General purpose registers

There are 32 general purpose registers on the Atmel AVR microprocessors.

Futurlec is a good site for parts.

LDR, our first sensor

Some good code and information here.
Light dependent resistor circuit. See http://www.ladyada.net/learn/sensors/cds.html

Arduino RS232

The Tx and Rx signals from the Arduino are not true RS232. Check out the MAX232 chip that translates voltages.