The presence of a pulse at the CTRL input produces an audio signal of duration determined by the 2.2 uF capacitor. This signal has the frequency given by the capacitor of 1 uF, which can be changed in the range of 220 nF to 2.2 uF. The R resistor in series with the BCS48 transistor determines the audio level, typically ranging from 47 to 220 ohms. The 7400 is powered by pins 14 and 7. Remember that the 74121 consists of a monostable, triggered externally by pins 3 and 4. The circuit can be used as a shield.
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