This ding-dong door bell circuit might become the simplest and cost-effective door bell circuit. The circuit is designed based on single IC 8021-2 which manufactured Formox Semiconductors. It is an 8-pin DIP (Dual Inline Package) IC whose only four pins, as shown in the above circuit design, have been used.
The IC, by default, has an in-built circuitry to generate ding-dong sound when the pin #3 of this IC is pulled low. The sound is stored in the IC as bits, as in a ROM. The sound output from the IC can”t however drive a speaker directly, as this puts strain on the device. Therefore a complementary-pair, two-transistor amplifier is used to amplify the sound to a fair level of audiblity. You may either use a piezo tweeter or an 8-ohm, 500mW speaker at the output.
During the standby period, the IC consumes nominal current of a few microamperes (mA) only. Therefore switch S1 may be kept closed. Each time switch S2 is pressed, ding dong sound is produced twice. If you try to press switch S2 a second time when the first ding-dong sound is still being generated, it has no effect whatever and the two ding-dong bell sounds will be invariably produced.
This circuit work only using 3V DC supply, you may easily use 2 pieces of 1.5 general battery (AA or AAA) to supply the circuit. Assemble the circuit in the box, make another small box just for push-on switch. Put the circuit box inside your house, while, of course the switch should be installed outside the house (door)