PSK31 Interface
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This is my PSK31 interface I built from plans found on the Rascal PSK interface website (now defunct).  It is a fairly simple way to interface a computer soundcard to an HF rig to operate PSK31.  This interface also works for SSTV & RTTY, and I use this interface with my Kenwood TS-140. 

The interface requires no external power and is operated by the computer's serial port.  The circuit consists of an audio transformer, a capacitor, a PC-mount 10K potentiometer, an opto-coupler IC such as a 4N25 or 4N35, a 100 Ohm resistor, a 10uF capacitor, and an LED.  The optical coupler IC and audio transformers provide isolation between the HF rig and the soundcard. 

I scrounged all the parts except for the opto-isolator chip from my junk box.  The audio transformer (the black box in the photo) was removed from an old computer modem.  The interface was put into an Altoids box and fastened inside with double-sided foam tape.  An audio cable was hardwired to the PC board, as was an old mouse cable used as a serial cable.  A DB-9 connector was used on the other end of the serial cable.  I used an old microphone cord and soldered an 8-pin connector on the end to connect it to the radio.  I used a minature 3mm LED for a transmit light in the front side of the box.  The Altoids box was painted with textured black paint prior to installing the interface inside. 
In the above schematic, we can see that the interface is built in three parts.  The top portion shows the audio in from the radio to the computer's sound card. Isolation is provided by a 1:1 audio transformer.  If one cannot be found, it can be constructed using a toroid and some wire, but you will have to either use a large toroid or make many windings around a smaller one.  You'll notice in the photo above that this part of the interface is not included in the one I built.  This is because I already had the audio from the radio connected to the computer to decode various digital modes on HF.  It is connected separate from the interface.

The second part of the schematic shows the TX audio from the sound card's output to the radio's mic input.  A second 1:1 audio transformer is used.  The TX audio level is adjusted using the 10K pot.

The third part of the schematic shows the PTT function provided by an opto isolator chip which is triggered by a + voltage on the serial port's RTS line, or pin 7 in the DB9 connector.  The voltage is provided by the computer itself, which turns on the LED and opto isolator chip.   The LED inside the chip turns on the transistor and keys the PTT line.  Pins 3 and 6 have no connection.  The second LED can be omitted if you wish, but I find it handy to have a visual indicator of when the PTT line is keyed by the computer.

You should refer to your transciever's manual to determine which pins to make the proper connections to the mic plug.
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