An electric generator comprising a rotor housing; a rotor mounted in the rotor housing, with two opposed main faces; permanent magnets circumferentially arranged in front of one of the main faces of the rotor; coils circumferentially arranged in front of the other one of the main faces of the rotor; the rotor being configured for, upon rotation, vary a magnetic flux through the coils produced by the permanent magnets, so as to generate electromotive forces in said coils; wherein the coils are arranged outside of the rotor housing. Also a valve for gas cylinder, equipped with a corresponding electric generator.