A small Tesla coil put together with the Radionic electronics construction system.
A relaxation oscillator based around a unijunction transistor creates a series of pulses fed into a monostable to neaten the pulse shape.
The oscillator frequency can be adjusted using a variable capacitor (the frequency is around 20kHz).
The monostable allows the pulse width to be chosen using a potentiometer.
The pulses from the monostable drive a step-up ferrite transformer via a MOSFET.
The transformer output is rectified and charges a high voltage 10nF capacitor.
When the voltage reaches 3.5kV the gas discharge tube breaks down and dumps the capacitor contents into the Tesla coil primary.
This sets off a short burst of high frequency oscillations in both primary and secondary, and a second voltage step-up.
The resultant high voltage at high frequency on the top load (sphere) can be observed.
