The sensing part of this project uses reed switches (one at the beginning of the garage door opener track, and one at the end) to determine if the garage door is fully open,  fully closed, or somewhere in between.

The action of closing the garage door is done by closing a relay briefly on the pushbutton circuit of the garage door opener.

The first iteration of the project is done with a Raspberry Pi. This is overkill, but it allows for quick implementation and can host a web interface on the same device as the monitor and controller.