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G8MNY > TECH 14.07.03 19:52l 38 Lines 1958 Bytes #999 (0) @ WW
BID : 5638_GB7CIP
Read: DL9NDQ DB0FHN GUEST OE7FMI
Subj: A Simple V.O.X. Circuit
Path: DB0FHN<DB0RGB<OK0PPL<RZ6HXA<HA3PG<JE7YGF<7M3TJZ<ON0AR<VK3TE<GB7YFS<
GB7CIP
Sent: 030714/1729Z @:GB7CIP.#32.GBR.EU #:5638 [Caterham] $:5638_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY
Here is a 3 transistors 4 diodes circuit that is a Voice Operated Tx PTT
control as well as an ANTI VOX circuit to stop mis-operation from Rx noise.
MIC AMP VOX ANTI VOX
---------------------------------------------------------+VE
! ! +! ! ! PNP e ! !+ !
10K --- C 1u ! ! !/!-->10K C 10u ---
! /_\ ! C u1 100K<-10K---! POT ! /_\
! ! 10K ! POT ! !\ ! ! ! +10u
!----! ! ! ! ! 10M ! !-------!>!-----C--LS
10M !--C---!>!----------- ! !
! ! u1 !---(-----------------------PTT
! !/ 10K !/ !
---C--FB-! BC109 ! --! NPN ---back
u1 !\!e 10K !\!e /_\ emf
MIC ! ! ! !
--------------------------------------------------------------------GND
PTT DRIVER
The BC109 Mic amp is RF protected by the Ferrite Bead on it base lead. Its' O/P
is voltage doubled and the DC stored in the double time constant to give fast
response & slow decay. The rectified DC is referenced from the +ve rail is put
across the 100K VOX gain control.
A sample of this DC switches on the PNP which turns on the NPN PTT drive
transistor. The 10Mê on the PNP base to NPN collector, gives positive feedback
hysteresis if needed (SCHMITT).
The LS audio is also voltage doubled to give DC that feeds the ANTI VOX control
this is used on the emitter of the PNP to stop the PTT operating from loud RX
LS audio that is heard by the mic.
All diodes are 1N4148 or Germanium point contact type.
/QSL
73 De John, G8MNY @ GB7CIP
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