The Model D-4B 4-Channel, 4-Button Handheld Transmitter is a four-channel transmitter that can initiate four separate functions. It is used in many remote switching applications where a portable four-channel transmitter is required. The first two digits of the digital code sent will be off or on depending on which transmitter button is pressed. Each transmission causes the red LED indicator on the front of the transmitter to light, showing that the transmitter is sending a signal and that the battery is in good condition.