The advent of transistors greatly reduced the battery requirements, since the current requirements at low voltage were greatly reduced and the high voltage battery was eliminated. Low cost systems employed a superregenerative transistor receiver sensitive to a specific audio tone modulation, the latter greatly reducing interference from 27 MHz Citizens' band radio communications on nearby frequencies. Use of an output transistor further increased reliability by eliminating the sensitive output relay, a device subject to both motor-induced vibration and stray dust contamination.