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General Category => Technical Corner => Topic started by: 26AM081 on August 26, 2021, 07:14:22 pm

Title: Simple Digi-Vox Circuit
Post by: 26AM081 on August 26, 2021, 07:14:22 pm
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

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Title: Re: Simple Digi-Vox Circuit
Post by: 26AM081 on September 08, 2021, 05:09:46 pm
Finally finished the Digi-Vox unit which is fully compatible with all CB radios. The whole unit is constructed in true "Home-brew" fashion, which is obvious to see from the pictures below.

The only deviations from the original schematic above, is firstly the use of a rotary switch to select when operating in either Digital or Voice modes, this will make life much easier with not having to plug and un-plug the microphone and speaker every time you wish to switch.

All connections can be permanently attached, with the microphone being plugged into the front of the Digi-Vox, and external speaker being plugged in via the rear 3.5mm jack socket. The unit doesn't even need to be power if operating in Voice only.

Secondly as I used a "Double Pole / Double throw" relay, I was able to add Send / Receive LEDs on the front panel for TX/RX operation.
 
Front Panel. Showing all the controls for operating. Easily toggle between Voice or Digital via the rotary selector switch.

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Rear Panel. Showing all input and output connections to your PC / Laptop sound card via the 3.5mm jack sockets. Also the main 6 pin aviation control socket to be connected to your CB radio.  A custom patch lead is required for the control socket as it carries all connections for the MIC, PTT, TX, RX and SPK. ( example to be posted soon ) Finally the last 3.5mm jack socket is used for an external speaker when operating in Voice Mode.
 
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Inside View. A typical copper clad, hand etched track work using a Dremel. The final result is not the neatest, but certainly the quickest. Plus without the use of etching with chemicals etc.

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