Re: noise indicator

[Federico Miyara <fmiyara@xxxxxxxxxxxxx>]

Lutfullah,

>Would certainly like to have your circuit diagrams.

The following is a simple block diagram for my version of the noise 
indicator. The AD 536 yields true RMS (root mean square), and, what
is best, it has a logarithmic output at a rate of -3 mV/dB. The 
amplifier next has gaim G=10, so that its output is -30 mV/dB. The
voltages V1 through V4 are obtained from a simple voltage divider
in such a way that you have 10 dB, that is 300 mV between each other. 
For example, if Vcc = 12 V, then R = 100 Kohm and R1 = R2 = R3 = R4 =
3 Kohm. You may use a readily available quad operational amplifier 
such as the LM324 as a series of comparators. The SCR may be a 
thyristor (direct current) or a triac (alternating current) switch.
The idea is quite embryonic and needs much polishing (for example 
some level shifting hear and there, some positive feedback into the
comparators, some protective diodes), but I guess it will work. 


                                 Vcc
                                  |                     Vlamp 
                                  |                       |
                               R | |                     (X) BULB
           ________              |_|                      |          
          |        |   |\  G      |             R       __|__ 
 from   __| DC/RMS |___|  \ ______)___|\      _____    |_____|  SCR
MIC AMP   | AD 536 |log|  /  |    |   |- \___|     |____/ |
          |________|   |/    |  V |___|+ /   |_____|      |
                             |   1|   |/                ----- 
                             |    |                      ---  
                             | R | |                      -  
                             |  1|_|
                             |    |
                             |____)___|\
                             |    |   |- \___  Same as preceding
                             |  V |___|+ /
                             |   2|   |/
                             |    |
                             | R | |                           
                             |  2|_|
                             |    |
                             |____)___|\
                             |    |   |- \___  Same as preceding
                             |  V |___|+ /
                             |   3|   |/
                             |    |
                             | R | |                           
                             |  3|_|
                             |    |
                             |____)___|\
                                  |   |- \___  Same as preceding
                                V |___|+ /
                                 4|   |/
                                  |     LM 324
                               R | |                           
                                4|_|
                                  |
                                  |                             
                                -----      
                                 ---
                                  -


If Vcc is adequately decoupled (by a capacitive short circuit to 
ground), there is no reason to get the SCR switching transients 
feed back into the mic amplifier. The AD 536 is about U$S 20 and 
it is manufactured by Analog Devices and by MAXIM, which you may
find at the web.

I hope this will help you.

Regards, 

Federico Miyara

PD1: Your idea of the vu-meter is a good one. My sole concern about 
it is that those consumer electronics indicators very likely do not
yield RMS but peak or average value, since it is simpler and cheaper.

PD2: You may insert a filter between the mic amp and the RMS converter 
to get A weighted dB.



  

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