I recently posted an instructable for an FLDigi compatible interface. It was largely based on a Hamcomm interface I designed as a hand out for a Ham / PC Usergroup talk I gave in 2005.
The main difference between HamComm and FLDigi is that FLDigi can use a tone on the right channel to key the rig eliminating the need for a serial port to drive a relay that Hamcomm used.
I wanted to try echolink and I found that this interface worked perfectly except Echolink likes the serial PTT option.
This instructable is a combination of all options in one box and its built from over 90% recycled parts so its Eco friendly, hence the name.
I wanted to hide the USB serial and USB sound dongles in the box so I looked through my junk box for a USB Hub. I found a 4 port hub so I decided to add a USB memory key.
The key contains all install images for echolink, FLDigi and supporting programs as well as some rig control software and the drivers for the Trendnet Serial adapter. I included versions for Mac PC and LInux. I wont post the contents for the memory stick because you have to register for echolink yourself.
The serial control lines are used to drive the relay but the TX and RX lines are brought out to a DB9 for other uses like controlling a newer radio.
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.
Step 2: Back Panel USB Connector
Many times I want to hide a USB dongle in a project box. I found these A to B adapters that seem perfect, except for the price. I can still make a half dozen on these on protoboard for the same price as one of the commercial ones. If you want a shortcut try one.
Step 3: Build the Serial PTT Relay Control Board
I used a standard PC mount DB9F connector but drilled out the threaded inserts so I could screw the serial adapter to it from behind. The DB9's I use have the rear terminals floating free which allows you to easily form them to fit a standard .1 spacing protoboard.
The wiring is pretty straight forward just be sure to add the kickback diode to the relay in the correct polarity. I once had a problem and I thought the the protection diode was backwards but it turned out the relay had an internal diode. Be careful to test any scrounged parts.
I use a variable power supply so find the actual pull in and drop out points for the relay. Some 5V relays don't pull in till 4.5V. That's not good enough if you use a long USB cable. Luckily I had a bunch that pulled in at 3.5V
Step 4: Assembling the Audio Board
This was pretty easy but if you want more detail see my instructable for the SWL Interface. Its the same board. You don't have to build this on separate boards if you don't want to but I had an extra SWL interface laying around so I used it here.
If you use a large enough board you can wire tie the USB Hub and Memory Stick to the board..I use these smaller boards because they're only $10 a hundred on eBay and I have a lot of them.
Step 5: Final Assembly
Once you have all your boards and parts together pick a case that they will easily fit into. I used an old leased line modem case I had lying around. I used an 1/8 inch jack for the Opto PTT and a 1/4 inch jack for the relay PTT.
The USB hub and memory key are attached with hot glue. I wanted to use some plastic wire clips on them but they're out in the garage and we just got almost 2 feet of snow here so that will have to wait for the thaw. you also could screw a small strip of metal to the floor of the case and use some wire ties.
There is still one USB port left on the hub. I haven't thought of a use for it yet. I have to go try this with Harv's Hamshack Linux Bootable CD. It should work with most modes. That was one of the handout CD's from 2005. Even then it was a bit dated but so is RTTY so it still works fine even on the lowest of PC's. Check it out if you're bored its still out there for download....