Ok, thank you @barrymossel

@BigBadHodad Please open a new topic and add more details about your setup.

Instead of using the "enable_uart=1" line in config.txt, I found out it might be more optimal to activate the voltage on pin 8 from inside the retropie>raspi-config>interfacing options>Serial.

An ATTiny85 is driving the LED. The LED is connected in series with a 220 Ohm resistor directly to a GPIO pin and GND.

@XD3l I don't know if there stuff is also in the US but some electronic warehouses sell Xmas calendars for electronics.
There are 24 doors to open and behind every door there is a new (very small) project to build. Here is a list with experiments :) and I think there also is a kit for the Raspberry Pi online :)

Edit:
The kit with RPi experiments