Fritzing

Just a very small board which still contains space in-between the various parts. The HK692 is a 4-pin doorbell IC. Press the button and Ding-Dong!

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The HK592 operates on 3V so two AA/AAA batteries or a buttoncell would do. 

Other parts that are required are a speaker, a resistor with a value between 500-10K and an NPN transistor, preferably an 8050.

And a button to press and to keep the board small, I used a small button. But in the ideal situation you would use wires to connect a button in some different location, just like the battery and speaker will be connected to the board through wires.

I considered including holes into the board but decided not to do so, since they would require a slightly bigger board. At 22x22 mm is happens to be very small, perfect to become part of a small container. It is slightly larger than a CR2032 battery, which would be perfect for this project.

The largest part is probably the speaker.