The Tesla coil is a self-oscillating 24V SSTC with feedback winding and NE555 interruptor. The half-bridge consists of NPN transistors. The pulse frequency of the NE555 can be configured via a potentiometer.
The driver has been retrofitted to allow the feeding of an interruptor signal for audio modulation. The coil can be modulated via a TOSLINK fiber (safely, without a risk of high voltage on the USB ports).