Change the variable resistor. It turns the Triac(2diodes, 2NPN, 2PNP transistors) "off" using pulses from AC to the 1k-ohm resistor and 3.3uF capacitor turns the gate "off" and the Diac(2NPN, 2PNP transistors) Turns it back "on" from the variable resistor and it cuts it off as it gets deeper "off", because of the minimum voltage.
