The Lightning Bulb circuit uses a quadrac (see Fig. 1)–a device that combines a triac and a diac trigger in a single package–to control the supplied current. Figure 1A shows the schematic symbol for the quadrac, while figure 1B shows the pinout for the unit used in our circuit. Note that quadracs are increasingly difficult to come by; if one can not be located, a discrete triac/diac combination of equal or higher rating can be used in its place.

Source:
http://www.uoguelph.ca/~antoon/circ/hv/l-bulb/l-bulb.html