Abstract: A transformer unit connects to an ac outlet and provides ac controlled by an opto-coupled triac to another ac outlet. The transformer unit includes a transformer and a third connector for connection to a control unit separate from the transformer unit. The third connector provides the control signal from the control unit, a low voltage supply for the control unit derived from the transformer, and the timing of the ac waveform, provided by a zero crossing signal or an ac or rectified ac signal which can constitute the low voltage supply. The control unit, electrically isolated from the ac supply, can produce the control signal under manual control or automatically to gradually change the conduction phase angle of the triac. The brightness of a lamp connected to the ac outlet is thereby controlled manually or automatically to provide a simulated dawn or dusk.