You can use a wiper motor with 2 rubber caster type wheels mounted on the ends of a shaft as the motor turns the wheels hit the door and push it open or just knock depending on how far back the motor assmble is from the door. I don't have a picture but I use it to open and close my thumping coffin and a motion sensor to turn on the 12v battery chargher. Lots of example circuits on the web for the 555 here is a good site.
http://home.cogeco.ca/~rpaisley4/LM555.html
Do a google for 555 timer you will find others.. Good luck!
http://home.cogeco.ca/~rpaisley4/LM555.html
Do a google for 555 timer you will find others.. Good luck!