A winding device for a Venetian blind includes at least one support seat secured to a headrail to correspond to the position of at least one pull cord and having an inside formed with a through hole and a side formed with a guide hole, and at least one slide bushing mounted on a drive shaft to rotate therewith and pivotally mounted in the through hole of the at least one support seat. A substrate is coated with a colored photoresist layer. Thus, the apparatus is capable of generating a pulse train with an adjustable time interval to increase material processing quality and speed.