Simple and common

Available
Electronics

74HC595 Shift-Register (DIP)

Texas Instruments
ELC74HC595
€1.44
The very popular 595 shift register chip of the 74 series is used a lot by hobbyis because of its wide availability, good speed and simplicity. Just plug it on an arduino like shown in the tutorials and youi have 8 more outputs, put two and you have 16 more outputs ! (up to a lot of them actually, with only three pins !)
 2
Available
Electronics

CD4021 8bit Shift Register (parallel to serial)

Texas Instruments
ELCD4021B
€1.44
CD4021 is a cascadable 8bit parallel to serial shift register (basically the inverse of the 74hc595). Sometimes you'll end up needing more digital input than the pins on your microcontroller board can readily handle. Using a parallel to serial shift register allows you collect information from 8 or more switches while only using 3 of the pins on your...
 2
Available
Electronics

PC817 4 PIN Opto-coupler PHOTOTRANSISTOR...

ECJTAOPC817
€0.48
PC817/EL817  4pin  DIP  General Purpose Photocoupler. This Opto-coupler consist of one piece of Ga as emitter and one piece of NPN transistor.
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