In 1981, Kraftwerk sold a nice merchandise item on their concerts: a Casio VL-80 calculator with Kraftwerk print on pouch and display, complete with a Kraftwerk songsheet.
This Casio calculator is capable of playing a total of 100 musical keys over two octaves. To play a melody, the user has to type in the keys one-by-one. It is possible to speed up or slow down the playback, but the calculator has no rhythm capabilities. So the sound is, in a time of polyphonic ringtones, very crude.

This technological wonder of the early eighties was quite expensive, so only a few were sold at the concerts. The Kraftwerk notations are for the following songs:

Taschenrechner
Computerwelt
Radioaktivität
Neonlicht
Autobahn
Computerliebe
Die Roboter
Das Model
Metropolis
Trans Europa Express
Schaufensterpuppen

As the sound is primitive, the melodies are sometimes hardly recognizable when played by the calculator - owing in part to the slow playback. To give an impression, listen to 'Computerwelt' as played by the Casio VL-80. A full-size scan of the notation sheet is also available.