Abstract: A fail safe system for preventing erroneous activation of a device, such as a respirator, is provided to inhibit continuous or free running operation of the device as well as precluding activation of the device within an initial, predetermined portion of a phase in a cycle of operation. A circuit is provided for inhibiting the production of an output activation signal in the event that an input signal is continuously present or present at least at the start of a phase. In addition, a further circuit is provided for establishing a predetermined delay period commencing with the start of a phase for precluding the production of an output activation signal during such a period. Thus, only upon sensing the occurrence of a particular external condition after the aforementioned delay period has expired, will an output activation signal be produced.