A Chlandi plate is a very flat peice of metal that vibrates up and down at a high frequency, used for educational purposes to visually demonstrate standing waves and resonance.
It is usually powered by a mechanical solenoid attached to the middle of the plate, and the vibrations cause standing waves that run through the plate and cause areas of no vibration, and areas of strong vibration. A fine powder is spread across the plate, and the standing waves can be seen clearly forming intricate patterns. A professional chlandi plate is shown below:
These Chladni plates often cost upwards of £200, even more for higher quality, educational peices. Here, I will attempt to mimick their patterns for less than 2% of that price margin.
A mechanical osciallator is often used to create these vibrations. This is a cored out magnet with a coil of wire inside it, as shown with the blue ring being the magnet, and the yellow coil inside it:
A weak AC current is passed through the coil, AC being a type of current that switches from positive to negative very quickly. When the coil has positive current flowing through it, the magnet is pushed one way, from a generalised version of Lorentz's force law. When the current is negative, the magnet is pushed in the opposite direction. When these positive and negative currents are switched very quickly, the magnet oscillates up and down, and the frequency of these oscillations is entirely dependant on the frequency of the switching of the input current.
Unfortunately, mechanical oscillators usually cost upwards of £30, as due to the economics of scale, the demand for them isn't high enough for them to be cheap. Luckily, another device that uses this exact mechanism is a speaker: more specifically, a bass speaker. A bass speaker is meant to output the 'low end' frequencies of music and movies, which mean the amplitude of their drivers is higher. In layman terms, this means the magnet has more room to oscillate up and down, so it goes further on each oscillation. A speaker has the exact same principle as a mechanical oscillator, but with a cone around the oscillator to help create sound rather than movement.
For my goal of creating a cheap, miniature chladni plate for schools to use, a cheap, easily replaceable oscillator was needed, and by now it was clear that a speaker would be the best option. I ordered two speakers from Aliexpress that were advertised as bass loudspeakers, for less than 1/15th of the price of one mechanical oscillator.
The issue with these speakers was their relatively high power consumption. I wouldn't just be able to solder audio leads to the speaker's terminals without amplification, so after some digging, I found the PAM8403, which was perfect due to its 5V power input and 3W power output.
I only had one speaker I was planning to use, so my plan was to connect only the left channels, meaning to only connect L and G on the base of the board, and only use one of the 3W outputs on the top of the board. The 5V input was perfect as the output voltage of USB-C is also 5V, so I ordered a USB-C breakout board, which is a USB-C port that gives you clear access to the 5V terminals.
I ordered the parts, and wired them together like so:
Next, I designed a neat enlosure to hide all the wires and provide a method of mounting the speaker that would allow it to oscillate freely. This enclosure would need to have an aux-in port and a USB-C port, and an adequate hole for the speaker to pressfit into. I settled on this design that would be printed in two parts and glued together.
I also modelled an adaptor to fit inside the hole of the speaker that oscillates the most. This would interface it to a thin, flat 10cmx10cm metal plate that I also ordered from Aliexpress.
The final assembly looked liked this, without the plate (speaker assembly only), and with the plate attached:
The results are shown below:
Cost summary:
3D filament : £0.30
USB-C breakout board: £0.40
PAM8403: £1.10
Speaker: £1.05
Metal plate: £0.45
Aux in: £0.25
Total: £3.55
