Photoelectric control circuit for digital magnetic compass

For use in a digital magnetic compass having a compass disc containing a plurality of optically coded representations of compass headings thereon, means for illuminating a selected portion of said disc, and photosensor means for providing electrical signals representative of compass headings, a photoelectric control circuit comprising:

• an illumination source;
• at least one photosensor receiving light from said source and through said disc;
• comparator means receiving an electrical signal from said at least one photosensor and a reference voltage and providing a digital output signal of one level when said photosensor signal is less than said reference voltage and of another level when said photosensor signal is at least equal to said reference voltage;
• and
• latch means operative in response to said digital output signal from said comparator means to provide an energizing signal to said illumination source from a start time until switching of said comparator means from said one level to said another level thereby to energize said illumination source only for a duration sufficient to permit detection of a photosensor signal at least equal to said reference voltage;
• said latch means providing no energizing signal to said illumination source from said switching of said comparator means until the next start time.