VoxPTT

Vox PTT - To control amateur radio without CAT control, mainly for older and classic transceivers

Vox PTT v1.3

Taken from and credited to https://freecircuitdiagram.com/2266-basic-vox-circuit-controls-ptt/

This uses a dual op-amp, first stage is an amplifier, second a comparator for the level to trigger the PTT transistor

Layout built in DIY Layout Creator https://diy-fever.com/software/diylc

Schematic created and tested in SimulIDE https://simulide.com/p/

Explanation of circuit
So, the input from the USB Audio Speaker terminal goes to R1 to limit the audio volume sent to the Transceiver's Mic socket

It is then decoupled as the inverting input on IC1-1 pin 2 is at half the supply voltage matching the non-inverting input pin 3
IC1-1 amplifies the signal by 100 gain.

IC1-2 is a voltage comparator, R7 has a 330k resistor at either end, R6, R8. The voltage on the slider is adjustable from ~5V to ~7V. If the incoming signal pin 5 is above this threshold pin 6 it will send the output on pin 7 high.

When Pin 7 goes high this biases the transistor Q1 on, when it goes low, the turn-off delay time is controlled by the setting of R9 and C2, from 0-3 seconds approximately.

LM358/TL072 pinout

Parts List

Most of these components I had lying around, I have linked what I ordered from Ebay.

You will need to vary connections for your radio, please refer to appropriate documentation to avoid possible damage..

Veroboard 5. X 2.1 inches 0.1 inch pitch cut from any larger sheet

C1 0.1uF Ceramic

C2 220uF Electrolytic

D1 1N314 fast signal diode

IC1 LM358/TL072 Op Amp low PSU voltage

LED1 Green power, may need R12 altered to suit if changed

LED2 Red PTT, may need R12 altered to suit if changed

All fixed resistors 1/8 Watt 1 %

R1 100KOhm variable resistor logarithmic

R2 47KOhm

R3 1KOhm

R4 47KOhm

R5 100KOhm

R6 300KOhm

R7 100KOhm variable resistor logarithmic

R8 300KOhm

R9 100KOhm variable resistor logarithmic

R10 20KOhm (not critical)

R11 1KOhm

R12 40Ohm

Q1 BC547 or equivalent, if relay PTT circuit then fit 1N314 with Anode to PTT and Cathode to 0V

Miscellaneous parts

7 way pcb header (from unused Raspberry PI/Pico)

8 pin Din plug https://www.ebay.co.uk/itm/197758921688

8 pin socket https://www.ebay.co.uk/itm/197758921688

if you wish to use with different radios

4 x RCA Phono Male Inline Cable Plugs Solder Connectors HiFi Audio Red Black

https://www.ebay.co.uk/itm/233752069991

ABS project box

10m Figure 8 Screened Audio Cable (more than needed) https://www.ebay.co.uk/itm/333159705457

1PCS 100 x 68 x 50 mm Waterproof Junction Box ABS Plastic Electric Project Black Enclosure Housing https://www.ebay.co.uk/itm/156287982326?var=457315171393

8 Pin Din plug and socket wiring from rear view of plug (suggestion only) your needs may vary

Some pictures in various stages of assembly

Testing using my laptop and WSJT-X to inject the audio simulating TX

The underside of the Veroboard, be sure to cut certain tracks to avoid shorts

Conservatory electronics workshop, who knew?

Testing a loose hookup on the bench, Oh, so many times

Almost finished assembly, it was in and out many times

Rear view complete with surplus holes from previous attempts

I hope that you are able to have a go at building this and let me know in the comments

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