I recently completed a popular maker DIY where I recreated a product called SlowDance. You have see the original in action here https://player.vimeo.com/video/262079608
This product functions on the premise that an Electromagnet vibrates the object in the frame at around 80hz and then strobes LEDs lights slightly slower than 80hz producing the illusion of slow motion (the LED strobing is not discernible to your naked eye). For my DIY version I used the electromagnet, Arduino sketch, and PCB available in this kit:
https://www.tindie.com/products/Nick64/jf-slomo-slow-motion-frame-diy-kit/
Now I'm interested in creating a scaled-up version of this frame but lack the know-how from a electrical engineering standpoint.
The intent for the scaled-up version would be to vibrate a 4-6ft long string of unknown composition at 80hz. I would then anchor the string taught at one side of the frame and attached it to the electromagnet at the other.
My question is fairly simple, what type of electromagnet would be best suited to accomplish vibrating a 4-6ft long string at 80Hz and getting an appreciable range of motion out of the string (and any object I subsequently attached to it)?
Best Answer
In the case of a string, it will be resonant.
You can probably use a pair of supermagnets either side of the string (old hard drive) , and put your AC current through the string. You want a heavy guitar/bass string. In fact get a guitar and start playing with that on the bass string.
The string is the electromagnet. The magnets field must be across the string (i.e. these magnets are attracting each other), and this makes the string move from side to side, parallel to the gap as you pass current though it.
The magnets will be down fairly close to the nut or bridge. Here are two small 3/8" fridge super magnets, held 3.2mm apart with some hotmelt. The signal generator is connected to each end of the string, and is putting 130mA through the string.
It is very hard to tune the signal generator onto the fundamental, the resonance is so narrow - about 0.2Hz at 100Hz
You cannot make a string move, except in a resonant fashion. Away from its resonant frequency it just sits there - that's what resonance is all about.
This also means that you have to make the string set the frequency, not the electronics.
You can also do it mechanically see Wheel Harp or Hurdy Gurdy. You could also make a little plucker than plucks it every few seconds - which would give the satisfying view of the vibration decaying.
BTW, the challenge of this is making a strobe frequency that is 0.3Hz off from the strings frequency.
Another approach is the skipping rope one.
Yet another is to use flourescent nylon for your string (Weed whipper line), and UV/Blue leds for the strobe. (attach a tiny super magnet to the line, or use hard-drive coil, or loudspeaker cone etc etc)