The issue is not so much with LM2596, rather, it’s the fact that this is deriving power all from the 24VAC power supply, and it might not have sufficient power to drive both solenoids and RPi. For example, the 24VAC adapter we sell is only rated 500mA — so that’s about 15Watt, while at peak power RPi B+ could draw close to 15Watt, leaving no room for solenoids.