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problem with the VU meter circuit
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Adhith:
Hello everyone...
I have posted a previous thread on seeking help  for the circuit for a VU meter. It has been made now and was working fine but now I encounted a new problem. The lighting of the LED was now very weird as if there is some noise in the input audio. Its lighting in more over like in the bar mode option(even when the pin 9 is floating) but with very low lighting response. What could be the  possible reason?? Does the peak detector at the input side is creating some sort of noise so that the response is bad. On the other hand it works perfectly some time and goes back to the bad condition again after. there is no loose joints or soldering since i checked it thoroughly.

just checked the pin 9 of the IC and its found that there is a voltage around 10.5 V with respect to ground. Removed the IC from the base and then probed the IC base leg for pin 9 and there is no voltage. So its clear that somehow the voltage is getting to the pin 9 through the IC. Is this a normal thing or it the cause of my problem??

The circuit diagram is attached below.
Audioguru:
Instead of using pins 7 and 8 on the LM3915 as a voltage regulator for the pin 6 reference voltage, you use a pot on the 12V supply voltage that is probably noisy.
My LM3915 VU meter works fine in BAR or in DOT modes. You copied my peak detector circuit but I can't remember which opamp you used. My peak detector is fed from an opamp used as a mic preamp.
Are you using a shielded audio cable for the input source?
Adhith:

--- Quote from: Audioguru on June 18, 2018, 12:25:16 am ---Instead of using pins 7 and 8 on the LM3915 as a voltage regulator for the pin 6 reference voltage, you use a pot on the 12V supply voltage that is probably noisy.
My LM3915 VU meter works fine in BAR or in DOT modes. You copied my peak detector circuit but I can't remember which opamp you used. My peak detector is fed from an opamp used as a mic preamp.
Are you using a shielded audio cable for the input source?

--- End quote ---
Thank you very much for your reply. The circuit was working very fine in both bar and dot without any problem for the past few days. The problem arised just a day ago. Its seems very much like a loose contact problem since it will be working perfectly sometimes and sometimes it wont. But there is no loose contact issue since i checked it thoroughly and also re soldered every spot and still no use. So I guess the potentiometer wont be an issues.

Yes i coiped the peak detector that you have recommended for me in the previous thread. The IC used is LM358.
The LED display is lighting up and no issue in that but the problem is with the response of the lighting up. The LEDs are just blinking with poor response as if I have fed a really bad audio input and thus i was under an assumption that the peak detector is adding up any noise or something in the input audio.

No I'm not using a shielded cable but fed the audio input from another source and still the problem persist.


Adhith:
founded out another problem which i think is the cause of the weird response of the VU meter. All the 10 emitter legs of the transistors are connected together which is then connected to the +Ve output terminal of the LED driver. The R, G, B output terminals from the driver are directly connected to the LED strips. But now I disconnected the +Ve supply and the R,G,B connection from the driver and instead connected a +12v terminal from a DC adapter to the transistor emitter legs and the negative terminal to the Red terminal of the strips. now it is found that all the strips are flickering continuously without any audio signal or even when the VU meter is turned off.
On further look it is found that while removing the LM3915 IC from the base there is no LED light up or flickering. So its clear that Somehow the IC is producing some noise at its output legs.. So I think this would be the problem but I don't know how to solve it.
Audioguru:
Test that the LM3915 can turn off all the LED strips by shorting its pin5 input to ground. If the LEDs do not turn off then replace the LM34915 IC.
If the LEDs do turn off then short the input of the peak detector (disconnect the signal source from C4 and replace it with a short to ground). If the LED strips do not turn off then replace the LM358.
Did you properly disable the unused opamp in the LM358 so it does not oscillate and cause interference?

You MUST use a shielded audio cable to feed the signal source to the peak detector to avoid mains hum in the air causing an interference signal.
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