An relaxation oscillator is made with a resistor, capacitor, and a neon lamp.
A neon lamp is put in parallel with a capacitor in a RC circuit. When the capacitor is charged to approximately 80v the neon lamp flashes, partially draining the capacitor and starting the process over again. The period of oscillation is RC dependent and can be varied by changing either the value of the resistance or capacitance. A amount of applied voltage will also effect the period of oscillation.
Neon lamp, decade resistance box, decade capacitance box, and battery stack (approximately 10-11 9v batteries), a DC power supply can also be used.