to this and put the car in a neutral state if it indicates that the Bluetooth connection has dropped. Another method (the one that I will go with) is to keep track of how long it's been since an instruction was received. If more than a certain amount of time passes and no instructions are received stop the car.

The millis() function returns how much time has passed since the Arduino was started in milliseconds. Every time an instruction is received we can save the value returned by this function and use it as a reference for knowing how long it's been since an instruction was received. With each execution of the main loop the the millis() function is called again and compared to the saved value. The difference between the two values is the amount of time passed since the last instruction. Once the difference has become greater than a certain amount we know it's been a while since an instruction was received. The car will be put in a neutral state. If an instruction is received before this condition occurs then the saved value would have been updated so the difference between the saved time value and the current time value would not be great enough to reach the expiration period.


Control Interface

All the code needed for sending control signals to the car and to have the car interpret and act on them has been presented. I skipped over discussing a user interface for controlling the car though. Some of the other phone controlled systems I've seen use the accelrometer for control. This is a matter of personal preference, but I don't like the accelerometer based approaches. When playing games on my phone the method of control that prefer second is the virtual joystick with a dead zone that is centered on whereever one's finger happen come in contact with the screen. My most preferred method of control is a physical controller.

I made an extremely simple virtual joystick for the phone in the form of a user control. When some one first place their finger down on the control that point becomes the new center point for the dead zone. Sliding ones finger will move the virtual joystick out of the dead zone and be registered as a direction. Lifting one's finger off of the control will set the virtual joystick back to it's neutral position.



What's Next

Writing additional code so that the car can be controlled by an Android phone isn't much effort. The same concepts carry over. When time allows I may update this article or add another one to explain what needs to be done to control the car with an Android phone. Also keep in mind that my personal goal is to make the car more autonomous. This means that the car must be able to know something about it's environment and position. In the next update I plan to walk through the various sensors that will be attached to the car and how to use them. These include GPS, distance sensors, and motion sensors such as an accelerometer and gyrometer/rate sensor. Because of the low distance range on bluetooth radios I'll be using a different radio (Wi-Fi) so that more distance can be put between the phone and the car. I may eventually move up to adding a GSM radio

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