Also you can ajust blank frequency and refine pics frequency with just one more capacitor. More the capacitor is hight, more the blank frequency is hight.
Around 4uf is a good value.
For very hights frequency put a led with 470 k resistor instead of the speaker. With the potentiometre knob, turn until the led fade, turn off, go back before she turn off, you have reach the maximulm transistor frequency. Depend on c value also , 470 pf is a good choice.
With 2n2222 it is 300 000 000 hertz.
The coil is necessary, it is in joule thief configuration . You can make the coil with a caducee coil for betters results, the output voltage can be very hight with caducee coil around a ferrite donut.
Minimum turns : 5 turns