This 4 -digit 7-segment display section is wired around four common-cathode 7-segment LED displays, and four BC547 npn transistors. The 1K resistors are used for base current limiting, and the 390R ...

Available in 3, 4, or 5-digit varieties, these multiplexed 7-segment displays provided countless hours of entertainment to those who would spell upside-down words on their pocket calculators.

Technically it’s not a seven-segment display at all. [F.W. Wood] designed these really nice-looking “4”s with the diagonal heads, and so he needed eight segments per digit.