so ive been messing around with this idea for a while and finally started putting something together. basically i have 4 antennas on the roof — a 40m dipole, a 6m yagi, a 2m/70cm colinear, and a random wire for the low bands — and im constantly walking back to the shack to flip the manual coax switch every time i want to change bands. its getting old fast.

i picked up an arduino mega a few months back thinking id use it for something eventually and this seems like the perfect project. the plan is to have it read the band output from my IC-7300 (the CI-V bus or maybe just the band voltage from the ACC port, still deciding) and then automatically key the right relay to connect the correct antenna. i already have a 4-relay module from a random aliexpress order sitting in the bin.

the part im not totally sure about is isolation. i dont want RF getting back into the arduino and frying it, and i know relay coils can cause spikes too. been reading about using optocouplers between the control lines and the relay board but the relay module i have might already have them built in, hard to tell from the docs. does anyone have experience with this kind of setup? also curious if a pi would be better than the arduino for this — i feel like the arduino is simpler for something that just needs to flip relays but maybe im missing something.

yeah ive done almost exactly this with a nano actually. the relay modules from aliexpress are hit or miss on the optocoupler thing — some have them some dont, you can usually tell by looking at the board and seeing if there are little IC chips near the relay inputs. if the inputs go straight to the relay coil driver transistor with no isolation chip in between then you got the cheap version. doesnt mean it wont work but id add a series resistor at minimum on the control lines.

for the CI-V stuff the 7300 is pretty easy to poll, theres a ton of arduino libraries already written for it. ive seen guys do it over USB-serial with a cheap ftdi cable and it works great. band voltage from the ACC port is simpler code wise but you have to do the ADC reading and figure out the voltage thresholds yourself which isnt that bad either.

as for pi vs arduino for this specific thing — arduino all the way. the pi is overkill and honestly the boot time alone would annoy me. if the power hiccups your pi takes 30 seconds to come back up and your relay is just sitting there doing nothing. arduino wakes up instantly. save the pi for something that actually needs linux.

RF isolation on the relay side is the thing people skip and then wonder why their arduino resets when they key up. keep the relay board powered from a separate 5v supply if you can, even a little usb wall wart, dont run everything off the same arduino voltage reg. common ground is fine just dont share the supply rail with the relay coils. i learned this the hard way on a different project, wasnt even a radio project, just had some inductive load and it was causing brownouts on the micro.

also if your coax switch is going anywhere near a decent power level just double check the relay contacts are rated for it. those little SRD-05VDC-SL-C relays everyone uses are fine for receive only or QRP but if youre running 100w and the antenna gets a bad SWR the voltage on the coax can get spiky. probably fine in practice for most HF work but worth knowing.