Re: door opener circuit..
First there is a safety issue which the laws of your country specify. Generally, the door opening should not cause damage to people and animals.
Then there is the sensor. There are several types. One is a pressure sensor on the floor in front of the door. There is the light or IR beam across the area in front of the door. There are more sophisticated types which read a card people put in a reader. There are noncontact readers with different ranges. There is also a transmitter which the user activates. This is commonly used for girage doors and security gates at parking areas.