Apparatus for the control of a number of remotely located irrigation or sprinkler valves from a central location by means of control signals encoded onto a single pair of power transmission lines linking a central encoder and a number of remote decoders. The encoder includes a multiplexer for selecting on/off signals supplied by a controller, an address generator to drive the multiplexer and to generate decoder addresses for transmission, and coupling circuitry to encode the generated decoder addresses and on/off signals onto an alternating-current power signal, by clipping half-portions of the signal to represent zero values. Each decoder includes a bridge rectifier, from which clock pulses and a serial data stream are derived. The data stream is continually shifted through a shift register, and a comparator compares a decoder address field in the register with the conditions of a set of manual switches indicative of the decoder address. When the comparator finds a match, the on/off signal is gated into a flip-flop and utilized to generate a valve control signal.