This is unwanted advice, I know, but here is what I would do: I'd make it so that both the transmitter has a very high output impedance (so that plugging in an headphone causes its output to drop to near zero) and the receiver has very high input impedance (so that it doesn't bog the tx down).
It will cause some noise issues, because a very high impedance front end will pick up stray radio transmissions, but I think it's doable given a small enough bandwidth
just in case this wasn't clear, plugging headphones or other audio equipment into these 3.5mm jacks is not the intention of this design; rather, i want to minimize the chance of destroying expensive equipment if it happens by accident
noise is pretty irrelevant on the power rail. but if i put a high constant impedance on the power rail—say, a 100Ω resistor in front of 5 volts—none of the devices powered from it can draw much power, 25mA being the limit in that case
(for the data line, sure, resistance is not at all futile, resistance is a perfectly good solution, along with some clamping diodes)
what i have in mind is two things:
1. maybe use a trrs connector and use the second sleeve for the power rail? that way the only thing that ever gets plugged into it by accident is a microphone, which i think is unlikely to burn out at 5 volts (even if normally it's not subjected to more than 2.2 volts)
2. also, limit the current with a linear current source. this design simulates as having about 10Ω impedance over the 0–100mA range and a few kilohms once you exceed the current limit: http://tinyurl.com/23qvuylw
> 'super simple' excludes power negotiation schemes like usb-c
like, i think you're thinking about devices one or two orders of magnitude bigger than what i'm talking about. i want to be able to wire up a 35¢ device like an attiny4 https://www.digikey.com/en/products/detail/microchip-technol... with maybe three 1¢ resistors and two 1¢ diodes for protection http://tinyurl.com/26ngjdzh or maybe just soldered directly to the jack. it has 512 bytes of flash for the code and 32 bytes of ram; that would have to contain both whatever is necessary for the communication protocol and whatever application i want to run on it (in that case maybe it could handle a capacitive touch button or run a couple of 5-volt pwm channels)
i'm not sure the attiny4 is powerful enough. but i'm pretty sure i can make the attiny45, the ch32v003, the pms150c, the attiny202, the stm8s003f3, or the mb95f264hpft work
otoh in usb-c a lot of the power negotiation consists of things putting particular resistors across the line, which you could do even at that small granularity. a bus topology won't let you do that, though; if you had five devices plugged in, all their resistors would be in parallel
(incidentally, despite the name, usb is not a bus; it's a bunch of point-to-point links. that's why it can do that)
It will cause some noise issues, because a very high impedance front end will pick up stray radio transmissions, but I think it's doable given a small enough bandwidth