Figure 1.  Circuit Diagram for a Simple Light Dimmer

This simple 'light dimmer' is a circuit that uses only a couple of transistors to control the brightness of a bulb.  This circuit is only for filament-type bulbs and can not be used for incandescent lamps.

The brightness of a bulb depends on the amount of current flowing through its filament.  Thus, a light bulb can be dimmed or made brighter by decreasing or increasing the current through its filament, respectively.

The light dimmer circuit above uses a medium-power NPN transistor (such as the 2N3053) to control the current flowing through the PNP transistor (such as the MJE 2955).  The PNP transistor is the one supplying the bulb with current, so it must be a power transistor with an adequate heat sink.

The base and load currents of the PNP transistor (and therefore the brightness of the bulb as well) can be adjusted by adjusting the base current of the NPN transistor.  This is done by adjusting the 1K potentiometer, which simply acts as a voltage divider at the base of the NPN transistor.  Increasing the base voltage of the NPN transistor causes it to draw more base current from the PNP transistor, which makes the latter more conductive.  This, in turn, causes a larger current to flow through the bulb and make it brighter.

The obvious advantage of this circuit is its simplicity.  It also does not generate RF interferences like dimmer circuits that vary the brightness of the bulb by varying the duty cycle of its oscillating load transistor.  A disadvantage of this simple circuit is its inefficiency.

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