Transmitters

A garage door opener is a motorized device that opens and closes garage doors. Most are controlled by switches on the garage wall, as well as by remote controls carried by the owner. ... The first garage door opener remote controls were simple and consisted of a very simple transmitter (the remote) and receiver which controlled the opener mechanism. The transmitter would transmit on a designated frequency; the receiver would listen for the radio signal, then open or close the garage, depending on the door position. ... The third stage of garage door opener market uses a frequency spectrum range between 100-400 MHz and most of the transmitter/receivers rely on hopping or rolling code technology. This approach prevents perpetrators from recording a code and replaying it to open a garage door. Since the signal is supposed to be significantly different from that of any other garage door remote control, manufacturers claim it is impossible for someone other than the owner of the remote to open the garage. When the transmitter sends a code, it generates a new code using an encoder. The receiver, after receiving a correct code, uses the same encoder with the same original seed to generate a new code that it will accept in the future. Because there is a high probability that someone might accidentally push the open button while not in range and desynchronize the code, the receiver generates look-a-head codes ahead of time. [Garage door opener. Wikipedia]