I want to build a vactrol. That means I have to build a device that converts the incoming audio signal into brightness levels. I found this.

I tried building this:

enter image description here

This does not seem to work. The LED does not change its brightness.

Do you have an idea what could be the problem or do you have a better idea?

  • \$\begingroup\$ What are the characteristics of your input signal and what is providing it? \$\endgroup\$
    – vir
    Sep 13 at 22:24
  • \$\begingroup\$ What wires did you connect between your headphone output and your circuit? Did you connect shield to battery negative (ground on your schematic)? \$\endgroup\$
    – Transistor
    Sep 13 at 22:27
  • \$\begingroup\$ The audio signal comes from my computer and ranges around +0.5V and -0.5V \$\endgroup\$
    – Joh
    Sep 13 at 22:32
  • \$\begingroup\$ I wired the right and left channel together as IN in the schematic. Audio ground to battery ground \$\endgroup\$
    – Joh
    Sep 13 at 22:33
  • \$\begingroup\$ Try disconnecting the two channels, then driving with only a single channel. And turn the volume up as far as possible. \$\endgroup\$ Sep 14 at 0:56

Your signal averages at 0VDC, it does not rectify and filter the input signal. Then the average brightness is with its input at 0VDC. You need a "peak detector" which rectifies the signal and filters the resulting DC voltage that represents the loudness of the input signal. Here is my peak detector: peak detector


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