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.
HOW IT WORKS
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
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.