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LED Lamp Dimmer

LED Lamp Dimmer

Introduction
This circuit provides 32 steps of brightness control from 0 to 100% for a backlight or instrument panel using just two general purpose microprocessor signals. In addition, very little board space is required since only three SOT23's and a uMax package are used. Although shown in the figure below driving white LED's, the load could also be a DC motor or an incandescent lamp.

Theory of Operation
The circuit is based upon a modified Schmitt Trigger relaxation oscillator shown in Figure 2.

Figure 2
Figure 2

The output of U1 will be high according to:

Output of U1

Where: k is the wiper position, Vcc is the supply voltage, VD is the Diode voltage, and VN and VP are the threshold voltages of the Schmitt Trigger. The U1 output is low according to:

U1 Output low1

U1 Output low2

U1 Output low3
 

Initial inspection of the above equations may make the substitution to solve for the duty cycle appear as a tedious algebraic exercise. But, when Vcc, VD, VN and VP are substituted into the two logarithm terms, it turns out that their results are very close in value. So, the expanded solution to the Duty Cycle equation can be simplified to:

Suty Cycle Equation

Based on the above, we see that the duty cycle of this circuit is not only linear but also independent of the components selected. What is affected by component selection is the center frequency of oscillation. An approximate equation for the operating frequency based on the above is:

Operating Frequency

As mentioned the oscillation frequency does move with the tap position also:

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