Abstract: A stack height control apparatus for sheet feeders in sheet processing machines. The apparatus comprises a 2:1 divider connected to stack height sensors, a comparison timing stage connected to the divider, a comparator stage connected to the sensors, to the divider and to the comparison timing stage, a forward-backward shift register connected to the comparator stage and at least two timing stages each connected to the shift register and to the comparator stage. The apparatus allows automatic feeding of sheets to machines such as employed for example as printing presses.