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AMS-02 is an astroparticle experiment designed for a very precise measurement of the primary cosmic rays spectrum. The experiment will operate on board the ISS at a 400 km altitude for a period of about 3 years. The main scientific goals of the experiment are the search for antimatter, for dark matter and the study of gamma rays. In AMS-02 the electromagnetic calorimeter plays a key role for its high capability to measure e+, e- and gamma spectra and to suppress the background generated by p and He nuclei. To directly detect photons the calorimeter must provide a stand alone trigger. The proposed trigger, presented in this article, will have the capability to detect photons with energy above 4 GeV with an efficiency of about 100%. The hardware implementation comprises an analog section, comparing the photomultiplier signal with a given threshold, and a digital section applying the trigger selection algorithm.