OK. So this is probably electronics 101, but I missed this lecture!
So up to now, when I have wanted to say power an Arduino (now Spark, of course) which controls a 12v device and a 24v device and maybe a 9v device using relays, I would have used three wall warts powering the different devices.
But surely there is a less cumbersome way. Can I just use one power supply at the highest voltage and then reduce the voltage for the other components with resistors? Or is it more complicated than this?