I was using Fldigi for some time but hated the long audio cables. Those 3.5mm plugs are pretty noisy when you rustle the cables and besides I didn't want to use my internal soundcard. I looked at the commercially available interfaces but none fit my needs. I weeded through a bunch of different DIY options but I wanted a box with USB and Rig connectors and nothing else. No power plug either.
This is about as simple as it gets with an Opto Coupled PTT and transformer isolated rig interface. all for less than $20
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Step 1: Choosing an Audio Dongle
I got a few different USB sound cards from ebay for about $3 each. Be careful which ones you get. The ones with the extra caps also have a resistor divider on the audio signals. They only put out 100mV and wont turn on the Opto Coupler. You can see the difference in the picture. Simpler really is better. The one that looks bare outputs almost the full 5V on left and right channels.
You could just leave the usb connector intact and poke it through the rear apron. I had extra B connectors around so I unsoldered it. They didn't even bother to solder the ground tabs at the factory so if you are keeping the A connector just solder those tabs down.
After removing the usb connector I soldered pigtails to all the USB connections and the tops of the 3.5mm Jacks. Dont bother with the center of the microphone connector its not used here..
Step 3: Packaging
I wanted a nice metal box for good shielding so I checked Craigslist and sure enough I found a guy giving away free parallel port data switches. He was only 2 miles away and had 3 of them.
Just had to drill a few more holes in the front. I used a peel and stick floor tile to make a new rear apron. I drew a quick front panel in Inkscape and printed it on a self stick label sheet. a few coats of clear spray will protect it. I only had satin so its not as nice as a glossy finish would be.
I've also used polyurethane on other front panel layouts. It makes the label a bit translucent so if you're going to try it lay down an even coat of paint before you apply the sticker. It does look like its covered in plexi when its dried.
I also put a piece of that floor tile behind the front panel to space out the pots and switches for a nicer look without using a bunch of washers. Besides the floor tile is a bit soft and helps lock the parts in place.
Step 4: Final Assembly
I assembled the parts on a small protoboard. The schematic is simple enough. I used a socket for the opto coupler just in case I fry it with a tube rig, the schematic shows a 4N35 but I used an IL-5 since I had plenty of those.
for the rig connector use a 6 pin din or 3 seperate connectors as you please. even a db-9, RJ45 or similar will do.
the transformers are 1:1 telco transformers. almost anything from 100 to 1000 ohms will work.