Abstract: A re-settable apparatus for producing a signal for triggering off a safety measure on an installation with at least a pneumatic feed in response to a drop in the pneumatic feed pressure comprises a body providing an internal chamber connected by a first duct to a first orifice for connection thereof to the pneumatic feed, by a second duct to at least one member for outputting a signal for control of the triggering effect, and by a third duct to an exhaust orifice. The first duct opens into the chamber through a first seat and the third duct opens into the chamber through a second seat of larger diameter than the first seat. A valve member is mounted in the chamber movably between a first position in which it is applied against the first seat and a second position in which it is applied against the second seat, while a resilient return member between the body and the valve member urges the valve member towards its first position with a force of given intensity.