The unit of frequency is the Hertx ( 1 Hertz = 1 cycle of on / off per second )  and so a very rapid on/off is heard as a continuous sound - all the individual sounds merging together.  Different notes  are simply a different number of 'on/offs' per second..  Using this we can arrange for the 555 time output to make a different sound ( a different musical note )  for each key on our mini-piano - if each key is connected to a different combined value of resistor and capacitor.  ( Work out the values or use the download programme below.)

If the 'wandering lead is touched on any of the resistor ends a particular note will be heard.  As each of the resistors shown is a pre-set resistor that can be adjusted to give a particular value then the frequency of each note can be adjusted until a full octave is heard from the mini piano.  Finally,  instead of a wandering lead,  a push to make switch or a membrane
contact switch can be added onto the end of each pre-set resistor.  This will now give us a much more easily played piano - even though it may not sound that great in a concert hall !

A CNC-cut case can now be designed - or alternatively a press-formed or vacuum formed case can be used to make the whole project have the look of a professionally made product.