I realize this is an ancient thread, but I am attempting to build this exact circuit. I am going to control a pioneer car stereo head unit via its wired interface (a 3.5mm stereo jack). I have confirmed that putting a 16k resistor from tip to sleeve does volume down and 24k resistor does volume up. I am a software guy by day and thought about doing this with arduino, but also don't want foul ups to crank my volume way up so ended up finding this thread. I built the circuit as @AnalogKid shows above except I didn't have 2N7002's laying around so I used 2N7000s instead. I also think from my tinkering w/the arduino version that my pulses need to be at least 50ms or so for the unit to pick up. I tried many different C4 values and just can't seem to get this to work. I also have fairly limited understanding as to what the upper left part of the schematic is doing. Has anyone built this or something similar before? Hopefully this image of my terribly messy breadboarding will help.
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