the ACT-34B is a four-channel transmitter that comes with long-life lithium batteries and a quick-disconnect key ring. Linear's 10-secont time-out feature prevents the system from shutting down by a single transmitter that runs continuously. Programmed in MegaCode format, which uses a million different codes for security, the possibility of code duplication is practically zero. These ACT-34B transmitters can be programmed locally or remotely, and the receiver learns the code on the spot. They operate on a 318 MHz frequency, and they're incredibly compact and convenient to carry. In fact, each ACT-34B transmitter measures only 1.25 inches wide, 2.25 inches high and 0.48 inches deep; they're the same size, if not smaller than, your ordinary vehicle remote locking system.