Most modern garage doors have photoelectric 'eyes' as safety sensors that emit invisible beams across the opening, stopping the descent of the door in case of obstruction. However, not every old model will have one, and they may break over time, and if you'd like to install one without going through the trouble of replacing the entire garage door opening system, follow these next few steps:
First: look over the instructions that came from the manufacturer of the product. The system will only work if correctly installed, so you must double-check the required height for the sensors, usually between five and six feet off the ground.
Second: Cut the power and remove the old sensor, if any. It's very important that there not be electricity running through the system while you work on it. Carefully remove the old sensors, leaving the mountings for the new ones if they match the height.
Third: Install the new sensors. Follow the instructions carefully, and be sure they are perfectly aligned before actually screwing them in.
Fourth: Test your new eyes. Close your garage door with something obstructing the way, like a broom handle, to be sure that the door will reverse itself and open correctly upon sensing the broom's presence.