Inexpensive, Easy to Build Sound Card Interface. IT WORKS!

Mouser Opto-Isolator                   78-4N25             $0.27     X1     $0.27 

Mouser 6 Pin IC socket                 571-1825093-1       $0.53     X1     $0.53

Mouser Bud Project Box (3.1x2.1x1.1")  563-CU-1941         $2.10     X1     $2.10

Mouser Shipping                        Shipping            $4.99     X1     $4.99

Parts-Express perf board (2.3"x1.9")   320-450             $0.80     X1     $0.87

                                                                     TOTAL $15.96 

Who designed it:

Louis Burke, W7JI, designed this circuitry for a sound card interface. It is simple and can be built in one evening. 

Unless you are hell bent on spending $75-$100 on a sound card interface , this may be a low cost way for you to get into the digital communications world! 

The plus is parts are inexpensive and best of all ... IT WORKS!

I used a 4N25 in place of the 4N33 as that was what I had available in my spare parts box. 

The 4N33 is a Darlington switch and the 4N25 is just a simple switch.

Where it is used:

Several of the W7JI interfaces are in use in the BCVHFA Communications Center. We also have a West Mountain Sound Card Interface and comparisons have been made with no noticeable difference in performance between this circuit and the commercial sound card interface. 

The W7JI interface performed adequately.

This circuit works for PSK-31/64, RTTY, ALE and all other sound card modes. 

Features and use:

The trimmer potentiometers allow setting the sound card audio level without having to reset the microphone gain or audio gain on either the rig or computer. 

This is a great feature to have built into the interface as NONE of your computer sound card settings have to be changed and none of the gain settings on your rig have to be change.   

CAUTION: The computer audio output is fed into the microphone input on the rig and in sound card mode audio is generated from the computer, then the operating system dings, dongs, and alert sounds may be transmitted if the sound card interface is in transmit mode. 

NOTE:  If you are going to use a USB inverter that only has 5 volts available, the 1K resistor needs to be changed to a 82 ohm resistor for the opto-isolator to trigger PTT function. 

However if you want to be able to also use a standard RS-232 12 volt port, then the 1K IS required so the opto-isolator does not get damaged. 

You can use a 3 pole jumper with a manual push on shorting bar and use the center pin as common with the 1K left of center and the 82 ohm right of center like what is used on a mother board of a computer to switch between the two voltages. 

Or add a mini SPDT switch to accomplish the same thing.