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This dimmer makes the light vary in brightness by adjusting how much time it is on and how much time it is off, called the duty cycle, which is controlled by the potentiometer. The duty cycle in this circuit varies from 100% to 0%. To have a duty cycle of 50%, set the potentiometer to the middle. Although the lamp will appear to flicker on and off, it will actually stay constant due to the heat capacity of the lamp’s filament. Even if it is receiving half the power, it will not be half as bright since the lower filament temperature will shift the spectrum into the infrared where we cannot see. It might have a faint red or coppery glow, but not much more than that. In real life, the op amp will control a MOSFET switch which will apply power from the 60Hz mains or block the power depending on the output of the op amp.
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