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. (via
Kraftwerk: 1981 Merchandise pocket calculator)