Oddball design question:
I have a pin that's used for audio input (allowing up to 12Vpp AC), and for testing/utility purposes I'd also like to be able to use this pin for a reliable UART TX output as well (0-3.3V). (Not at the same time of course, but without a hardware change).
They key is that this circuit that injects the UART signal can't alter the audio signal when it's inactive. So it has to be something high impedance when "off" and seeing anything from -6 to 6V. This rules out simple transistor circuits or a MUX as far as I can tell.
[ The internal circuitry runs at 0, 3.3V, 5V, and 24V, no bipolar ]
A relay should work, of course, but I'd hope there was a smaller/cheaper/more clever way to do this.
[ The UART input/RX was easy enough, just using a comparator to sense voltage. I can't figure out a good way to do this with the output though. ]