The transmitter and receiver circuits are quite simple and use picaxe chips. These single chip microcontrollers can sense analog voltages, turn things on and off and can transmit data. See instructables http://www.instructables.com/id/Control-real-world-devices-with-your-PC/
for a description of how to program picaxe chips. With a radio link as well as an interface to a PC it is possible to sense data remotely and transmit it anywhere in the world.