This electronic light dimmer circuit can be used to adjust the brightness of mains lights and the speed of AC motors. It uses a triac, diac and has a radio-frequency interference (RFI) noise suppression circuit built into it as well.
A triac may be considered as two SCR's (Silicon Controlled Rectifiers) connected in opposite directions. A diac is a gate trigger device. Triacs, diacs & SCR's are different types of Thyristors.
This light dimmer circuit controls the average power to a load through the triac by phase control. The ac supply is applied to the load for only a controlled fraction of each cycle. The triac is held in an OFF condition for a portion of its cycle then is triggered ON at a time determined by the circuit.
Each time the triac is turned on the load current changes very quickly - a few micro seconds - from zero to a value determined by the lamp resistance and the value of the mains voltage at that instant in time. This transition generates RFI. It is greatest when the triac is triggered at 90o and least when it is triggered at close to zero or 180o of the mains AC waveform.
Since there may be long lengths of mains wire between the triac and the lamp load which will radiate this RFI an L-C RFI suppression network is usually built into these types of circuits.

Short out the choke coil and notice that the RFI increases. The wire-wound coil and C1 provide the RFI suppression network.
Triac used for this project can be 2N6075 or BT136-500D 1and diac used for this circuit can be HT-32 .
Input voltage for this dimmer circuit must be 220-240 volts and the fuse used must support 2-3 A .