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Topic: Opamp circuit behaving strange. (Read 1 time) previous topic - next topic

adilmalik

I built a Vu meter using the LM324 (http://4.bp.blogspot.com/_S8fE2DbYBlQ/Sbf_fcnxTBI/AAAAAAAAAJ0/Dkmfywgfoh0/s400/4LEDvumeterlm324.GIF)
But it doesnt seem to work. Without my ipod as input, it works fine; i adjust the input voltage using a pot, and the vu meter works fine, but as soon as i use an audio source, all the led light up and flicker to the beat of my music :\. I have also triple checked the voltages the op amp is comparing but again when i use my ipod as a source it literally stops comparing and lights all outputs high.

Thanks in advance.

RPCoyle

if you are tied into the audio out then it sounds like it is working fine. If you are not, then where is the input from the ipod coming from?

adilmalik

Sorry i dont understand you. my ipods audio out jack serves as the input here. Seeing the voltage levels in the circuit it should work fine, but instead it lights up all the leds at once :\

majenko

Place a 22µF capacitor in-line with the audio input (series).

Magician

Try to connect ground wire of your audio cable to negative battery terminal.

Grumpy_Mike

Quote
Place a 22µF capacitor in-line with the audio input (series).

Or in fact any value capacitor. The bigger the value the lower the frequency response but you could even use a 0.1uF and get good results.
This is called AC coupling.

adilmalik

No luck i tried the capacitor, but it had no effect. Im stumped.

Grumpy_Mike

Explain what you mean by "no effect", was it the same as before you inserted it.
If so then either you have not got it in right or you have some other problem. Either way you need that capacitor in.

MarkT

That's an awful graphic, hard to read.  However its clear that
(1) The input must be DC coupled for the circuit to work at all.
(2) The 50k variable resistor has to be correctly adjusted.
(3) the opamp inputs must be connected the right way round, incoming signal to the inverting inputs, resistor divider string to the non-inverting inputs.  A quad comparator would be better than an opamp here.  Some opamps cannot tolerate large differences in voltage between inputs, note.
(4) If the source is AC-coupled then it should be applied directly to the inverting-inputs of the amps and the input terminals shorted.
[ I won't respond to messages, use the forum please ]

DuaneB

Hi,
   I have also built a LM324 based VU Meter and while it works, an LM3914, LM3915 or LM3916 is a much better alternative giving 10 levels with only one external component required.

   Look them up on your favorite video site.

Duane B

rcarduino.blogspot.com
Read this
http://rcarduino.blogspot.com/2012/04/servo-problems-with-arduino-part-1.html
then watch this
http://rcarduino.blogspot.com/2012/04/servo-problems-part-2-demonstration.html

Rcarduino.blogspot.com

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