My new homebrew "DigiComm" interface using a USB port and VOX PTT

[Lamewolf]

I've built a few interfaces for digital comms in the past but they have all been for computers with a serial port.  My new Dell laptop has no serial port and I wanted to simplify connections to just one cable from the laptop to the interface and one cable from the interface to the radio.  Since I usually only run qrp power levels, i didn't worry about using isolation tranformers in my design and have even built interfaces in the past without them and have never had any problems.  Audio is coupled through capacitors though so I guess it is somewhat isolated that way.  I looked at various vox circuits on the web and built a few of them to key the ptt with but just couldn't seem to get them to work.  The core of the interface is a very cheap USB soundcard for audio connections and a 2 transistor vox circuit to key the PTT line.  One transistor a 2N3904 acts as an amplifier to drive another 2N3904 a a switch to activated the PTT line.  In various circuits I found on the web, they all used either 1uF or 2.2uF coupling capacitors in the circuit but I could not get the circuits to work even after checking and double checking everything to find it all done right.  After a lot of head scratching, troubleshooting, and study, I decided to design my own circuit.  With other designs, the best I could do with the PTT was to make it chatter in and out of transmit and I found that it was the low value caps being used, so I included 10uF caps in place of the lower values and only have 2 transistors where a lot of them had 3.  With the new circuit, it keys the rig very quickly and returns a split second later after transmission to receive as it should with on air reports of clean sounding audio.  The USB soundcard is mounted inside the enclosure with the vox circuit and the USB port protrudes through a hole in the back of the enclosure for connection of a USB extension cable.  Also on the back is a 5 pin DIN plug for connections of cables made up for various radios, so to change rigs only requires changing cables.  So far I have a cable that will fit my Icom 703, 706Mk2, and my 718, one for my little Yaesu FT-817, and one for my old Yaesu FT-840.  So far I have tested the interface for short periods up to 100 watts without any problems but the circuit also has .01uF bypass caps on all output connects to bleed RF off of it which seems to work well.  Included is a schematic of the circuit in case someone would want to duplicate it.  The 5 volt DC power connection, ground, audio in and out to the soundcard are made by opening the case of the soundcard and soldering small wires direct to their proper place.  The soundcard is then closed back up and mounted using double sided tape to the enclosure.

UPDATE:  Been notified I omitted R1, 2 & 3 values !  R1 & R3 are 4.7K ohm and R2 is 100K ohm - sorry !

[cockpitbob]

Sweet!  I didn't know those little USB sound cards existed.  I need to get out more.

Thanks for posting your design!  This is the kid of stuff that makes for a good forum.

[Lamewolf]

Sweet!  I didn't know those little USB sound cards existed.  I need to get out more.

Thanks for posting your design!  This is the kid of stuff that makes for a good forum.

Funny thing about this little project, about the same time I got it finished I got a Tigertronics Signalink USB in a trade.  I only built it because I didn't want to buy an expensive interface but me and the other party in the trade are happy with our new toys.  Now I'll just keep the homebrew in the go bag with the laptop.

[Hamman147]

On the usb sound card did you use the TIP for both in and out put And this is just the thing I was looking for I have one up and run using RS 232... ;D

[Lamewolf]

On the usb sound card did you use the TIP for both in and out put And this is just the thing I was looking for I have one up and run using RS 232... ;D

Sorry I didn't include that in the details, but yes I did use the tip on both sides and they were real easy to solder to.  I tried that interface out on Winlink over the weekend and it did just as well on the Winlink 2000 HF email as my Signalink USB does !  Running 5 watts from my Icom 703, the interface and my laptop all on battery power I was able to connect and send email over radio.  The Signalink resides on my shack desk and the homebrew interface now goes in my go bag with my field gear !

[Lamewolf]

Gang,

I just updated the original post to include resistors 1, 2, & 3 values that I had omitted in the schematic.

[gil]

Thanks. I might make one for my RT-320..

Gil

[STN]

My eyes are not what they used to be :) Nevertheless this is an Awesome build!
Thanks for sharing

[chq]

Greetings....  I'm new to RP's and noticed this post yesterday.  I already have several interfaces that I can use but I'm curious about the circuit and how it differs from one called "The USBlink" from G4ILO.  I don't see anything when I click on the BMP file.  I've gone to the OP's website but he does not have this particular circuit in his list of DIY.  Do any of you have a copy of the schematic?  If not, no a big deal.... I was just curious. 
Best 72  -  chq