Re: A simple VOX control circuit for the BitX-40 to allow it to be used for FT-8

R. Kuehn

Hi John,

Interesting simple design. You could probably get away without the isolator or extra SSR, just connect the base of a transistor to the op-amp output and you'd be good to go.

I went with a slightly more complex design after G4ILO,, that rectifies the audio signal to turn on a transistor switch (schematic attached), however I could not get the base voltage on the transistor high enough to 'turn on the switch' so to speak. So after much fiddling and noodling I cheated and put in a comparator to drive the base of the transistor. 

There are many designs out there like a dual stage op-amp, from N1HFX,, and another discrete transistor type from G8MNY, If I would do it again I would go with the simpler version of the G8MNY as the final NPN will always be driven fully on when the vox is activated.

At some point I want to modify the bitx40 code to enable CAT but it is far down on my workbench list, and maybe someone will beat me to it ;^)


On Tue, Mar 20, 2018 at 7:32 AM, John P <j.m.price@...> wrote:
As promised, I uploaded the description of the simple VOX interface between my PC and the BitX-40 that I've been using for FT-8. I suppose it will also work for other digital modes, and should really be able to work with almost any radio provided the PTT voltage & current don't exceed the specifications of the SSR I used.

Those of you who know me know that I'm a real hacker when it comes to hardware, so all suggestions for improvement are always welcome (email: my call at Or post here so all can kibitz!

John - WA2FZW

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