I am working on my first robot project. It would be a simple differential drive robot, plus two photoresistor to avoid dark or light.
I choose to use Arduino as the controller, and two modified Hitec continues rotary servo to drive the wheel. I believe I will keep on other robot projects after this (like a bigger ones with more functions, and I will try DC motors, too), and I hope my investment now could also be used later.
For the Arduino, I believe it's better to use a power from 7V to 12V; while the best for the servo is 6v; and of course, it's also better to use two powers, one for the controllers, and one for motor/servos.
I will use a 9V battery (like this one: http://en.wikipedia.org/wiki/PP3_battery
) for the controller, I didn't find a rechargable one yet (how do I recharge this kind of battery??).
I am still not sure what to use for the servos. One option is to buy a battery pack (like this one: http://www.robotshop.ca/lynxmotion-bat-05-battery.html
), then I will have to buy a charger for it, too, and they don't have any stock in RobotShop.ca (I am in Canada), I checked local The Source (the store in Canada, just like Radio Shack) and they don't have it either. I thought about battery holders (like this one:http://www.robotshop.ca/sfe-battery-holder-4xaa.html), but I can only find those to hold either 4 or 8, which would be either 4.8V or 9.6V. I also found this battery pack (http://www.liquidware.com/shop/show/BP/Lithium+Backpack+Stealth
), it's quite impressive (especially the fact that I can recharge it using USB), while the price is quite impressive, too, cause I have to pay about $20 dollars to ship it to Canada (and don't know how much I have to pay for the custom yet).