The circuit is not in this thread.
A triac is used in an AC light dimmer. It turns itself off each half-cycle so it needs to be turned on again over and over if you want lights to remain on.
An SCR is half-wave so it lights light bulbs dimmly with AC. It turns itself off each cycle so it also needs to be turned on again over and over if you want lights to remain on.
An SCR is used in a DC alarm system. It stays turned on after it is triggered for a moment.
Transistoirs or Mosfets turn on DC traffic lights and are driven by a sequencer circuit.