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The first 555 is a slow oscillator, the second one is a fast oscillator. The second one produces the tone in the buzzer (loudspeaker) and the first one makes it go on and off slowly. If you miss out the first 555 it will give a contuinuous tone instead of going on and off repeatedly.
Brian.
The circuit diagram in post 1 shows a loudspeaker, not a buzzer. A loudspeaker does not work on DC, so it needs the 555 oscillator.cant understand why buzzer needs pulses, if it can be operated on DC
The circuit diagram in post 1 shows a loudspeaker, not a buzzer. A loudspeaker does not work on DC, so it needs the 555 oscillator.
If you use a buzzer, you don't need the second 555. You still need the first one because the output from TR2 is not strong enough for the buzzer.