Making a Fully Automatic, Gravity-Powered Door Closer

One’s room is a holy place. A home within a home. A place where one can relax, let their guard down, and enjoy themselves. True especially for the younger generations, one’s room should be a place free of worry and strife, a safe haven for one to think and be themselves.

Lately, I’ve been dealing with a rather nagging problem. And although I know there’s no one to blame but dismissable forgetfulness, both my parents and siblings simply never close my door! They open it to tell me something and just walk away. After years of asking them to close it, almost every one of their trips to my room still ends with this “present”.

I was surprised to find that this problem wasn’t just one I had. Just search up “parents never close door” on YouTube and you’ll find videos with thousands to millions of views regarding the phenomenon!

Let me dispel some misconceptions that leaving one’s door open is supposedly a beneficial and a necessary practice. There’s nothing wrong with closing one’s door. For starters, such a practice is very valuable during a fire, preventing flames from spreading. Don’t believe me? Just take a look at this article. Additionally, closing doors stifles noise and allows for more focus. Most importantly, at least in my eyes, is that it helps keep warmth from escaping. I simply cannot handle the cold!

What makes a room a room is that it is an enclosed area sectioned off from the rest of the house, a separate part of the house, not merely an extension of it. Determined to preserve the autonomy of my room, to defend its sanctity, I concluded I needed to address the issue of left-open doors. I needed to build some sort of door-closing device.

Baffling Brainstorming

With the idea fresh in my mind, I began to think. What specific mechanism would cause the door to close? What would activate it? Would it be manual or automatic? Alexa-activated perhaps?

I began by researching online and observing the systems in place around me. Most of the doors in my school use a Commercial Door Closer, a rather bulky silvery box that mounts on the top of the door and uses two metal arms to pull itself shut. However, these devices were difficult to install, expensive (just look below!), and simply not for the homely-room environment, more for the professional workplace.

The top results after searching for “commercial door closer”

Another idea I stumbled across was the use of a large motor and a rod of some sort to “push” the door shut. However, this approach required constant access to power (not environmentally friendly), utilized a lot of electronics and controlling mechanisms (convoluted), and jutted out in a rather ugly manner from the wall. Additionally, most of the 5V DC motors* I had at home lacked the torque** to push the door shut.

To see the overcomplicated and unsuccessful endeavors in using a motor, take a look at this sample motor tutorial and notice the aesthetic and performance of the final product towards the video’s end.

Clearly, that approach won’t work. Perhaps something using pneumatics or hydraulics? A piston that would expand and release on command…

It was at this point, when I found myself asking my friend if she could give me a hydraulic piston for Christmas, that I realized I was barking up the wrong tree. I was making this problem way too complicated.

A robotics mentor of mine once told me that the most effective design ideas are the simplest. There’s no need for an elaborate electronic or hydraulic device to solve the problem at hand. I had been trying to fly a kite by using military-grade GPS navigation and a pair of rocket engines.

All we want to do, something even a baby could do, is to close a door.

* The 5V DC Motor is the standard motor, found in many toys and household products

** Torque is the “turning force”, a measure of how much “twist” the motor can exert

A Brief Background

Let’s stop to take a look at some of the context during our investigation.

Just as my imagination was taking off, our Gravitational Potential Energy and Pulleys unit was coming to a close in AP Physics I. And that’s when it hit me. In my quest to close the door, the power of gravity is a force that is always present, a force that I could harness to serve my purpose. Perhaps there was no need for motors or pistons after all…

The premise of gravitational potential energy is rather simple. The higher an object is off the surface of the earth, the more gravitational potential energy it possesses. This potential energy can be harnessed to do work*, namely on closing our door in this scenario. It can be thought of as a ball sitting on a hill. The ball is not currently doing anything, but it bears the potential energy (due to the Earth) to do some serious work if it starts to roll down.

An introduction to pulleys is also paramount in understanding the design process of this automatic door-closing device. Pulleys essentially allow one to change the direction in which forces are exerted. If a pulley is mounted vertically and a string is run over it, when an individual tugs one end of the string downward, the other end will experience an upward force exerted on it. By configuring different arrangements of pulleys, one can redirect applied forces.

Knowing that I had gravity and the ability to channel it on our side, the question now became how I would harness the force to close our door.

The rationale I developed was that when the door was pushed open from the other side, some object would elevate off the ground (increasing its Gravitational Potential Energy, its ability to do work). When this applied force was removed, for example when the person who opened the door walked into the room, the force of gravity would take over and pull the object back down, closing the door automatically.

Now, I simply had to figure out how to connect some weight to the door so that its downward movement will lead to the door’s closing.

In order to close the door, I needed some sort of pulling force. If I were to attach a string to the top of the door and pull it towards the door-frame, the door would close, as shown here.

However, I wanted the system to remain within the room, and not extend out into the hallway. In order to redirect the force, a horizontal pulley could be attached as shown on the left to the top of the door frame. Once the string is run through it, closing the door can be achieved by simply pulling the string away from the frame, regardless of the direction.

After this step, this tug still needed to be redirected downwards (since gravity does not act horizontally). To achieve this, another pulley can be attached to the adjacent wall, this one oriented vertically. The string will be run through it, and now, voila, a downward tug on the string will lead to the closure of the door!

As can be seen in the above diagrams, gravity pulling down on the weight will cause the open door to swing towards the frame (meaning it will close!) The red arrows represent the direction of the tension force, or the pull, within the strings.

* Work in physics is defined as the ability of a force to move an object a distance

Careful Construction

With these schematics drawn out, I began to build. I knew that eventually, all the pulley components would have to be mounted onto the walls and the door. In such, I decided to use LEGO Technic pieces as my building materials. These pieces were easy to work with and have pre-built holes which are the perfect size for screwing and nailing.

I began to list out the specific components needed:

1. Something on the door attached to one end of the string
2. A horizontal pulley on the door frame
3. A vertical pulley on the adjacent wall
4. The weight itself, attached to the other end of the string

I dove in and began to build. Each component wasn’t too difficult to make, just a few beams and a simple wheel to act as the pulley. Below are pictures of my final products, but if you wish to design your own, I listed some helpful pieces of wisdom I learned along the way. Hopefully they save you from some of the same mishaps I encountered!

Component 1 — Door Attachment

These images show the component (red) that attached the string to the door.

1. I initially used velcro strips. However, these were too flimsy so I proceeded to use four small screws to fasten it to the door. Adhesives will likely not be effective.
2. The base of the door connector was rather small at first. Thus, when the string was tugged at the top, the bottom of the connector piece would sometimes come loose. After some thought, I realized why. The door itself can be thought of as a hinge, with a tension force being applied to its top. Thus, the longer the portion of the connector on the door itself, the less force the screws have to exert to keep the connector in place (similar to how it is easier to open a door by pushing on the knob than the hinge). I then added an extra downward portion (golden screws).

Component 2 — Horizontal Pulley

These images show the horizontal pulley (blue) attached to the door-connector (red).

1. Sometimes, the string would come loose and fall off the pulley. In order to prevent this from happening, a black beam was extended outwards, past the pulley, to serve as a “bottom”. It can be seen clearly in the third image.

Component 3 — Vertical Pulley + Guiding Piece*

These images show the vertical pulley (green) and the string’s guiding piece* (red).

1. *One piece of improvement was the addition of a small guiding mechanism to ensure the string did not veer and the block did not swing. The small piece helped ensure that the block was only free to move in the vertical direction of motion.

Component 4 — Wooden Block + Entire System

These images show the wooden block (weight) and the entire string-pulley system.

1. The exact weight one should use depends on the type of door they have and the quality/state of its hinge. However, personally, since there are often a lot of small children in my house (thanks to my younger brother), I decided to play it safe and go with a lighter wooden block to ensure a gradual and smooth closing of the door. Additionally, this places less stress on the components and the string.
2. In regard to the string, the mockup initially used yarn. However, this material was too rough and too unwieldy to use (too much friction against the pulley). I found that the fishing line worked rather well and was strong, small, and transparent.

Exciting Evaluations

The endeavor was a wild success! The door-closing device functioned and continues to function smoothly, soundly — without a hiccup.

For less than $10 and a few hours of time, anyone can build their own automatic door-closing system! Only four small components were utilized, rendering the system inconspicuous. A nice little touch of creativity and uniqueness to one’s room, this system is green and is rather simple yet clever in practice. I distinctly remember the reactions of my friends and family, all in awe of this dazzling room upgrade.

Image credits:

• “Problem: Two Masses on a Pulley.” Phyley, www.phyley.com/two-masses-on-pulley.
• Types of Energy, Energy Transport, Energy Balance, www.atmo.arizona.edu/students/courselinks/fall16/atmo170a1s2/lecture_notes/energy_temp_heat/energy_intro.html.
• Dorkashop.com. “NEW Lego 15 x Brown Seats Chairs 2x2 4079 Lego Pieces”. www.dorkashop.com/index.php?main_page=product_info&products_id=101119.