The Door buzzer sound circuit was two-tone sound. With the switch S1. working of circuit,The IC1 is a frequency dividing circuit, and generator a sound frequency came out.by the C1, R1 are the frequency generator. This audio signal from pin 7 and pin 4 into the pin 1 and pin 8 of the IC2a, IC2c respectively.
The IC2b of a frequency converter to control IC2c.When the control signal during the low (Low) 1.25 kHz frequency matches will IC2a.At the same time, the control signal is in a state IC2b high (High), frequency 300 Hz matches will IC2c. Output at pin 3 of low IC2a in the same condition,IC2a and IC2c output is sent to the IC2d.Which acts as both a signal and output to the transistors, Q1 acts as switches and amplifiers.The tones can be heard as two strokes.
The R3 acts as a volume, you add more value to make the sound softer, but smoother, a +6 volt power supply.