Abstract: A printer having a printing mechanism for placing printed matter on a substrate, the printing mechanism including character printing elements and an electrically driven hammer unit which is connected to receive an actuating current and is operative for causing the printing elements to print characters on the substrate by impacting against the elements with an impact force dependent on the magnitude of the actuating current, the printer being further provided with: a substrate thickness monitoring device mounted in the printer for monitoring the thickness of a substrate disposed to be printed upon by the mechanism and for producing an electrical output signal indicative of the substrate thickness within a predetermined amount of time; and an actuating current control circuit connected to receive the output signal from the monitoring device and to control the actuating current in a manner to reduce the impact force when the substrate thickness is less than a selected value.