This circuit produces a light effect that "ripples" between maximum and minimum points smoothly, with frequency basically determined by the two capacitors. When capacitor C1 is charged, the voltage rises by also charging C2 via R4, so that the field effect transistor smoothly conducts. However, as soon as it reaches the saturation time, C1 is discharged by triggering the unijunction and C2 is discharged slowly by R5, causing the transistor to go slowly to cut, turning off the lamp. With the values indicated in the diagram, we have a smooth oscillation of the brightness of the lamp. The reader can experiment with capacitors and resistors to get exactly the effect he wants.
