Silence Detector Circuit

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The basic principles are as follows:
- Detect sound level for example a rectified signal from a microphone (its a level so only look at a single polarity)
- Use a comparator or something similar to set your threshhold
- Send the output to a monostable 555 or similar set for 5 seconds
- Invert the output from the 555 to light the LED (so it goes off when triggered and lights again after it times out from inactivity)