El escenario
A visitor presses the normally open push button; the UM66 melody generator powers up and drives a BC547 transistor to sound the 8Ω speaker.
Qué hay en este dibujo
Lee las decisiones que hay detrás.
01
Placed the push button in series with the battery so the UM66 only draws current while pressed.
02
Used a base resistor and base pull-down to keep the BC547 off when the melody IC output is quiet.
03
Connected the speaker between the transistor collector and VCC for proper drive.
Reuse for any battery-powered call bell, UM66-based melody player, or simple 3V audible alert circuit.