The electronic bell needs no power supply. Most of the resistors are not critical, although C2, R2, and R3 work best at the values given. Leaving out Rl will make the unit ring louder. The piezo buzzer may vary from store to store. If it has two leads, connect the red lead to the collector and the black lead to the emitter of Ql
If a third (blue) lead is present, connect it to the base of Ql.