The indicated circuit is an intermittent one in which we use two 555 integrated ones, one as a low frequency oscillator (audio) and the other as a modulator. As we can see, the first integrated 555 (CI-1) operates at a very low frequency, given by capacitor C1 and adjusted in potentiometer P1. This oscillator modulates, that is, it interrupts the signal produced by the integrated second at regular intervals, as applied to pin 4. When pin 4 is at the high level, the second oscillator is released (CI-2), producing the tone which is set in potentiometer P2. The power obtained at the 555's output is good, but it does not excite a loudspeaker, due to its impedance.
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