4021 8-Stage Parallel-In/Serial-Out Shift Register

A CD4021 CMOS integrated circuit providing a 8-stage parallel-in serial-out shift register. Reads eight parallel inputs and shifts them out serially. The standard way to read many buttons or switches with only three microcontroller pins, and widely used in Arduino projects for exactly that. Being 4000-series CMOS it runs on anything from 3 to 18 volts, which makes it far more flexible than 5V-only TTL logic and lets it work directly in battery and higher voltage circuits. It also draws almost no current when idle. The trade-off is speed: CMOS of this family is slower than 74HC logic, so it suits control, timing and teaching applications rather than fast data paths. Unused inputs must be tied to a supply rail, never left floating. Wide 3-18V supply range, unlike 5V-only TTL; Extremely low quiescent current suits battery circuits; High noise immunity from the wide logic swing; Standard 16-pin DIP fits breadboards and sockets