I have made a robot that moves like the logo turtle. It can even draw using a ‘Penup’ and ‘Pendown’ function like in logo. Hence, it is named ‘Logo DrawBot’. The microcontroller is an Arduino Uno R3.
The robot uses a unique motor-drive system using a ‘servo-switch’. This part has been explained in detail later on.
An optional modification too is possible that uses IR Reflectance sensors and a black and white paper coding strip.

Step 1: Parts List

Basic Parts:
1. Arduino Uno R3 Microcontroller
2. 2 x Tower Pro SG 90 Servo Motor
3. 2 x 3volt G7 Geared motors
4. 2 x Wheels
5. 1 x Ball Caster
6. 1 x Slide Switch
7. 4 x AA Battery
8. 1 x 9 volt battery
9. 1 x 3.7 volt BL-5C mobile battery (I’ll talk about this later)
10. 3 CDs
Accessories:
1. Arduino to PC cable
2. 2.1 mm power jack for Arduino
3. Male to Male jumper wires
4. Other wires
5. AA battery holder, 9 volt battery clip
6. Screws
7. Other junk for construction.

Soldering iron, heatshrink tubing, zip ties, super glue, etc were also used.

Step 2: The Body

3 CDs were used for the basic structure. They were supported by screws and plastic tubing at the centre. The central hole was used for accommodating the pen.
The ball caster and the 3.7 volt mobile phone battery are to be attached on the underside of the 1st CD. I have made customized holder for the 3.7 volt battery used plywood, metal brushes. This battery was used to power the 2 DC motors.
The 2 DC motors,slide swirch and the 9volt battery is to be fixed on the top side. Double sided tape is very convienient as usage is quick and gives nice suspesion as well! Thus this is used for securing the motors and slide switch. My slide switch is a 2-channel one, can be used to connect two pairs of wires. The 9volt battery however somehow stays in place.
The Arduino, pen up-down servo and the motor-switch servo will come on the top of the 2nd CD.

Step 3: Prepare the 1st CD:

The 3.7 volt battery pack was attached on the underside of the first CD. The ball caster was attached here as well.
Attach the wheels to the motors with screws. Fix the motors to the sides of the 1st CD. Make sure the alignment is right. The positive motor terminals should be attached to the +ve of the 3.7 volt battery pack. Route the –ve from the battery through 1 channel of the slide switch to the 2nd CD.
Also keep space for for the 9 volt battery. This battery will be used to power the Arduino. Connect this battery to the Arduino through the second channel of the slide switch.

Step 4: The ‘Servo Switch’:

This is an entirely new idea used in this project. Usually DC motor control using an Arduino requires motor drivers. I decided to try out something new. (Refer to diagram for clarity)
(In the diagram the +ve wire from motor 2 touches the +ve of the battery)

Let me make one thing clear: The 3 servo pins come from the Arduino and are NOT attached to this circuit.

The parallelogram piece atop the servo is the horn on which the –ve wire from the 3 volt pack is tied. Now if the servo horn and wire makes contact with both wires, the circuit is completed for both motors and the robot will move in a forward direction. But if the horn touches only one wire the corresponding motor will run.
This servo ‘switch’ will be controlled with the help of the Arduino.
This is perhaps a manual technique but is absolutely failsafe and effective.

My future idea is to use a relay (or 2) in place of this mechanism. Just to make it a bit more tidy!!

Step 5: Powering the Arduino R3 and Servo:

The R3 is powered by the 9 volt battery through the same slide switch as used for the DC motor circuit.

I did not use any other shield on the Arduino. Thus I fixed jumper wires to 5V and GND and attached the wires from the servo to them. The Signal pins from the servo are attached to PWM output pins 9 and 10. Heatshrink tubing works wonders while soldering these wires.

View this link for circuit diagram:
http://www.robotoid.com/appnotes/arduino-operating-two-servos.html

Thus I connected the two servo motors to the Arduino. The first one for the pen, the second for the motor switch.

Step 6: Penup/Pendown:

I used the central bore of the CDs for the pen. I found some nice piece of chord, it is from the badminton racquet gutting. The nice part is that it threads through the servo horns perfectly.
A little bit of experimenting with the program for this servo is required such that the pen touches the ground nicely and lifts up as well.

Step 7: Powerpack for the Servoes:

The servo motors take in a lot of amperage which the arduino cannot provide directly. Hence I attached another power pack of 4 AA batteries atop the 3rd CD. The 4.8 volts (rechargeable) that the batteries put out is used to power the servoes.
For additional reference visit:
http://www.robotoid.com/appnotes/arduino-operating-two-servos.html

Step 8:

The robot is ready. A bit of experimenting is required to fix the degree inputs for the servo motors. Thus you have to find out for yourself at which position (angle) the pen comes up (and goes down) and for which angle contact is made with the motor wires(for the servo switch).

I also plan to add some decals on the top and sides of the robot. To make it nice and conceal the inner wiring.

PS: Please vote for my project in the arduino contest. Also an suggestions/feedback is welcome