The lighting dimmer control is done by the potentiometer, you need to turn right or left to adjust the lamp’s brightness.
The lamp dimmer has no switch, you can see only a fuse for safety. You can add a switch in series with the fuse, or just insert this dimmer circuit between the existing swith and the lamp.
For more lighting dimmer circuits check the related links bellow.
Capacitor C4 is charged via resistors R3, R4, P1, and R5. After a certain time, dependant on the potentiometer, the charge contained in C4 is large enough for diac D to start conducting, so that a firing pulse is applied to the gate of the triac. Consequently the triac conducts and power is transferred to the lamp.