I have been looking for suitable, cheap audio isolation transformers and while reading about them and looking them up, I came across a few digimode radio interfaces; some isolated, others not so much… All in all though, they do provide access to an interesting operating mode with ‘commodity’ transceivers (ie. ones that don’t natively support digital modes of operation).

The one that really caught my eye was the “USBlink” presented by G4ILO which combines the use of a cheap USB sound card, and some transistors working as an amplifier and a switch in order to provide VOX circuitry for PTT (push-to-talk) operation without the requirement for a serial or USB switching interface.

The real benefit of this circuit may not be immediately apparent for someone who does not have an interest in radio, or even for someone who is just beginning in radio as a hobby; as was the case with me. This circuit would allow a would be operator to run a “radio gateway” for use with the Free Radio Network, EchoLink or any of the various other radio-internet-radio systems without the need for too much work or additional cost. I feel cost is also something worth mentioning here as the total cost of the project in my case is, well… free… all of the parts required to complete this circuit I had available in my parts bin. If I did have to purchase them, even in small quantity volumes, the cost would be significantly less than that of a commercial one, a ballpark figure I’ve just made up off the top of my head is around €3 in components, another €3 in connectors, around €5 for the enclosure, and the USB cards vary in price depending on where you get them, but can generally be had for less than €5 delivered.

All in all, I have put all of my parts together into what I like to call a “project bag”, which is just a bag of parts that I’ve labeled and ear marked for a specific project and will proceed to make it when I can come up with a stripboard layout I find to be both efficient and appealing in both size and operation.

I will post and update when I have the unit constructed and in some sort of working form. As I am not currently licensed, I will not be able to gauge how appropriate it is for digital transmissions, however I will be able to review how it works as a FRN gateway interface.