> A photocell and lamp or LED is another commonly used approach.
> Old and low tech, but it works nicely.
> --
>

True! Search for Vactec VTL5C2. It uses an LED and a CDS photoresistor.
I've used them as audio switches. Also made an audio compressor out of
them. With a voltage follower driving the LED through a current limit
resistor, the photocell is across the input to the voltage follower.
There's a series resistor between the source audio and the photocell. As
the audio gets louder, the LED gets brighter, the CDS cell resistance
drops, shunting the audio and dropping its level. Because CDS cells are
slow, you get a reasonable compressor attach and release time.

This does remind me of a very old technique of using an incandescent lamp
as a variable resistor. This was used to control the loop gain in many
audio oscillators, including those from HP. In addition, old radios would
sometimes have an incandescent lamp across the audio at some point in the
circuit. This acted as a volume expander (opposite of compressor). Louder
audio would light the lamp more brightly, increasing its resistance,
shunting the audio less. Conversely, quiet audio would light the lamp
less, decreasing its resistance, shunting the audio more, decreasing the
audio more. This was a simple noise reduction system.

Anyway, the CDS cell is a good idea!

Harold
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