An alarm system as extrinsic sensor on optical fibers for detecting and controlling inflammable liquids based on thermosensitive proprieties of the PN1PAAm hydrogel is presented. The changes on the optical proprieties of the PN1PAAm with the temperature (being its LCST 32°C), induce abrupt changes on the light intensity and they act as an alarm signal, which is transmitted by optical fibers and after they will be processed by an optoelectronic circuit, responsible to active an alarm. An appropriate system consists of the hydrogel connected between its ends to two segments of plastic optical fibers (source and receiver) and they turn on the alarm when a photo detector does not receive light when the hydrogel becomes when it reaches threshold of temperature. The characterization of the hydrogel and the experimental results are presented for a prototype.