The UJT, capacitor, and the resistors make up an oscillator that supplies an ac voltage to the SCR's gate. From my understanding, the SCR wouldn't turn off when the gate pulse become zero. It will only turn off when negative pulse is applied to it. However in this situation, the gate pulse is fed by the Base1 of UJT which is shown in the diagram. The Base1 pulse is only from level 0V to certain positive level voltage (no negative voltage level). My question is, how will the pulse turn off the SCR and eventually control the speed of motor? Thanks in advance...