Abstract: A fluid-pressure operated pilot valve produces a cyclic modulation of pressure in a control duct, for operating a relay valve, by introducing a predetermined delay between the displacement of a first valve element from, and its return to, a first position against a first valve seat by a fluid-pressure operated timer device, which includes flow restrictor means and a pressure accumulator, acting upon the larger area side of a differential area actuator which vents the pressure in the control duct to an exhaust duct in a second position of the first valve element; the relay valve being operable in response to the cyclic pressure changes in the control duct to admit an irrigating liquid to an irrigation spray nozzle.