Having upgraded my Acer Aspire One netbook to an Acer V5 laptop which has a faster processor and larger screen, I was surprised to see it had a single connector for Audio Input/output and was therefore incompatible with all the Interface cables I had made for the K3/IC706/FT817. As a quick fix I plugged in my USB soundcard into the V5 which did have separate audio in and out while I made some cables
I had a suspicion the TRRS connector on the Acer was wired like the one on the Nexus 7 as I discovered when building the PSK Interface . I searched the Acer manual but that was unhelpful. I called Acer helpline but even their Tier 2 technical support had not a clue about the question, so I assumed it was the same
I found a Mono 3.5mm lead for the K3 Line In connection, a stereo 3.5mm lead for the Line Out and a 4 pole TRRS plug. I wired up the connector to feed audio into the K3 and that worked fine. Feeding audio out of the K3 into the computer was a failure. It was unsuccessful and it was impossible to disable the Laptops Internal Microphone. Measuring the open circuit voltage on the Mic Pin showed 2.8V, obviously intended for an Electret Microphone. When the K3 was attached this voltage dropped to 0.8V, the Line input obviously had a DC path. Searching the internet I found a few articles on using a resistor of a few hundred ohms on the mic input to control Play, forward and stop of a media player on a Nexus 7. I wondered if a load resistor must be used to emulate the current drawn by an Electret Microphone to disable the Internal Microphone. Electrets are typically 1600 ohm, so I put a resistor across the mic input and used a 10uF capacitor to stop the K3 DC loading the port. The final circuit is as follows:-
This worked fine, but now it needed Tidying up. By using a 1600 ohm 0805 size resistor and an 0805 size 10uF ceramic capacitor the components could be fitted inside the plug shells.
NOTE: No PTT signal was needed via this interface as the K3 was put onto Transmit by CAT signals