LED VU meter question

[scwhiteley1]

Hello!

I just ordered a few LM3916 IC's that I intend to use to build a LED audio meter. My plan is to connect it to a stereo I've built using TDA7240 amp IC's (20W).

My approach is pretty straight forward, I'm planning to use the typical application schematic out of the spec sheet, which you can see here:

http://www.national.com/images/pf/LM3916/00797101.pdf

The full spec sheet is here:

http://www.national.com/ds/LM/LM3916.pdf

The typical application design is for a 0V to 10V VU meter. The amp supply signal is from my mp3 player. So it seems clear to me that I don't hook the signal from the mp3 player directly into the LM3916 IC, as it's nowhere near 10V. So is the signal source I put into the LM3916 simply the speaker output from the amp? The IC's are cheap, but I'd prefer not to burn any of them up if I don't have to...

Thanks!
-Chad

 

[audioguru2]

You probably want to drive the VU meter from the output of your MP3 player so that the volume control on the amplifier does not affect the range of the VU meter.

The LM391x ICs have an adjustable voltage reference using pins 7 and 8. When pin 8 is connected to ground and R2 is not used then the 10th LED lights with an input of 1.25V peak which is 0.88V RMS.
The LEDs will be a dim blur unless one of the peak detector circuits are used.
I use this peak detector circuit since the transistor one does not work properly with a low input signal and their opamp ones need a dual-polarity supply:

View attachment 40948

 

[Scott S1]

Wouldn't the 7.5 volt circuit last just as long as a 6 volt one ? The 7.5v draws 200mA, the 6v draws 400mA. 7.5v: 200mA drawn until 6v would last roughly 4 hours. 6v: 400mA drawn until 3v would also last roughly 4 hours. I'm not trying to start an arguement, just trying to point that out.

 

[audioguru2]

I don't see a 7.5V circuit nor a 6V circuit in this thread.

I used a "9V" Ni-Cad battery that is actually 7.2V. Then when the battery failed I replaced it with a Ni-MH "9V" battery that is actually 8.4V.
I used two 1.8V red LEDs in series at each output and the current is 26mA at each output.

The rechargeable battery is trickle-charged by a "9V" wall-wart power supply that is actually 9V to 11V. The wall-wart powers the VU meter most of the time.

 

[Scott S1]

sorry i was looking at two differnt post and replied to the wrong one.
once again, sorry.  :-[