tiny circuit comprising of a single 3 terminal IC UM66
can be built small enough to be placed inside a greeting
card and operated off a single 3V flat button cell.
There is not much to the circuit. The UM66 is connected
to its supply and its output fed to a transistor for
amplification. You can either use a 4ohm speaker or
a " flat" piezoelectric tweeter like the one
found in alarm wrist watches.
If you use the piezo,
then it can be connected directly between the output
pin 1 and ground pin 3 without the transistor.
The UM66 looks like a transistor with 3 terminals.
It is a complete miniature tone generator with a ROM
of 64 notes, oscillator and a preamplifier. When it
first came into market, it was programmed for the "Jingle
bells" tune. Now they come with a wide variety
of different tunes.