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Right part of schematics (diac, triac ..) are standard dimmer circuit.
Changing opto couplers diode current, you will change resistor resistance and circuit will work, bulb will gets more voltage in case when current through coupler is greater.
Left part is soft start circuit, so when you close switch, voltage on C1 will slowly start to increase (time constant is R3*C1), so T1 will slowly increasing current trough LED diode of coupler and thats it.
TRIAC
It work like 2 anti parallel connected thyristors with single gate (terminals are A1, A2 and G(ate)). When you put impulse on gate regards to A1 terminal triac opens and let current flow. Impulse can be positive or negative, no diference.
DIAC
Is like two serial oposite connected zenner diodes, meaning it's only 2 pins on diac, it opens on defined voltage (e.g. 33V or similar). So when voltage on diac is smaller than "open voltage" diac do not allow current flow, and when voltage is enough diac opens and let current flow.
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