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LED comparator for RF circuits

  1. Sep 22, 2010 #1
    I have a question about making an LED light up when sensing audio. Right now, I'm using Super-Regenerative receivers and testing various components of them. Using a signal generator, my audio speaker squeals, as it should, when on the right frequency. When it's not on the right frequency, I hear oscillations (white noise). I did a little research and found one circuit with a comparator and an op-amp.

    Basically, does anyone have any ideas on how to detect audio vs. white noise? As in, the signal gets quiet during white noise and is loud when on the right frequency.

    It's just highly annoying to myself and my classmates having it squeal all the time.
  2. jcsd
  3. Sep 22, 2010 #2
    You could use a low pass filter and a high pass filter with the crossover frequency above the upper end of your audio. When you have significantly more output out of the low pass filter than the high pass, you know you have audio.

    The outputs of the two filters could be rectified and compared in a comparator. I would attenuate the low pass filter so that the comparator won't indicate audio unless it is clearly present. Connect the LED to the output of the comparator so that when audio is present, it lights up.
  4. Sep 23, 2010 #3


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    Science Advisor

    I made some super-regen circuits a long time ago. They are notoriously noisy when they are not receiving a signal, but give amazing sensitivity for simple circuits.

    On thing I noticed on mine was that the DC current drawn reduced a lot when it was receiving a signal.

    It would be worth finding out if yours is the same. It may be possible to use this effect to mute the audio amplifier when there is no signal being received.
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