OBSERVATIONS FROM SCOTLAND                            09_June 2008                                GM1SXX

A general purpose Isolated data interface for data-modes.

For those who like to play with SSTV, Packet, RTTY, PSK31, CW  and other common data modes, here's a simple to build, reliable interface to connect your radio to your PC, yet maintain total electrical isolation between the two. It's nothing more than a couple of small signal transformers to provide isolation in the signal path, plus an opto-coupled PTT switch.

Almost any small transformers will do in this circuit. I used some small signal transformers but matching transformers scavenged from an old transistor radio will also work.   The Opto-coupler is similarly unfussy, just so long as you get the connections correct :-)  I used a superbright Blue LED as the transmit indicator. It's very handy to be able to see when the rig is in transmit.  Nothing about this circuit is critical and I used ordinary stranded twinflex to make all the leads.

This interface works beautifully with MIXW and should work well with any software that can toggle the RTS line on the serial port of your PC to control the rig's PTT line. If you find that it is not switching the PTT line.... (the LED should stay on while the PTT is activated), try swapping the pair of wires going to pins 4&5 of the opto-coupler.

 I used a small potting box (from Maplin) of 4*1*1 inch in size, and I just epoxied the two transformers to the bottom of the box with pins facing upwards. The PTT circuit so simple that I just soldered the LED and resistor directly to the pins of the coupler along with the leads going to the serial port connector. In fact I used the lead scavenged from an old computer mouse to make the PTT cable to save the bother of actually putting one together.

If you use several different radios, a better approach would be to use a DIN socket on the interface and make custom cables for each of your  radios. This way, you can share the interface between different radios.

73 Al.

GM1SXX