LED pulse width modulator

Posted on Feb 6, 2014

Fairly early on in my work with high-power LEDs such the Luxeon 1 and 3 watt devices I considered that PWM (Pulse Width Modulation) techniques would be an interesting means of modulation the LED. In theory, this should be capable of producing the lowest distortion modulation on an LED because the linearity of the LED`s `Current-versus-Luminous Out

LED pulse width modulator
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put" curve would be irrelevant: The apparent brightness would be integrated by the receiver to produce a voltage proportional to the duty cycle. In addition to high-power LEDs, this unit is also suitable for driving inexpensive laser pointers, provided that suitable circuitry (e. g. voltage/current regulation) is used with the laser pointer. In other words, it will NOT safely drive a laser pointer directly! For a "steady state" DC output (that is, no waveforem being generated) a 50% duty cycle square wave is generated at a frequency several times higher than the highest-frequency component in the audio to be reproduced. While this frequency could theoretically be as low as just twice the highest audio frequency, it is usually several times higher than that to simplify lowpass filtering and to cut costs. To increase the output voltage, the duty cycle of this square wave is increased, with 100% being "full on. " Conversely, to decrease the voltage, the duty cycle would be decrease, down to 0% being completely off. In reality, most PWM circuits avoid getting too close to either 0% or 100% as either extreme would produce objectionable "hard" clipping. The PWM output is filtered to average out the square wave, the ultimate result being a voltage that is directly proportional to the duty cycle of the original square wave. As it turns out, the linearity of a PWM generator could, in theory, be absolutely perfect as the duty cycle...

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