Grumpy: so I was supposed to leave the other resistors already in series at the same time? Nobody had mentioned that, so I took them out when I tried the 1M parallel.

dc42: Thanks a bunch! That was very helpful. Since the Arduinix board doesn't have much room for expansion and I'm not confident in building an additional power supply, I think I will use your solution. I did some calculations for the resistor values I have on hand (exactly 180K and 390K aren't among them) and found some values that work similarly.

Most importantly, I then tested it and it worked out great!

Thanks to everyone, especially dc42.