This audio peak detector allows a pair of stereo channels to be monitored on a single LED. Identical circuitry is used in the left and right channels. Use is made of the switching levels of Schmitt trigger NAND gates inside the familiar 4093 IC.
The threshold level for gate IC1.An (IC1.B) is set with the aid of preset P1, which supplies a high-impedance bias level via R2 (R1). When, owing to the instantaneous level of the audio signal superimposed on the bias voltage by C3 (C2), the dc level at pins 1 and 2 (5 and 6) of the Schmitt trigger gate drops below a certain level, the output of IC1. An (IC1.B) will go High. This level is copied to the input of IC1.C via D2 (D1) and due to the inverting action of IC1.C, LED D3 will light.
Network R3-C1 provides some delay to enable very short audio peaks to be reliably indicated. Initially turn the wiper of P1 to the +12 V extreme — LED D3 should remain out. Then apply ‘line’ level audio to K1 and K3, preferably music with lots of peaks (for example, drum ‘n bass). Carefully adjust P1 until the peaks in the music are indicated by D3. The circuit has double RCA connectors for the left and right channels to obviate the use of that rare and expensive audio splitter (‘Y’) cables.