Need help with LM3916 LED volume meter

[Mahdi.H]

Hi all,

I am currently building a vu meter with an LM3916 chip on a breadboard. I am using an electret mic with an LM386 chip for amplification.

When in "dot mode" multiple LED's still light up (they flicker actually) and it basically looks like bar mode. I'm fairly sure this is because the input is an AC signal, not a DC voltage. I tried to add a half wave peak detector (Figure 6 in the LM3916 datasheet), however now only the highest LED lights up.

Any help would be much appreciated

This is the basic circuit schematic I have been using (this does not include the peak detector): https://www.instructables.com/files/orig/FX5/67GS/FLROK12R/FX567GSFLROK12R.gif

LM3916 datasheet: https://www.electro-tech-online.com/custompdfs/2013/01/lm3916.pdf
LM386 datasheet: https://www.electro-tech-online.com/custompdfs/2013/01/lm386.pdf

 

[alec_t]

I tried to add a half wave peak detector (Figure 6 in the LM3916 datasheet), however now only the highest LED lights up.

The decay time-constant of the peak-detector could be too long, or the amplifier gain could be too high.

 

[audioguru]

Like most Instructables, the circuit is completely wrong (look at the IC datasheets):
1) The LM386 is a power amp, not a mic preamp.
2) Each IC is missing a very important supply bypass capacitor so they are probably oscillating.
3) R5 is not a volume control. Instead R5 plus R3 should be a volume control.
4) Pin 7 of the LM3916 is supposed to have a resistor to ground to set the current in the LEDs but instead the resistor is at pin 8 (with a wrong value) which is supposed to be connected to ground.

The figure 6 peak detector works only when pin 7 and pin 8 are set for an input range of 10V which is not done.

The LM391x ICs detect positive input voltages and ignor negative input voltages so an input rectifier is not needed.

 

[HD ELEKTRONİK]

LM386 pin3 added 100KΩ to chassis.



(sorry my eng.)

 

[fernando_g]

Like most Instructables, the circuit is completely wrong (look at the IC datasheets):
.

Agree with you 100%. There are several instructables which are right, but many others that are so wrong, that it is really a coin toss to build a circuit succesfully as shown, without one actually understanding how the thing works.

And this means -mostly- reading the datasheets. A lot of people find it boring, but if one does not have the mindset to read and understand a datasheet, then it is unlikely to build a succesful circuit.

 

[audioguru]

LM386 pin3 added 100KΩ to chassis.

It will not do anything and is not needed.
The LM386 IS NOT AN OPAMP! Instead it is a little power amplifier that has all resistors needed to make it work.
The schematic of it on its datasheet shows that it already has a 50k resistor to ground on each input.

 

[audioguru]

Agree with you 100%. There are several instructables which are right, but many others that are so wrong, that it is really a coin toss to build a circuit succesfully as shown, without one actually understanding how the thing works.

And this means -mostly- reading the datasheets.

The idiots who make WRONG Instructables should read the datasheets.

 

[HD ELEKTRONİK]

LM386 is "Low Voltage Audio Power Amplifier"
https://www.electro-tech-online.com/custompdfs/2013/02/lm386.pdf

 

[fernando_g]

LM386 is "Low Voltage Audio Power Amplifier"

Power is the keyword here.... it is different from an operational amplifier (opamp).

 

[audioguru]

The cheap noisy LM386 power amplifier is designed to drive a speaker. Instead a low noise audio opamp should be used for a preamp.

Also, the LM3916 should be connected as shown on its datasheet.