Doppler Effect Model

Summer 2019, Exploratorium

The project

For my third summer in the Exploratorium shop, my mentor wanted to challenge me by giving me a project that didn’t already have plans. He and I worked to come up with a topic for an exhibit that would be engaging, easy to model and relevant to the public. He had wanted to develop a model for the doppler effect in the past, but had never been able to figure out how to effectively demonstrate the phenomenon, so he gave the project to me and let me loose.

When he had first thought of this project, my mentor’s intention was to use coins dropped on a belt to demonstrate how the doppler effect worked. As the coin dropper moved with or against the belt, the spacing of the coins on the belt would change as they were dropped at a constant rate. This effectively models the doppler effect because it shows how the sound or light waves get compressed and stretched as an object moves in relation to the source. However, coin handling is extremely difficult. The coins need to consistently be dropped flat on the belt. Additionally, they need to be reloaded into the dropping mechanism from some lower point. This cycle of reloading and dropping should ideally be continuous, allowing the exhibit to run constantly, instead of requiring reloading or presetting by the visitors or museum staff.

Confronted with these problems of coin-handling, I quickly decided that the best path forward was to scrap that idea entirely. The idea of dropping something on a moving belt was good, but the item being dropped needed to be easier to reload, drop accurately and run continuously.

At this point in the project, I was joined by another intern, Hazel. Hazel was starting her first summer at the Exploratorium, and my mentor paired her with me so I could teach her design and manufacturing skills. We began brainstorming alternate ideas to coins. The first idea that seemed promising was using a pen making dots on a piece of paper.

We liked the pen idea for several reasons. One, pen and paper is cheap and easy to come by. Second, a disposable paper potentially allows the visitor to have a souvenir. And third, a mechanism to have a pen make a dot is relatively simple to build. However, we quickly encountered several problems with this. One, pens need to be replaced and reloaded, which means that museum staff need to constantly monitor the exhibit. Second, if the pen is exposed to the public, vandalism is extremely likely. We hoped to make the exhibit free-standing and self-sufficient, meaning we did not want there to be parts that needed replacing daily, so we moved on from a regular pen and paper.

Related to pen and paper, but slightly more aligned with what we were looking for was a whiteboard marker and whiteboard vinyl belt. This worked well because the vinyl is easy to erase, making the belt infinitely reusable. We built a mock-up of this design using some 80-20 aluminum and quick machined parts, shown below.

This prototype featured a rotating cam to drop the pen at regular intervals and a vinyl belt. This produced okay results, but not quite the definition and flexibility that we were looking for.

Since we thought we could do better, we tried some crazier ideas. We decided to try using a UV laser and a reactive vinyl belt. The belt could be erased using UV light easily, the laser would have near infinite life, and nearly every variable could be changed, allowing ultimate flexibility. We weren’t sure that this would work, but parts were cheap, so we built a prototype. Ultimately the laser produced the best result, and we decided to move forward with that design for our floor-ready version.

As we began to work on a floor-ready design, we really wanted to focus on visitor engagement with the exhibit. Both Hazel and I agreed that an important part of any exhibit experience is the tactile and mechanical feel. Unfortunately, the laser didn’t really offer us this, as it is computer controlled, and the belt is automated and the speed is not adjustable. To help incorporate the tactile feel, as well as provide more interest in the exhibit, we mounted the laser on a chain-driven track so the visitor could turn a crank and move the motor along the track. This allows the user to manually move the laser and physically interact with the exhibit. The belt is mounted primarily below the table, allowing for a clean aesthetic and visualization of the effect.

See below for photos of the build process and floor prototype.

REflections

This project was both very fun and very challenging. Initially, the coin handling proved an impossible task, but was very engaging to work on. As we progressed into the design process, we refined the problem and the scope, but things never quite worked as well as we hoped them to. The work felt messy and not up to the same standards as the other projects I have produced in the past. However, that was part of the point of this project - to expose me to something new and encourage me to step outside of my comfort zone. Additionally, I had a chance to mentor a younger student on this project. This was a challenge because I am used to just doing things, and often there were times where I knew I could make something easily, but had to force myself to step back and encourage Hazel to learn and try on her own. I really learned a lot on this project and am proud of my ability to take an overwhelming and seemingly impossible design challenge and come out with an acceptable prototype at the end.