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Help with LM3915 LED VU meter

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Youran

New Member
Hi,

This is my first post on Electro Tech Online, because I couldn't figure things out by myself.

Here's the problem:
I've successfully built the LM3915 circuit as shown in the datasheet. Everything's the same except I used 10k instead of 8.06k for R2 and I've left out the 2.2 uF capacitor.
When I turn it on without an audio source, all LEDS light up, so I don't think I've made a mistake in the solder work.
When I plug in an audio source (headphone out from my stereo or computer), there are always 7 or 8 LEDs lit, regardless of the volume that is outputted by my stereo/computer. When I turn the volume up, LEDs 8,9 and sometimes 10 work as they should. So I conclude that my setup works? I just need to configure it so that it uses all the LEDs and when the volume is turned down, none of them are lit.

I'm not using a rectifier circuit, however I've built one along with a booster circuit but that didn't give a better result.
I'm thinking of building a new (better?) one from this site: LED Audio VU Meter
Would it help in any way?

I've tried using all kinds of different values for resistors but to no avail. I've also tried to work out the exact value I need according to Vref but I find the calculations in the datasheet somewhat confusing.

FYI: the signal on my headphone out fluctuates around 2.5V with peaks to 3V (is this correct?) and I have an adapter with adjustable voltage (3-12V) however when choosing different values, I don't really see a difference.

Hopefully someone can help me because I really want to finish this project.

Thanks in advance,
Youran
 

audioguru

Well-Known Member
Most Helpful Member
The LM3915 needs to have 0VDC on its input with the audio coupled to it through a capacitor:
1) Connect a 1M resistor from the input pin 5 to ground.
2) Connect a 0.1uF (100nF film type) capacitor in series with pin 5.
3) Connect the input signal to the free end of the capacitor and connect the ground from the signal source to the ground of the LM3915 circuit.

The values of the resistors R1 and R2 set the max input level needed to light all 10 LEDs and set the brightness of the LEDs.

Since LEDs can turn on and turn off much quicker than you can see them then the circuit looks much better if one of the peak detector circuits is used.
 

Youran

New Member
Thanks, works much better now !
I'll try playing around with the values of the resistors now, I'll post back later.
 
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