Dingdong door bell circuit based programmed IC 8021-2.
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.