Seeing the pictures from the door-stopper with easy catch-and-release function immediately makes you understand how it works. Really easy and very practical:
1. While opening a door push it over the wooden rail until it sticks.
2. To release it again, pull gently and the door sets free
The origin for this design lies in a bicycle shed that is daily frequented a number of times for parking bicycles. An installed door-closer gave additional hindrance when accessing the shed with a bike at the hand.
What was needed was an easy to operate system that didn't require a clumsy action like lifting a catch or some other kind of stopping mechanism. Just pull gently and that's it.
The design as depicted works perfectly and is very reliable.
Prerequisite is that enough space is available under the door, and that the bottom side is flat (no weather strip attached).
Further steps in this instructable deal with the detailed design of the door-stopper (Step 1), concepts for a next-generation door-stopper (Step 2) and finally some words on how to take this product further (Step 3).
Step 1: The Design of the Door-stopper
The design of this door-stopper consists of a wooden block of a height slightly lower than the bottom of the door itself. If the rail is placed on the block, it should be higher than the bottom of the door. Thus, the height of the block depends on both the free space under the door and the thickness of the rail. In this specific design, the block height plus 75% of the thickness of the rail equals the available space under the door (block height is 41 mm or 1.61 in, rail thickness is 8 mm or 0.31 in and the space under the door is 47 mm or 1.85 in). In this design both elements (the door and the rail) do not fit by 2 mm (0.08 in, the other 25% of the rail thickness), which is resulting in a friction that is just enough to keep the door in an open position.
The rail design is important: it should be shaped downward in order to make the door hovering above the rail when it comes close to the rail. Only when the door is being pushed further towards the block it touches the rail and sticks there by friction. The material used for the rail in this design originates from a wooden slat out of a bed rail, which is bent and springy from itself. Its length is about 30 cm.
Note that this design is not just a wedge, or a wedge turned round. The innovative part of this concept lies in the springy nature of the wooden rail. The free space under this rail allows it to bend down, thereby exerting an upward force to the door, by which the door sticks to the wooden rail.
The rail sticks out from under the door when it is in its open position. This is on purpose: it allows to push the rail down with a foot to release the door. In practice however this functionality appears hardly used: it's more convenient to pull the door with a free hand. The next-generation door-stopper better should not stick out, as it may be a cause of injury, especially when people are bare-footed. Sharp edges should be rounded.
Another design flaw is the inclined rounding at the end of the rail. It has no function in this configuration and can be omitted in a next-generation design.