You don't need a high voltage to trigger a triac itself. I think most of the designs you're referring to use a microcontroller to monitor the zero crossing of the AC wave, and then trigger the triac with a logic pulse at the desired time point in the AC cycle.
In your circuit the diac has a high breakover voltage and therefore by itself triggers the triac at a defined point in the AC cycle. I'm not sure what you can use to achieve the same effect at low AC voltages.
