[Describe the design of a simple home automation system]

I designed a sensor interface for a computer if that helps.

http://sites.google.com/site/angophoraelectronics/projects/temperature-humidity-sensor

  Micro-controller                    ATMEGA8A
  Humidity sensor                    HONEYWELL HCH-1000-002
  Temperature sensor              LM335AZ
  Crystal                                  3.6864 MHz
  Serial interface                      MAX232
  Humidity Sensor Interface    74HC4060

Component cost is about $36 USD.