The circuit shown in the figure is triggered by an input voltage determined by the setting of P1. Of course, the voltage applied to this input must be equal to or greater than that required for the triggering of the Diac. The P2 adjustment is made so that we have the current necessary to saturate the transistor when the diac drives, without danger of excessive base current causing damage. The relay will depend on the voltage used in the supply and on the type of load to be controlled. We remember once again the latch action of this circuit, which means that it keeps conducting when the input voltage falls below the trigger threshold. To disconnect, the input voltage must drop to zero.
