It is a lot of fun watching some ultrasonic levitation videos, but it seems all these videos have almost identical Arduino sketch and hardware, so I decided to try to simplify it.
What I come up with is to set timer 0 in CTC mode and use it to drive OC0A pin output. This can avoid possible delays by background interrupt with other (interrupt driven) approaches. Also simplified hardware, just using a NPN transistor, specifically, 2N3904. Probably any other model would work, even MOSFETs, etc. The key part is to use this transistor to short the ultrasonic transducers. Of course, the draw back is wasted power consumption with the current limit resistor. So it is not a good design for permanent setup.
However, if you have an Arduino around, a transistor, a couple of resistors, and of course, a pair of ultrasonic transducers, try it, it is a lot of fun. Here is video
