This lighting-control unit will fade out one lamp while simultaneously increasing the light output of another. The two loads track each other accurately without adjustments. The gate of SCRl, a silicon-controlied rectifier, is driven from a standard phase-control circuit, based, for example, on a unijunction transistor or a diac. It controls the brightness of lamp LI directly
Whenever SCRl is not on, a small current flows through LI, Dl, and Rl, permitting SCR2 to fire. When SCRl turns on, current flow ceases through Dl and Rl; the energy stored in Cl produces a negative spike that turns SCR2 off.