so ive been messing around with an arduino mega for a few months now and i finally got to the point where i think i can actually do something useful with it for the shack. the idea is pretty straightforward — i want it to read which band my radio is on via the band data output on the back of my IC-7300 and then automatically switch my antenna relay box to the right antenna without me having to remember to do it manually.

the 7300 puts out BCD band data on a mini DIN connector which is actually pretty well documented, so reading that part with the arduino should be doable. im more fuzzy on the relay side of things. im planning to use some SRD-05VDC-SL-C relays i grabbed off amazon, they should handle the RF switching fine for HF at least. but i keep reading conflicting stuff about whether i need opto-isolation between the arduino and the relay module or if the cheap relay boards already have that built in.

has anyone actually built something like this? curious what pitfalls i ran into before i start frying things. also thinking about adding a small OLED display to show which antenna is selected but that might be scope creep at this point

yeah done almost exactly this with a 7300, its actually one of the cleaner projects to start with because the band data is so well documented. the BCD lines are open collector outputs so you do want pullup resistors on the arduino inputs, 10k should be fine. the cheap relay boards from amazon almost all have the opto-isolation built in now, look for the EL817 or PC817 optocoupler chip on the board, if its there youre good. if its just a ULN2003 driver chip with no opto then id add a separate barrier between that and the arduino just to be safe.

one thing i ran into — the band data changes like instantly when you spin the VFO through bands and if youre not debouncing or adding a small delay in your code the relays will chatter like crazy while you tune across band edges. i added a 200ms delay before the relay actually switches and that cleaned it up completely. the OLED is totally worth adding, i used a 128x32 SSD1306 one and it adds like 20 lines of code with the adafruit library, dont skip it

i did something similar but went with a raspberry pi instead because i wanted to log which antenna was being used alongside my normal logging software. ended up being way more complicated than it needed to be honestly. the pi is overkill for just switching relays, arduino is the right call for that part. though if you ever want to tie it into flrig or something over the network the pi does make that easier.

also just a heads up on those SRD relays — they work fine for receive and low power but if youre running anything over like 100 watts be careful, ive seen the contacts arc and get pitted over time. might be worth looking at a proper RF relay like the Tohtsu or Dow-Key stuff if youre running the amp. just something to think about before you build the whole thing around those little boards