Linefeed add-on for HP Deskjet 520

A fine printer, really, that Deskjet 520 from Hewlett Packard, only it lacks a line feed button! This obvious deficiency is made good by the add-on circuit shown here. It is inserted in the Centronics link between the computer and the printer, and effectively supplies the code for a line feed (OA hex) plus the associated strobe pulse.