active low pass filter

Printed From: the12volt.com
Forum Name: Car Audio
Forum Discription: Car Stereos, Amplifiers, Crossovers, Processors, Speakers, Subwoofers, etc.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=134057
Printed Date: May 14, 2025 at 12:57 AM

Topic: active low pass filter

Posted By: myonus
Subject: active low pass filter
Date Posted: April 18, 2013 at 12:57 PM

Hello,

I was working on a circut that contolled some LEDs via sound capture. Similar to a color organ, but based on light intensity instead of color of the light. My trouble began when the transistor that completed the circut turned the light on and off/ it was so errratic, so I wanted to smooth it out. And It wa suggested that I use an op based active low pass filter to melow it out. I found some documents on this simple circut, but at a little loss because it relates to the crossover kind of filter, where i believe my application isnt? im just dealing with volts to switch right? but then the mic that puicks up the sound s triggering another transistor to open, so that where the sound plays in on the LED light? i guess im confused what is actually powering the led..voltage. then how would a filter for frequency pass calm down the opening and closing of a transistor?

anyway, if anyone has an insight, maybe i just ned a nudge in the right direction.

thanks

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Be not guided by morality, but rather, integrity.

Replies:

Posted By: oldspark
Date Posted: April 18, 2013 at 9:07 PM

So it's a sound volume modulates LED intensity display...

You said it was too "erratic" which presumably means it needs compression or slowing down.

The slowing is via the filter - smooth out the lot, or fitter out the frequencies you don't want.
Keep in mind color organs use filters to limit their frequency band - and you are seeing all bands go off at once.

As said elsewhere, the simplest version of what you describe is the LEDs connected across the speaker.
Filters or amps are added if required, and the tap taken from the preamplifier for volume setting independence.

Posted By: myonus
Date Posted: April 18, 2013 at 10:30 PM

ok, got some of that Spark, I think wht your sayiong is use the corrisponding cap that matches the frequecy that i want to round off, the other components swith it on and off or send it to ground. Sound like this is a perfgect spot for my variable resistor too. But I still have some reasding to do. That Texas Instruments 30 page pdf was so over my head, I think my intelligence circuts melted.

Thanks again!

myonus

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Be not guided by morality, but rather, integrity.

Posted By: myonus
Date Posted: April 18, 2013 at 11:42 PM

is it possible to just buy one alredy made?

im getting a headache :(

myonus

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Be not guided by morality, but rather, integrity.

Posted By: oldspark
Date Posted: April 19, 2013 at 10:17 PM

Have search for musicolors or light-organs etc.
They should include the required filtering and gain.