I’ve been volunteering at the Science Museum of Minnesota doing science demos. I pick one out, set it up, and just get to engage kids and their families for a few hours as they peruse the exhibits. Whether it’s solving tetromino puzzles, examining the luminescent properties of butterfly wings, or anything else, it’s so fun to see kids play and be their natural inquisitive selves!
In my short time there, however, I’ve come across a lot of demos that are derelict and broken, put together by some long-ago volunteer with minimal instructions or guidance. I’m not even sure what some of them are supposed to do! So I have really taken a liking to choosing a project at a time, fixing it up, documenting it, and rolling it back out. I hope to not only bring them back online one by one, but document them on the Science Museum’s internal pages for future referencing. Below are the projects I’ve worked on so far-
Projects
Musical Light Beam
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My first project here- the documentation in the box promised the ability to transmit audio over a light beam, the audio input being in this case (dating the demo somewhat) a portable CD player. Two Cinema BTL audio amps, each connected to it’s own box of custom circuitry powered two diodes at the base of 5” tubes mounted on ThorLabs lens mounts. No instructions.
I naively began by simply getting it to work. (Starting with finding new batteries for the CD player). The transmission LED glowed red when I flipped the switch on the custom circuitry box, but no audio came out of the speakers I’d attached to the receiver side. I puzzled over the circuitry while sitting in the paleontology lab (so cool!) for a while, and decided to bring them home to dissect and rebuild.
The Transmitter
The custom circuitry on the transmitter side is simple. The audio source (microphone, CD player, etc.) is amplified by the audio amp, then the circuit adds a DC bias (in the form of a 9V battery) to ensure unipolarity on the LED and adds the audio signal on top of this DC bias via a coupling capacitor.
The existing circuitry on both ends is hard to follow (the bias resistor was hidden inside some shrink tubing) and super glued in place. So I opted to transform this into a PCB for simplicity and reproducibility. Below is the circuit diagram and PCB layout of the board.
Schematic and PCB for transmitter stage
Disconnects are included to make it easier to troubleshoot and replace components in the future. The battery (CN1 pins 1&2) gives the 9V DC offset to the LED when the switch (U1) is closed, and the audio signal (pins 3&4 of CN1) put the audio signal on top of this via the 47 uF capacitor. The board layout is compact and fits easily into the existing space. Without an oscilloscope its hard to test whether the LED is working as intended (the audio oscillations are small, fast, and imperceptible on top of what appears as a constant LED brightness), but once I order and solder up the new board with new components I’ll be more confident this end is working properly.
The receiver
Wave Resonance
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Another demo I’ve tried to get working- a wave on a string, is defunct. The instructions are clearer, but refer to equipment no longer in the box. I want to set this up with a speaker and variable strobe-light so that in addition to putting a standing wave on the string I can “freeze” the wave by controlling the strobe frequency. (Will I be allowed to use a strobe light in public?)