Ray Wang wrote this post about voltage conversion from 24VAC to 5VDC.
Voltage conversion from 24VAC to 5VDC is quite useful, because a lot of home automation devices use 24VAC, including sprinkler solenoids, home surveillance systems etc. Having a conversion module makes it easy to use a single power supply, without a separate 5V adapter for your control circuit. There are plenty of resources you can find online about it. But these resources are rather scattered. So in this blog post I summarize and discuss the common choices.
24VAC to 5VDC Conversion – [Link]