KI4JJB Sound Card Interface

This is NOT intended as a 'How To'.

I decided to build my own sound card interface. I enjoy these type projects. There are no detailed instructions to be found here, just ideas I hope you can use to build your own project.

Parts I Used

  1 4N35 Optoisolator
  1 IC Socket
  2 POT 10K Ohm
  2 Transformer 600:600
  7 Terminal Block
  1 Diode 100v 4.0ns
  3 Resistors
  1 PC Board
  1 LED Red
  1 13 Pin Din Plug
  1 Audio Cable


Total cost was about $15 - $20. Most of my parts came from DigiKey, but a few came from Radio Shack. About half of the cost was the audio transformers. They were about $4 a piece.


This interface completely isolates the computer from the radio using 2 telecom transformers and a optoisolator. This helps prevent interference between the two.

I decided to allow for level adjustment on both the input and output lines.

A LED indicates when the board is in transmit mode. It is connected via a terminal block. This allows me to swap to a panel mount LED when mounted in a project box without major modification.

My wire to board connectors were selected to allow for multiple connects and disconnects during testing.

I originally planned to connect the ground of each audio circuit to the sweeper of the POT, however, I reversed this because of noise it introduced to the circuit. This may just be an issue specific to my hardware configuration.

There is a diode in the PTT circuit to protect the optoisolator.

I used a label maker with clear labels to finish out my board.

Now there is just one more thing for me to do...

Make my first contact.

73, KI4JJB

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