The circuit presented here (see figure) provides users with an indication of a high or low logic state. When the input is a logic 1 (high), the common-cathode display indicates "H." When the input is a logic 0 (low), the display indicates "L."
The circuit uses one gate out of four in IC N—a 7400 quad, two-input NAND gate— to invert the input and apply the output to the d, e, and f segments of the display via an emitter follower (T1). The diodes and the resistors suitably buffer the direct and indirect inputs (via the emitter follower) to segments e and f. The device operates from a regulated +5-V power supply.