R1 and C1 are a phase shift network - they produce a variable delay in the waveform applied to the ST4 and hence the triac. When the voltage across C1 reaches the breakdown voltage for the ST4, C1 partially discharges into the triac gate through the ST4. This pulse triggers the triac into conduction for the remainder of the half cycle.
This easy-to-build controller is ideal for dimming lights, and controlling the output of electric heating type appliances. The light or heater element etc is placed where the 'LOAD' is marked on the circuit.
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