Voltage levels and VU meters...

[Jaroslav Lukesh]

> On Tue, 27 Jul 1999, Chris Crosskey wrote:
> 
> > If it needs DC then what do I do, if I rectify it with signal diodes,
how do
> > I go about smoothing it when the signal might be anywhere between 20Hz
and
> > 20kHz?....I know that I'm unlikely to need the bottom octave of that or
the
> > top two but I want it to be capable of the full range....
> 
> You might want to use active rectifier to get rid of the 0.6V diode drop.
> For smoothing, make an RC lowpass filter with cutoff set low enough (say,

> 20Hz). 
> Disclaimer: I don't have any experience using VU meters so this advice
> might be wrong.
> 
> Antti

Hi,

Vu meter shoud have following charecteristic:

1. FAST response (<1 msec)

2. SLOW decay (0.05 to 0.2 sec)

How to do it:


in o--R1---+------+--------o out
           |      |
           C      R2
           |      |
          gnd    gnd        


decay: t=R2*C
response (rise time): t=R1*C

R1 may be omit, but output of prev. stage must have good current limiter
(should be used 741 type opamp or LM324 - quadruple 741, max. output
current is 25mA)

BUT Dont forget for equation

		C*U=I*t

if max detected voltage is for example 5 volts ie., C*5=0.025*0.001 ;
C=0.025*0.001/5=0.0052 
ie., C must be less than 0.0052F, if we use for example 10uF, condition is
OK, rise time is OK too.

Best Regards


Jerry LSH