This circuit detects the presence of audio (voice) on the output of a scanner. If the scanner stops on a `dead carrie
r" or noise, the circuit mutes the speaker to avoid annoying noise. Ul amplifies speech and drives rectifier D1/D2 and switch Ql. Comparator U3 drives speaker switch Ql and indicator LED1. Q2 completes the speaker path to ground. U2 is an audio amplifier to drive the speaker. R3 is a volume control. PL1 connects to the scanner speaker or to the headphone jack.