BCD to 7-Segment Display circuit

    The ABCD input of the BCD to 7-segment decoder driver are normally held at ground potential

    by resistors R1 through R4(1KOhm).  Switches Sa, Sb, Sc, Sd are normally open.  When a switch

    closes it connects the input to the +5V supply, applying a "1" to the input.

    The 7446 converts the BCD number presented to its input and selectively brings some of the a-g

    output low (via the R5 to R11 resistors- 220 Ohms).  An output going low allows current to flow

    resulting in the connected segment's LED to light.