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G8MNY  > TECH     16.08.18 08:00l 95 Lines 4665 Bytes #6 (0) @ WW
BID : 33733_GB7CIP
Read: GUEST
Subj: Eagle DB120 Sound Meter
Path: ED1ZAC<ED1ZAC<IZ3LSV<I0OJJ<GB7CIP
Sent: 180816/0557Z @:GB7CIP.#32.GBR.EURO #:33733 [Caterham Surrey GBR]

>From g8mny%gb7cip.#32.gbr.euro@i0ojj.ampr.org Thu Aug 16 07:58:39 2018
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>From: g8mny@gb7cip.#32.gbr.euro
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From: G8MNY@GB7CIP.#32.GBR.EURO
To  : TECH@WW

By G8MNY                                (Correction Feb 16)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

The is a simple cheap sound meter.
     ____                                        PCB LAYOUT
旼컴좔컴컨컴커                                旼컴컴컴컴컴컴
납~~~~~~~~~~납                                    /~~~\    
납 \        납                                쿜12쿘ETER쿣R2
납  \       납                                    \___/    
납__________납 40dBA-120dBA in 2 ranges                    
        ____ In built calibrator            쿣R1      __  
       /~~\                                 넬 Q5  /~  ~\
          납                                납    쿞WITCH
       \__/                                 납쿔C   \_  _/
        ~~~~                                냅   VR3 ~~  
                                                         
 EAGLE                                      읏   旼컴   粕
                                             쿣R5   쿣R4
                                             읕컴   읕컴
읕컴컴컴컴컴켸

CIRCUIT
 旼컴컴쩡컴쩡컴쩡컴2k2컴컴컴컴컴컴컴컫컴컴컴컴컴쩡<+ve
     56k                                         OPAMP
   +   /    +                                   The TD7120 provides
Mic캑쳐탠큂3  ===                              56k   fixed stable gain
  4u7  \e   33u                  7 旼컴        to drive the non the
     56k           Low           2~\3 56k       linear metering stage.
        VR2쳐)컫컴컴컴훟<2k2컴쩡컴큈A\  6     +
        Mic  14k4           150k  7120>컨컴컴)컴늘컴3k3>
        cal   쳐컴컴컴o       쳐컴  /읕쩡19k캑+ 10u
            220  High       +  5_/ 1     ===
                        10u===   4  ===3n3 10u
 읕컴컴좔컴좔컴좌좔컴컴컴컴컴컴컴좔컴컨컴컴좔컴컴좔<
                읕훆R1컴혃 Cal
                   200      旼컴컫컴컫컴컴쩡5k1컴<+ve
                       220k     33k 33k   
PREAMPS                    3k9         3k9
The 3 wire electret mic 읕컴컵컴늘\ /졍쳐캑
& preamp Q3 is powered        \    X    /
from a decoupled rail.       Q2쳐컴 읕컴큂1
                             e/ ASTABLE \e
VR2 calibrates the mic       읕컴컴컫컴컴컴
capsule & VR1 calibrates           100
the astable calibrate             컴좔컴<
oscillator Q1&2.

METER DRIVER                                                oOff
+ve<컴컴컴컴컴컴컫컴컴쩡컴컴컴컴컴컴컴컴컴컴컫컴풽  旼컫컴쩡o<컴컫컴
                10k /                    47u+  \ /  2k2 쳐oHigh  
                 쳐캑Q5                     === 컴쩡    읕oCal   
                __ \e         +10u           Q7쳐컴켸  旼oBat   
                \_/   쳐컴560컴쩡늘컴쩡컴               oOff  
                __   쳐커     +                             
     D1         \_/ /e       ===10u         __              
  旼>쳐        쳐캑Q6 100k                 /_\'          === +
             /   \       旼좔                          BATT
>컵2k2컵컴쩡컴큂4         __ __ __ __                   PP3
        100k \e         \_/ /_\ \_/ /_\            쳐컴    
  읕<쳐  읕컴컴)컴컴)컴+   쿏9 쿏8 쿏7 쿏6          VR3 680   
     D2        220    ===                        50k      
                      10u                              
<컴컴컴)컴컴컴쩡좔컴컨쩡좔컴)컴컨컴컨컴)컴좔컴컨컴컴컴)컴컨컴좔켸
          10u===     ===10u                            
      D3쳐<쳐컨컴VR5컴좔컴컴컨컴캑>쳐컴컴)컴컴컴컴컫컴컴캑
                  5k             D10      VR4 33u===  Meter
      D4읕>쳐컴컴컴컴컴컴컴컴컴컴컴컴컴컴좔2k컴컴컨컴컴켸

To get a usable 40dB scale on a meter quite a lot of compression is needed.
This circuit uses an AGC approach. Transistors Q4, 5 & 6 from a power amp to
drive the double voltage doubler bridge D6-10 rectified load of the meter &
also provide AGC with D3 & D4 making an variable input attenuator.

VR5 set the AGC log action & diodes D1 & D2 correct for over compression to get
the very linear 40dB scale. VR4 is the main calibrate & VR3 is the battery cal.


See my bul on "Sound Levels dBA dBB dBC".


Why don't U send an interesting bul?

73 de John G8MNY @ GB7CIP



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