It is often not possible to put the second bell in parallel to the existing one because the bell transformer is normally already fully loaded. The circuit proposed here offers a solution to this problem.
The buzzer sounds after the bell-push have been released, The energy for this is stored in capacitor C2 while the bell-push is pressed. This causes only a small (and brief) additional load on the bell transformer.At the same time, T1 ensures that T2 is off. When the bell-push is released, T1 is switched off and T2 begins to conduct. As long as the charge on C2 lasts, the buzzer will sound. The length of the sound depends on the value of C2 (the higher the value, the longer the sound). Since all the energy is supplied by C2, the buzzer cannot
be replaced by a normal bell or gong, since these draw far too large a current.