A Simple V.O.X. Circuit
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.
The BC109 Mic high gain amp is RF protected by the Ferrite Bead on the base lead. Its' O/P is voltage doubled and the DC stored in the double time constant to give fast response and slow decay time. The rectified DC is referenced from the +ve rail, is put across the 220K VOX gain control for a time determined by the 220K decay variable control that discharges the caps.
The gain controlled sample of the DC switches on the PNP which turns on the NPN PTT drive transistor. The 10Mê from the PNP base to NPN collector, gives positive feedback hysteresis if needed (SCHMITT) for snap action.
The LS audio is also voltage doubled to give DC that feeds the ANTI VOX control this DC 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.
73 De John, G8MNY @ GB7CIP
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