[HayZee518]

In its "normal" state, the door is kept closed by action of air and the cylinder ram. When the solenoid is de-energized the counter weight pulls the door open acting against the ram and retracts the ram into the cylinder. when the motion sensor timing circuit times out, the motion detector contacts open an auxillary relay's spring loaded contacts close, energizes the solenoid and admits air into the cylinder and the ram extends closing the door. when the motion sensor detects an object, its contacts swap closed and the aux relay is energized which disengages the power to the solenoid and the counter weight opens the door. this is FAIL SAFE open function so loss of power would automatically open the door.