I am totally intrigued by the idea of the 433 hz remote power switch addition to the opensprinkler. Question is, if I want to control front flower bed lighting, back flower be lighting, and maybe a fish tank or two, I don’t think that a single transmitter at the arduino would have the range. Can I wire in multiples in parallel ? I was figuring that I would run a set of cat 6 cables in my attic, to a few transmitters, maybe above each of the power switches. Is that possible ?
Wiring multiple transmitters will not help. However, you can extend the transmission range by 1) use an antenna of the appropriate length (17cm or 6.7inch, which is 1/4 of wavelength for 433MHz) and/or 2) use a higher voltage to drive the transmitter. On OpenSprinkler, the VCC pin of the transmitter is wired to 5V. The transmitter actually has an operating voltage of up to 12V. So if you need to maximize the range, you can connect the VCC pin of the transmitter to a 12V source — unfortunately there is no 12V source inside OpenSprinkler, so you would need to use a separate 12V power source if you do that.
I would suggest starting with tip 1) — you can also extend the antenna further to 1/2 wavelength. The transmission range might be sufficient without having to bump up the voltage.