Light is transmitted from the color LED, through the fiber optic cable, to control the NPN transistor (Q2) and red LED (D1). 2017 October 14, “Snap Circuits Light”, in Snap Circuits, Elenco, page 59:
An NPN, in conjunction with a PNP and resistors, can be used to create logic gates.
(abbreviation, alt-of, initialism) Initialism of non-protein nitrogen.