Well, I'm going to create a start-here robot..To learn more of Arduino and have fun in creating it and playing with it..I got some questions..
First, I can't get continuous rotation servos (even i can't get normal ones) or geared dc motors..So, will small ungeared dc motor (like the ones in Adafruit's lessons) work on a motor shield? (I think that geared and ungeared are the same..I think)
Second, Why some people uses extra batteries than the ones connected to the dc plug in the arduino? And which is the best way..4AA batteries connected to the shield (1.5V x 4) and 9V battery connected to the dc plug..Or something else? (Using a IR receiver for a IR controlled mode and a Ultrasonic sensor for obstacle avoiding)
Third, Is there any good way to connect all the things? And any good tutorials for the 2 modes code..Also i think that I'm confused in the design..2 CDs like a one from Let's Make Robots or a plastic food container?
-Nexusrex
Yes, an un-geared DC motor will work on a motor shield that same a a geared DC motor.
The problem with using un-geared DC motors is that their minimum speed is very high. It might work if the wheels are very small. With wheels smaller than the diameter of the motor you will need to mount the motor at an angle so the wheel can touch the ground without the motor dragging.
DC motors, and servos (which contain a DC motor), generally require more current than the Arduino's 5V regulator can supply and less voltage than the 7V the Arduino's 5V regulator needs to work. That's why people often supply 6V to the motors and 9V to the Arduino separately.
The choice of un/geared (or stepper) motors depends on what they have to drive. DC motors feel well only above a minimum speed/rpm.
Separate batteries allow to run the Arduino from a clean (undisturbed) supply, and each other device at its required voltage. A 5V Arduino can also run on 3 AA/AAA cells (4.5V) connected to Vcc, a 3.3V board on 2 cells. Motors may need more current, then fatter batteries (or rechargeables) may be a better choice. But everything can be run from one strong power source of at least 7V, using voltage regulators as required.
The (mechanical) design is up to you, but I'd start with a somewhat complete robot kit, and move to my own design later.