|The prototype Turtle|
The prototype is more complicated than it has to be, mostly due to the fact that Raspberry Pi model A doesn't have built in Wifi and can't even supply a USB Wifi dongle with enough power on its own. Today the amazing Raspberry Pi Zero W can be used instead.
Development EnvironmentThe Turtle is programmed using a Web IDE running on the robot itself.
|The Integrated Development Enviroment|
The IDE makes it possible to edit and run code on the Turtle. A history of executed programs are saved to make it easy to go back to an earlier version. It is also possible to save a program using a descriptive name. The console output of the running code is displayed in a separate text area to simplify debugging.
More than one person can program a single Turtle at the same time if they have one computer or tablet each and take turns running programs.
Mechanical DesignWheels from inline skates are perfect for the Turtle. They are narrow with good friction which makes the robot turn in a tight and controlled way. To attach a wheel to a stepper motor shaft you need a mounting hub . It is unlikely that you will find a hub that matches the holes on the wheels. I used a 1mm thick steel plate to adapt the mounting holes on the hub to the wheel.
Two stepper motors are used to drive the front wheels. The drawing pen needs to be centered between the motors for the Turtle to be able to turn on the spot without moving the pen. The pen should also be possible to lift to be able to move the robot without drawing a line. I found two copper plumbing parts in the local hardware store that fit perfectly together without getting stuck or fall through. I attached the pen to the narrow conical tube and placed six of the larger tubes between the motors. I strapped it all together with cable ties and used some blu-tac between the tubes to increase the friction.
On top of the engine pack I attached a 1mm thick steel plate to hold the electronics. Steel is probably not optimal for the Wifi signal and I would choose wood or plastic today.
As third supporting wheel I used a bearing ball but anything with low friction works fine.
The servo motor that is used to lift the pen is attached to a vertical steel plate that is glued to the base plate. Rubber bands push the pen down. The servo arm pulls a thread to lift the pen.
Electrical DesignThe Turtle is powered by a rechargeable racing pack delivering 12V DC. To get the 5V DC that the Raspberry Pi requires, dual 5V/1A regulators  are used, one powering the Raspberry Pi and one powering the Wifi dongle through the USB hub.
The stepper motors are 200 steps/revolution, 12V DC. I use Easy Driver  to drive the stepper motors, one per motor. The STEP and DIR pins are connected to Raspberry Pi GPIO.
The servo that lifts the pen runs on 5V DC and is connected to the same converter that drives the Raspberry Pi. The signal wire is connected to Raspberry Pi GPIO.
Software DesignInstall Raspbian: 
Setup headless networking:  
Change keyboard layout:
sudo vi /etc/default/keyboard
Install the IDE: