The sudden explosion of new digital modes has taken the Amateur Radio world by storm. New modes such as PSK31, PSK63, MFSK, Throb, MT63, Olivia and WSJT (JT65, FSK441, ISCAT etc) as well as established modes such as RTTY, SSTV, Hellschreiber and others have rocketed in popularity.
In order to use these modes all that is required is a simple computer-to-radio interface which couples the computer soundcard to the radio. Several options are available both commercially and as homebrew projects. Below is outlined the circuit which I have used to great effect. This circuit was obtained from the web pages of Ernie Mills WM2U, and all components are available in South Africa.
More on Ernie's Interfacing (including Boatanchor interfacing) can be found here


Whilst some people have achieved success by connecting their audio lines directly, I chose to isolate my sound card from the radio by means of small audio transformers, using the following circuit:

The 100:1 divider is very important because the output from the Sound Card can be in the order of 1volt into the microphone input which is nominally 10mvolt. It follows therefore that without the attenuator, microphone input overload can result, causing the normal narrow bandwidth to increase dramatically producing unwanted splatter.

Experience has shown that the value of the transformers is not critical: ideally if you are taking the audio feed from the external speaker jack or are using a slice from your speaker you will be looking at a low impedance source, so in that case T1 will be a 1K Ohm to 8 Ohm isolation audio transformer; If you are taking audio from the Acc socket on the back of your rig then it will be a high impedance source, in which case you will use a 600 ohm to 600 ohm, 1:1 turns ratio audio transformer. T2 will always be a 600 ohm to 600 ohm, 1:1 turns ratio audio transformer. Finding 1K to 8 Ohm isolation transformers is however near-on impossible in South Africa, so tests were done using 600 ohm to 600 transformers in their place, and results were quite satisfactory.

The 600 ohm to 600 ohm transformers are readily available in Johannesburg from AP Electronics in Commissioner Street, Jeppestown. They can also be purchased online from Yebo Electronics in Cape Town. Cost is around R15.


Now, a very simple optocoupler circuit can be added to automatically switch the PTT circuit on the Rig. This uses the RTS line in the active Comm. Port. The voltage swing on this is -12v/-5v through +12v/+5v so this line is always 'hot'. To isolate the RTS line from other devices being used on the same Comm. Port, you might need to incorporate a switch.
Important: Please check that the PTT line, and/or the Keying line on your rig has a positive voltage on it and it requires a pull-down to ground to activate. These circuits will not work otherwise.

The Opto Isolator is a 4N25, 4N29, ECG 3084, PS2601 or equivalent. I use the 4N25.

Transistor switching is also an option, using a simple 2N2222, BUT this does not offer complete isolation and you run the risk of ground loops. The circuit for transistor switching is as sollows:


Should yo wish to use CW keying with your interface, then an additional switching circuit needs to be included:


This will allow programs such as CWType or WinWarbler to hard-key the radio via the key jack, just as if it was a normal straight key!

Please bear in mind that if you are using audio for CW generation, such as can be done by MixW and other programs, transmission is via SSB and does not require this circuit.


For radios with a built-in FSK/RTTY mode, an additional switching circuit can be employed to control the FSK switching line. This circuit will be identical to the above circuits, with the exception that it will be connected to the TxD line and Ground (Pin 3 and Pin 5 respectively on a DB9 Connector).


Problems have been experienced trying to get the optocoupler interface to switch the Icom IC706MkIIG specifically. I have had success in this regard by replacing the 4N25 with a 4N35. Another reported fix is the addition of an NPN transistor like a 2N2222 on the output of the 4N25 opto as a Darlington. Connections are: opto pin 4 (emitter) to 2N2222 transistor base; opto pin 5 (collector) to 2N2222 transistor collector and to PTT+; 2N2222 transistor emitter to PTT Neg / Rig Ground. This may provide a solution for other transceivers with PTT problems as well

Thats all there is to it!! Good Luck and hope to see you on my screen soon!!!

73 de ZS2EZ