Thermal print mechanism comprising a printer chassis (2), a thermal printhead (1), a platen roller (3) having a conductive shaft (6), a motion means (11) to put the platen roller (3) in rotation, two lateral conductive contacts (4 and 5) 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 (6) for conducting of electrical current, thus forming an electrical switch. At least one non conductive part of one end of the conductive shaft (6) interacting with one two lateral conductive contact (4 or 5) or at least one non conductive element mounted on said one end of the conductive shaft (6), is arranged so as to open the switch when the platen roller (3) is rotating.