Pulsing LED?
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SnoopDogisUgly

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Hello

I am in the middle of breadboarding my first CMOY amp. I think I have one channel working except I have a concern.

The LED on the power circuit pulses with the music. For example, during a louder section of the music the LED gets brighter and vice-versa. I searched the DIY forum archives for pulsing LED's and came up empty. Is this normal? It makes me think there is a short somewhere causing this.

thanks
 
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eric343

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That, or the LED is in *series* with a power rail... Which would make more sense, since I doubt putting it on the output would make it do that...
 
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eric343

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I'm not so sure about that... since LEDs have a fixed voltage drop, and brightness is a function of current that they're allowed to pass (IIRC), then by putting the LED in series with either of the battery leads, you'd reduce the supply voltage by 1.7V (if it's a standard red LED) and the LED would get brighter the more current the amp draws (since the amp in this case is acting as a variable current-limiting resistor; the louder it gets the more current it passes). Actually a pretty darn brilliant way of making a 1-LED Vu meter, if I do say so myself... of course, it's really only good for home amps where power is in abudance, but hey, it's a cool idea


And in order for the LED to do that hooked up to the output, you'd have to have a DC-offset of 1.7V (this is off the top of my head) to keep the output from going below ground - since if it does that, the LED turns off, and that doesn't happen here. Obviously it would be possible for the output voltage swing to happen in such a way that it does some simple pulse width modulation; since the below-ground swing accounts for about half of the wave and the above-ground accounts for the other half, and the LED (being a diode) treats any negative voltage as zero, essentially you'd be halving the brightness of the LED. You could test for this by running a volt or two through a photosensor into some high-impedance cheap headphones, and pointing the LED at the photosensor- if you hear any music, the LED's on the output. Or at least it's *probably* on the output, since it could still do this if it was in series depending on how slow or small or nonexistent your power supply capacitors are.
 
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