Check out this great project shared by Paul in the Adafruit Support Forums:
I’m creating a project to pull a string when a cat enters a door to cause the door to close behind her. The door is a standard screen door with a standard door closer. I want to prop the door open with a small stick, attach a string between it and one of your 9v solenoids. I’m building a circuit to activate the solenoid when the cat crosses a laser beam….
Thanks for all the help. I ended up using the 24v DC converter/power supply. It worked very well with the solenoid pulling on one end of a stick holding the door open. I had to work on the door to get it to close and latch properly. Here are pics of the final rig (overview, & control center), plus a pic of my cat Stella, safely at home.