so ive been messing around with this idea for a while now and finally started actually building it. basically i have 4 antennas on the roof - a 40m dipole, a vertical for 20/15/10, a 2m yagi, and just a random wire i use for receiving sometimes - and right now im running out to the shack and manually flipping relays on a homemade switchbox every time i want to change bands. drives me nuts especially during contest season.

so i grabbed a arduino mega i had sitting in a drawer from some old project and started thinking about how to drive the relays off the digital pins. the plan is to read the band data output from my radio (IC-7300 puts out CI-V over USB) and then have the arduino interpret that and automatically switch to whatever antenna i have mapped for that band. seems pretty straightforward in theory but the relay driving part is making me second guess myself - do i need a transistor buffer between the digital pin and the relay coil or can i just run 5v relays direct off the pin? i dont want to fry the board obviously.

also wondering if anyone has done something similar with a raspberry pi instead. i feel like the pi might be overkill for just relay switching but then again if i want to add a web interface or logging or whatever down the road it might be worth the extra complexity. interested to hear what people have actually built, not just what sounds good on paper.

yeah you absolutely need transistors or at minimum a ULN2803 darlington array between the arduino and the relay coils. the digital pins on the mega are only rated something like 40mA absolute max and you really dont want to be anywhere near that, practically speaking i keep them under 20mA. relay coils can pull 50-100mA depending on the module and even if the relay module has onboard transistors (which most of the cheap ones from amazon do) its worth double checking the datasheet before you assume anything.

i did almost exactly this setup with a uno and an IC-7610. wrote a little sketch to parse the CI-V frequency data and map frequency ranges to relay outputs. took me a weekend to get the parsing right because the CI-V protocol has some quirks with the byte order but once i figured that out it was pretty solid. havent had a false switch in like 8 months of use. if you want i can dig up the sketch, its pretty rough but it works.

on the pi vs arduino question - i actually ended up running both eventually. arduino does the hard realtime relay switching because i didnt want any linux scheduler delays messing with my timing, and then i have a pi zero W talking to it over serial that runs a flask web app so i can see what antenna is active from my tablet. honestly the pi zero is cheap enough that its worth it just for the wifi convenience.

dont overlook the flyback diodes across your relay coils either if youre rolling your own board. learned that the hard way on my first relay project, kept getting random resets on the arduino and couldnt figure out why for like two weeks. turns out the inductive spike when the coil switches off was doing all kinds of weird stuff. just a 1N4007 across each coil oriented the right way fixes it completely. most prebuilt relay modules already have these but if youre wiring your own watch out.