analogue multiplexer for arduino

using a 74hc4067 was simpler than i expected, using these links gave me all the info needed:
- general muxing tutorial
- muxing for arduino
- other arduino pinout extensions
- 74hc4067 specs
- 74hc4067 general info
- 74hc4067 further info

readymade ones are available, too, but i preferred to go the learning way. the LEDs are a debugging help for the state of the multiplexer address bits and will likely not be on the final PCB.