The frequency of sound can be controlled with 5.6k and 10k resistors. The transistor 9013 is used to amplify the output of IC to drive an 8 ohms speaker. You can also use substitute transistor in the place of 9013. The circuit will produce ding dong bell sound by pressing the switch near the 1N41418 diodes. The operating voltage of the circuit is 4.5 volt DC.
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