Soundcard interface (including PTT) for Baofeng UV5R and other cheap handhelds
I was interested in using a couple of Baofeng UV5R radios for VHF/UHF digital amateur radio communication. This includes packet radio using the direwolf TNC as well as newer and more efficient protocols such as ARDOP.
I had two problems when trying to set up the radio/soundcard interface. My laptop soundcard doesn't work very well, so I wanted to use a cheap USB soundcard. The USB soundcard I have only provides a microphone input and not a line-level input. As a result, the received signal level going to the soundcard had to be very low (of the order of a few 10's of mV). The Baofeng radios are not very good at providing this low signal level because the only audio output on the radio is intended to drive a speaker with audio levels of a volt or two. Adjusting the speaker audio level to match the mic input via the Baofeng audio control was very tricky due to the sensitivity of the soundcard. I needed to be able to adjust the audio output of the radio with a reasonable margin for error.
My second problem was how to reliably do transmit/receive switching. The ARDOP protocol requires the radio to switch between transmit and receive with latencies of the order of 100 mS. The Baofeng's VOX mode is notoriously slow to drop out, hanging on for a few seconds in most cases. This is far too long for ARDOP exchanges, and so I needed a reliable way to switch the radio's PTT circuit.
Looking around, I found several possible solutions, but none was a complete soundcard interface for the Baofeng UV5R. The best of these were from VK7NTX and ZS2EZ. The VK7NTX link gives a nice description of how to do PTT with a Baofeng radio. Both the VK7NTX and ZS2EZ links describe audio interfacing to a soundcard line input port, but don't have details on interfacing to the soundcard mic input.
Using ideas from the VK7NTX and ZS2EZ pages, together with a bit of tinkering, I was able to develop a reliable soundcard interface for the Baofeng UV5R that had fast PTT switching and greatly reduced sensitivity to the radio's audio knob position for reliable reception of data. My soundcard interface is shown below (click the thumbnail for a larger version).
The transmit audio circuit uses a 1:1 audio isolation transformer to isolate the Baofeng 3.5 mm sleeve from ground to enable the PTT circuit to work. Resistors R1 and R2 are used to attenuate the transmit audio level so that the deviation of the transmitted signal is ~ 3 kHz for a 1200 baud packet signal at maximum speaker gain on the soundcard. Careful initial tuning of the deviation by setting R2 is required in order for your signals to be reliably copied by others.
The PTT circuit uses resistor R3 to limit current through the PC817 optocoupler LED. When the serial port RTS line goes high, the LED turns on and pulls the collector of the optocoupler low, shorting the sleeve of the 3.5 mm UV5R connector to ground and asserting PTT on the radio.
The receive audio circuit includes capacitor C1 to block any DC voltage from the soundcard (usually used for powering electret microphones). Resistor R4 is chosen to attenuate the audio signal from the Baofeng speaker (2.5 mm connector) to a level compatible with the soundcard microphone input. Assuming a typical soundcard microphone input impedance of 15 kOhms, a value of 82 kOhms attenuates the voltage by a factor of a approximately 6.5 (or 16.25 dB). Using this value of R4, together with a soundcard mic gain setting of between 7 and 10 in alsamixer, I obtained reliable reception with the Baofeng audio knob set anywhere between the 6 and 9 oclock positions. I also added a header (J1-J2) which can be shorted to bypass R4 if I want to switch to a soundcard with a line input instead of a mic input.
The soundcard interface works well with the 1200 baud packet radio and ARDOP protocols. For the TNC, I use direwolf by WB2OSZ for packet and ardopc by G8BPQ for ARDOP. I would also note that this interface could be used with a hardware packet TNC.