Here is a very simple project of a door bell circuit. The heart of the circuit is UM66 melody IC which is a great and very famous melody IC, comes in TO-92 transistor package. The IC has only three pins and it is a complete melody system requires no external components only connect
3 volt batteries (any size) and a small piezo and it will start generating pleasing melody. Now a days this IC comes in variety of different melodies. The transistor BC 548 is used to amplify the output of the IC to drive an 8 ohms speaker.