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Preamp 12th January 2011 06:55 PM

LED VU meter using existing bar graph
 
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Hello everybody,

I need some help on designing an LED VU meter. The well-known LM3915 is not an option this time, because I want to use an existing bar graph (see attached picture).
This thing actually accepts any voltage between 0V and 10V while the displayed range is adjustable (ie. 0V...1V or 5V...5,5V).
The graph is made of 2x 101 LEDs which should look pretty neat and give a fine resolved display of the volume level.

Now: What do I need to get my level displayed in dB? I was thinking of a range like -80dB to +6dB...

Thanks in advance,
Lasse

DF96 13th January 2011 09:44 AM

You need to rectify your signal to get a DC voltage, then take the logarithm to get dB - use specialist chip or BJT with an op-amp. I very much doubt you will get 86dB dynamic range, and why would you want to anyway - do you want the lowest few LEDs to respond to recording noise?

forr 13th January 2011 10:11 AM

Hi Preamp,
This is quite curious, I own almost similar meters, they are labeled MORS which used to be a french trademark I think. In mine, the displays are made of fluorescent bars, not leds. I'll probably use them soon, they're pretty looking and only need to be plugged in 220 voltage mains for the power supply.
To get dB of RMS voltage, in his hardware distorsion meter, Cyril Bateman used an AD536, a 1 kOhm resistor having a 3500 temperature coefficient and an OP1770. It needs calibration. The scale is 60 dB.

Preamp 13th January 2011 12:08 PM

Quote:

Originally Posted by DF96 (Post 2431544)
I very much doubt you will get 86dB dynamic range, and why would you want to anyway - do you want the lowest few LEDs to respond to recording noise?

I'm going to use a PGF3000 whose datasheet says it cuts off at -100dB, so I thought I could start at 80dB to still have some "action" while listening to small volumes and make some use of that 100 LEDs... 60dB will still be fine, of course.


Quote:

Originally Posted by forr (Post 2431562)
To get dB of RMS voltage, in his hardware distorsion meter, Cyril Bateman used an AD536, a 1 kOhm resistor having a 3500 temperature coefficient and an OP1770. It needs calibration. The scale is 60 dB.

Sounds quite complex. Do you have a link to that project? Or is it only available as printed media?

Preamp 13th January 2011 01:54 PM

EDIT: Just found the schematic in AD536 datasheet.


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