Most of the experiments have very modest hardware requirements. You will need an Arduino or compatible clone and a Blinkenlight shield.
Another option is the The Blinkenlight “Lernpaket”. It includes an Arduino compatible board with integrated Blinkenlights. This board is now also known as the Blinkenlighty.
If you want to roll your own shield I documented the schematics and the layout.
Some of the experiments will require some additional hardware though.
The frequency comparison experiments require at least two Arduinos because otherwise there is nothing to compare. One Blinkenlight Shield is sufficient though two are more fun of course.
The power grid monitoring experiments requires a cheap AC adapter some NPN transistor and one resistors. These parts and their values are by no means critical. I suggest a 12V AC wall adapter, a BC548 and a 10k resistor. But almost anything will do. Any transformer between 6 and 16V would be fine. Wall adapters have the advantage that they keep your fingers away from mains voltage. Just make sure that it says AC.