In SX Microcontrollers, SX/B Compiler and SX-Key Tool, mgreen wrote:

DigitalDJ,
I was going to suggest an audio transformer originally.  Pick up either an 8 ohm to maybe 300-1000 ohm transformer or a 1:1 transformer if you've got a line output and use a bridge rectifier to feed a small filter capacitor.  Decide on the time constant you want for the reaction time for sound on or sound off and pick a resistor to put across the capacitor to discharge it based on the time constant.  Connect the negative side to ground and use a 10K to 100K resistor between the positive side and the SX pin.  The SX will see a high any time the rectified voltage is above roughly 1.4V (without using the comparator).  If you want, you could put a volume control somewhere in the circuit to set the trigger level.
Mike

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