OK, since many users asked about this, I went ahead and did some test myself:
– 12V LED light: works fine, can be directly plugged into OSPi as a normal sprinkler zone (doesn’t even matter polarity because OSPi runs on AC voltage). Note that because the peak voltage of 24V AC is quite high, in practice I suggest connecting two 12V LED lights in series to halve the voltage on each.
The downside with LED light is that because it’s internally a diode, it only lights up on the positive cycles of the AC voltage, so it flickrs at 30Hz, which is noticable. You should try it out and see if this may be a concern.
– 24V light bulb: works fine, can be directly plugged into OSPi as a normal sprinkler zone.
– 12V light bulb: should not be directly plugged in, but if you put two of these in series then it should be fine.