The circuit is using a low cost CD4011 IC, that is wired as a multivibrator in the circuit, due to which the output at pin4 of the IC continuously goes high and low, that switches ON and OFF the 2N4401 transistor on continuous basis, hence the peizo buzzer produces beeps and the LED flashes. The operating voltage of the circuit is 9V DC but it can also run on lower voltages.
A simple and reliable beeper circuit can be built using CD4011 IC. The circuit can be used for alarming & indication purposes or at any place where a peizo buzzer is required. The circuit continuously switches ON and OFF the buzzer due to which it produces beeps in gaps.
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