A Simple V.O.X. Circuit


Here is a 3 transistors 6 diodes circuit that is a
Voice Operated Tx PTT control as well as an ANTI VOX circuit to stop mis-operation from Rx noise.

Image 1


The BC109 Mic high gain amp is RF protected by the Ferrite Bead on the base lead. Its' low level O/P is voltage doubled with Germanium diodes D2 & 3 into double time constants Caps to give fast response & slow decay time. The rectified DC is referenced from the +ve rail less the voltage drop of Silicon diode D1 together with the germanium detector diodes, this makes the Mic Audio detector very sensitive. The voltage is put across the 220k VOX gain pot, & the hang time determined by the 220k decay variable control that discharges the capacitors.

The gain controlled sample of the DC switches on the PNP which turns on the NPN PTT drive transistor Thats is protected from rig relay back EMF by D6. The 10MΩ from the PNP base to NPN collector, gives positive feedback hysteresis if needed (SCHMITT) for snap on-off action.

High level LS audio is also voltage doubled with Silicon diodes D4 & 5 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.