sound activated relay with single FET?

[neazoi]

Hi, can a single FET be used as a sound activated relay, taking it's input from the sound plaster of a PC and used as a solid state switch to a 300-500mA load?

 

[Audioguru]

A Jfet or Mosfet does not have much voltage gain. The gate needs 5V to 10V for it to turn on.
You do not need the very high gate resistance of a fet when it is driven from a sound blaster. So use an ordinary power transistor that can have a high voltage gain instead.

But sounds have a wide range of levels from faint to loud so your relay might turn on only when the sound is loud. If you need more sensitivity then an opamp with high gain can drive the transistor.
Sound levels fluctuate up and down all the time which might cause your relay to turn on and off many times all the time so you will need an averaging or peak detector circuit to drive the transistor or opamp. The opamp itself can be the detector that drives the transistor.