This circuit for a simple Digi-Vox unit will allow easy connection between any CB or HF transceiver that is not VOX capable. Below shows how the digital signal from a PC or Laptop can trigger the PTT on the radio, and then hold it on until the digi transmission has completed, which-then-after it has completed will drop the PTT.

The digital signal, “which is technically just an audio frequency”  will first pass through the first stage amplifier and then onto a double diode detector stage to be rectified into a steady DC voltage. The use of a DC blocking capacitor between the detector stage and 1st stage amplifier is there to prevent false trigger, or permanently supplying voltage to the base of the TIP121.

The steady all-be-it small voltage applied to the base of the TIP121 Darlington transistor is just enough to conduct, thus switching on the transistor and passing the supply voltage to the relay, which in-turn will switch on the relay and close the PTT switch to your transceiver.

It’s important to use a “double pole” type relay here as some transceivers / CB’s require the RX connection to be closed when in receiving mode, otherwise you may not hear any digital signal being returned for you to decode.

At the same time the digital signal is also being passed over to the mic input connection on your radio to be sent. The use of a small variable potentiometer has been employed to set the correct level that’s needed. If you have a mic level control on your radio, you may wish to just use this instead. However switching from voice to digital may be a nuisance when adjusting for both types of transmission. 

Finally the Digi-Input must be DC blocked from the rest of the circuit and transceiver, this is very important and is easily achieved via the 2x 1uF capacitors that sit on both AF connections to your computer. If the 1uf from the AF+ was omitted, there would be a DC offset being sent back to your computer. This is also the same for the AF- which would more than likely cause an unwanted ground-loop.

The circuit is very simple, free from snags and odd behavior, and shouldn't be a problem for someone to build that is competent in doing so.

73’s DE Bill

Why are you using expensive varicap diodes for your diode pump?  Or are you just using a diode symbol taken from elsewhere?  You can easily use any cheap 1N4148-type diode.

You need to add a diode across the relay coil terminals, with its anode to ground - this will prevent the destruction of the relay-driving transistor by the back-emf created by the collapsing magnetic field when the relay is switched off!