Transmitter Cards are used to connect a Sensor to a Sensor Controller. With a blank transmitter card, you can encode it to a channel in the Sensor Controller's GUI by placing it in the bottom-left slot and clicking [Encode]. Once a card has been encoded, placing it in the same slot of a Sensor will allow the device to transmit data to the Controller that encoded the card.
Crafting[]
Source: The Tekkit Wiki