The circuit presented here () 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.