Stereo Peak Indicator

The circuit diagram shows a low-cost, fast responding, peak holding, audio level indicator.The sensitivity is determined by preset P1, which (sort of) DC-biases the input of Schmitt-trigger gate IC1a to a level close to the switching threshold. When the negative peaks of the audio signal cause the output of the first Schmitt trigger gate to go high, the capacitor on the input of the second Schmitt trigger gate rapidly charges via diode D1. The charge can only disappear via the 1-Mohm resistor, R2, so a fast attack, slow decay is the result. In this way, even the fastest peaks will be caught and indicated by LED D2.