We find it refreshing to revisit circuits that solve real problems with just a few parts and some smart thinking. This simple 4093-based audio peak detector from December 2004 does exactly that. The straightforward circuit monitors music and turns on a small light when the sound gets loud. No microcontroller, no firmware.

The Circuit

With the audio peak detector, you can monitor a pair of stereo channels on a single LED. You can think of the circuit as a hair-trigger alarm for audio. With a tuning knob (P1) you can set the level where the circuit should react. When a loud audio peak pushes the signal past that level, it flips an electronic switch inside the 4093 chip and illuminates an LED. Because audio peaks happen quickly, a small capacitor acts like a timer, keeping the LED on just long enough for your eyes to see the flash.
 
Simple peak audio detector
 Audio peak detector
“Identical circuitry is used in the left and right channels,” Flemming Jensen noted. “Use is made of the switching levels of Schmitt trigger NAND gates inside the familiar 4093 IC.”

Take another look at the compact circuit diagram. "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," he explained.

The Simple Audio Peak Detector Project

The original article, “Simple Audio Peak Detector,” appeared in Elektor December 2004. Check out the original article.
Editor's Note: The article originally appeared in a December 2004 issue of Elektor. Due to the project’s age, some of the parts and components might not be available. Nevertheless, we believe the design will inspire you to start a project of your own.