simulate this circuit – Schematic created using CircuitLab

I have made a LED VU Meter using LM3915 and added a low pass filter too. Then I've tried a basic op amp circuit using LM324 (inverting and non inverting) to amplify the LM3915's input, but it doesn't work. It only lights up the first led, it doesn't work like a VU meter should. Can anybody show me how to use an op amp like the LM324 with the LM3915?

My LM3915:
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The two LM324 circuits I tried:

enter image description here

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    \$\begingroup\$ What exactly do you mean by "amplify LM3915"? \$\endgroup\$ – Hearth Oct 30 '18 at 19:29
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    \$\begingroup\$ Please provide a schematic and limit yourself to one period per sentence. \$\endgroup\$ – winny Oct 30 '18 at 19:35
  • \$\begingroup\$ I meant to increase the input signal of LM 3915...when I added low pass filter using 3.3 Kohm and 1 mikroF the leds only light up to third or fourth led on LM 3915... I want it to light up from first Led to the last Led when the bass sound is played... but it only light up first Led when I try to use op amp ...please help me.... \$\endgroup\$ – Nelson Oct 30 '18 at 19:35
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    \$\begingroup\$ Please give us a schematic for what you're using. There's a built-in schematic editor here on this site. \$\endgroup\$ – Hearth Oct 30 '18 at 19:41
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    \$\begingroup\$ You need to modify the schematics to show the actual component values you are using, and tell us about the power supply voltage. \$\endgroup\$ – Elliot Alderson Oct 30 '18 at 22:25

You haven't indicated what you did for the low pass filter, but what I've found with these devices when I used them 30+ years ago is that you really can't see the higher LEDs turning on with a straight signal going into them, since they are on for such a short time, they're very dim. For a proper VU meter, you need a peak hold circuit - mechanical VU meters had a fairly quick rise under the moving coil's force but were well damped so returned fairly slowly under the hairspring. The LM3915 datasheet shows a peak detector based around an op-amp, the size of that output cap will determine the response. enter image description here

The gain of the op-amp circuit can be adjusted to get a reasonable scaling, but you can also reduce the value of the reference voltage on Rhi (pin 6) to increase the sensitivity at the expense of accuracy. The datasheet shows examples of that too.

  • \$\begingroup\$ thank you for helping me Phil.... can I change LF351 with Lm 324 or any other op amp? I don't have LF 351 at the moment... and if I can change to LM 324, should the value be the same? \$\endgroup\$ – Nelson Oct 31 '18 at 7:19
  • \$\begingroup\$ thank you phil... it works with LM 324 ... this forum help me..... thank you again.... this forum will not know how much it helps me... LOL \$\endgroup\$ – Nelson Oct 31 '18 at 8:30
  • \$\begingroup\$ Pretty much any op-amp would work for this. I'm amazed that the LM3915 is still in common use, TI stopped production of it years ago - they don't even host the datasheet any more. \$\endgroup\$ – Phil G Oct 31 '18 at 13:44
  • \$\begingroup\$ then what did they use to make led vu meter nowdays???? LOL... I told you all I am a beginner in this kind of stuff..... I like vu meter when I nwatch people light up their house with leds and those leds light up based on musics... then I started get into this electronics,,,, day and night.. non stop... Thank You again Phill.... \$\endgroup\$ – Nelson Oct 31 '18 at 13:50
  • \$\begingroup\$ These are still fun projects for hobbyists, it's just with the focus on digitally stored audio now it's easy for manufacturers to derive the logic directly within the DSP (digital signal processor) and not use additional devices. Standalone all-analog amplifiers seem to be becoming a rarity now. If you get really interested in this subject, there are many Arduino-based LED projects, some of which use addressable multicolored LEDs. \$\endgroup\$ – Phil G Oct 31 '18 at 18:46

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