Mark sense detector with variable threshold

A mark sense detector with a threshold detector having a weighted and variable threshold discriminates reflective variations on an illuminated surface such as a hand-marked lottery entry card having a plurality of tracks with potential reflective variations occurring at discrete positions thereon. The card is fed relative to an array of photodetectors for the tracks, each photodetector producing an input signal for a respective one of the tracks in response to reflections of the illuminated surface, the input signal varying with marks and with parameters of the illumination and feeding. The track input signals are applied to follower amplifiers for each of the tracks. An integrator for each of the tracks averages the output of the follower amplifier, the average being divided and applied as the threshold input to a comparator for each of the tracks. The integrator has different time constants for charging and discharging and is blocked by a diode against discharging through the follower, thereby favoring detection of marks. The division of the average level applied as the comparator threshold can be switchable as a function of card parameters. One of the detector channesl can be used for edge detection.