If you just need to run garden lights, keep in mind that you may be able to use opensprinkler as is without any modification. This is because you can run dc lights on AC power – it will produce some flickering but usually not noticeable. With 24vac you can connect a light bulb rated 24dc and that should work fine (because 24vac is measured in terms of rms average). If you need to run 12vdc bulb instead, you can connect a 1n4007 diode in series to block half of the AC wave and hence reduce the equivalent rms voltage by half.