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Help: LED VU Meter

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Twiggy

New Member
Hey, I am currently trying to build this circuit, which is an LED Volume Meter. The schematic and guide says that it requires a PC soundcard as input. I was wondering whether an electret microphone would serve the same purpose. Also, the schematic also shows Pin 5 as being connected to the 1M resistor, and talks in the guide about a Node A? Any guidance here?
 
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audioguru

Well-Known Member
Most Helpful Member
Most instuctables are designed by NOOBs who know nothing about electronics.
This one is missing the peak detector circuit recommended and shown in the datasheet for the LM3915.
Its input must be +1.25V for the 10th LED to light but an electret mic produces a peak voltage of only 0.014V. You need a mic preamp circuit to feed the LM3915.

The LM3915 will become extremely hot in that circuit when all the LEDs are lighted because the power supply voltage is too high. Use 6V.

All the LEDs will stay lighted all the time since the LM3915 input is missing a 1M resistor to ground.
It is too bad that the author simply copied the wrong project at Aaron Cake's site.
 
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Twiggy

New Member
right thanks, im pretty sure that the right project at Aaroncakes is this. Can you recommend a mic preamp?
 
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bountyhunter

Well-Known Member
If this connects to a computer, the computer should have an audio out for headphones. On mine, I run my VU meter from that line. A "line level" output is about 500mV (RMS) for the zero dB level as I recall. That's about 900 mV (peak) voltage.

I have a schematic for a VU meter that has the peak detectors included if anybody wants it.
 
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audioguru

Well-Known Member
Most Helpful Member
I corrected Aaron Cake's circuit. It didn't work before.
Here is a preamp for an electret mic:

EDIT:
Look at the instructable schematic that was written by a NOOB who does not know anything about electronics. He shorted the +power supply wire to ground (to keep the electrons in?).
 

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Twiggy

New Member
great thanks, should have a go at this now.

EDIT:
Sorry, but still under the shackles of electronics noob. When 3 wires are connected to a potentiometer (as in this circuit), the middle one connects to the middle pin, right?
 
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ericgibbs

Well-Known Member
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great thanks, should have a go at this now.

EDIT:
Sorry, but still under the shackles of electronics noob. When 3 wires are connected to a potentiometer (as in this circuit), the middle one connects to the middle pin, right?
Thats right!.:)
 

Twiggy

New Member
good, just thought i'd check. :)
 

ericgibbs

Well-Known Member
Most Helpful Member
good, just thought i'd check. :)
hi,
I suggest you to take 'agu's advice regarding the original VU circuit, its got many errors and shortcomings.
Look at the LM3915 datasheet for applications.
 

Twiggy

New Member
i know, i'm using Aaron Cakes circuit now.
 

Twiggy

New Member
lol sorry to bump this up again, but what could i build this on. i wont order one off a manufacturer, because its really expensive i dont have anything to build it myself. and can i substitute the lm3915 for an lm3914.
 
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audioguru

Well-Known Member
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What is expensive?? An LM3915 costs only a buck and a half, less than a cup of coffee at a restaurant. I bought LM3915 ICs for only one dollar each at a surplus electronics parts store.

If you are in a very poor 3rd world country then I am sorry because I have never been there (But I went to Cuba and saw some poor people). Here everything is available and inexpensive.
 

Twiggy

New Member
lol, if england is a third world country then god help us. I simply found it easier to get my hands on an lm3914. can this be used to measure, for example the difference between 0 and 1mA? I was interested in using it for a lie detector if the VU meter wouldn't pan out.
 

Mikebits

Well-Known Member
Actually the 3914 sounds better for your application.
 

Twiggy

New Member
kk, im might swap to the lie detector then. would i simply swap the lm3915 for an lm 3914 in the circuit and have the input as the lie detector?
 

Twiggy

New Member
I was thinking of using this circuit of Aarons.
 

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Mikebits

Well-Known Member
Just understand, the 3914 increments as a 1 for 1 value, or linear. The 3915 increments logarithmically. If you desire your indicator to express change in emotion in the form of current, then I would think linear indication is best, in which case 3914 is best suited to you app. You must decide the parameters of how your design will perform...
 

Mikebits

Well-Known Member

Twiggy

New Member
lol, so very? i really only want a very simple circuit to work with the chip. Most people say this works well as a simple lie detector, skin resistance??
 
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