Door bell circuit
The IC has an in-built circuitry to produce ding-dong sound each time its pin 3 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 audibility. 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 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 produced, it has no effect whatever and the two ding-dong bell sounds will be invariably produced.
The article was first published in March 2002 and has recently been updated.