More Inputs#

The mirror of the output problem: a design needs to read more switches, buttons, or digital sensors than there are pins to spare. The same shift-register trick works in reverse — sample many inputs, then clock them out over a few wires.

The Part#

The go-to is the 74HC165, an 8-bit parallel-in, serial-out shift register. A load pulse captures all eight inputs at once into the register; the controller then clocks those bits out serially on a single data line. Like the 74HC595 it daisy-chains — each ‘165’s serial output feeds the next’s input — so three pins can read 8, 16, or more inputs. The ‘595 for outputs and the ‘165 for inputs are the standard complementary pair.

Alternatives#

• An I²C GPIO expander (MCP23017, PCF8574) — addressable input pins on a bus, and many expanders can generate an interrupt on change so the controller need not poll.
• A 4051 analog multiplexer when the “inputs” are analog voltages rather than logic levels — it routes one of several to a single ADC pin.
• Matrix scanning — arranging many switches into a row/column grid (as keypads and keyboards do) reads N×M switches with N+M pins, no expander required.