Thermal print mechanism comprising a printer chassis, a thermal printhead, a platen roller having a conductive shaft, a manner of imparting motion to put the platen roller in rotation, two lateral conductive contacts arranged on the printer chassis so as to be directly or indirectly in electrical contact with two opposite ends of the platen roller conductive shaft for conducting of electrical current, thus forming an electrical switch. At least one nonconductive part of one end of the conductive shaft interacting with one two lateral conductive contact or at least one nonconductive element mounted on said one end of the conductive shaft, is arranged so as to open and to close the switch when the platen roller is rotating.