Author: Josh <email@example.com>
Date: Fri, 19 Jul 2019 11:01:47 -0400
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+This Arduino sketch was written to create an interface between my computer and
+radio for packet radio transmissions. Feel free to get in touch with any
+questions you may have.
+The interface functions primarily as a switch and a voltage amplifier. Direwolf
+already generates AX.25 packets in audio form, so this interface serves as a
+converter from the 3.5mm aux output from the computer to the RJ12/6 input of my
+Yaesu FT-2980R radio.
+If you want to build something similar to this for your radio, here are some
+* Connect pin 2 to the PTT line in your radio input cable. For some reason,
+ my radio's PTT engages when pin 2 is set to LOW, and disengages when pin 2
+ is set to HIGH. If this is not the case for your radio, invert the values of
+ the digital writes to pin 2 in the sketch.
+* Run the left channel (usually the white wire) from the stripped end of a
+ 3.5mm aux cable into the TX line of your radio. The other end of the cable
+ should be plugged into your soundcard's LINE OUT.
+* Also run the signal from the left channel into a voltage amplifier, and from
+ the voltage amplifier into the A0 input pin of the Arduino.
+* I built a voltage amplifier by running the +5V pin of the Arduino into a
+ simple resistor voltage divider. You can easily find schematics for this
+ online. The idea is that you want the signal into the A0 pin to be centered
+ around ~2.5V, instead of around 0V (which is how it comes from the computer's
+ audio output jack). The Arduino's analog input pins cannot detect negative
+ voltage, so it is necessary to offset the audio signal from the computer in
+ this way.
+* Make sure to consult the schematics in your radio's manual to determine which
+ wires in the radio input cable correspond to PTT, TX, and GND.
+That is a general summary of how this can be built. It may take some tweaking
+and experimentation to get it to work correctly. Basically, the sketch turns
+PTT on when it detects signals in the aux cable that deviate from strongly from
+the baseline voltage. This works fairly well for my setup, but your results may
+If you find that your radio is transmitting when it shouldn't be, you can tweak
+VOLTAGE_LOW_BOUND and VOLTAGE_HIGH_BOUND at the top of the sketch until you've
+found the appropriate range for your particular setup. It may be useful to use
+the Serial Monitor to observe the values of A0 when no signal is being
+transmitted, as well as during signal transmission, to determine the correct
+Note that this interface only handles data transmissions; my Yaesu FT-2980R has
+a 3.5mm audio output jack, so I'm able to just run a 3.5mm cable from the radio
+to my soundcard's LINE IN in order to receive data packets.
+I hope these ideas are of some help to your own projects.