[Laser] PWM of LED in QST

[n5gui at cox.net]

OK, anybody with a better explanation feel free to chime.   That goes more than double for correction of any error.

First let me define "flicker" as something that can be seen, which for the moment let us assume that is slower than 10 Hz.   The intensity needs to vary over a period longer than 1/10th second.

In simple terms, over any period shorter than 100 milliseconds the intensity can vary and not be seen.  As long as for longer periods, the ON-time to OFF-time ratio should be the same.  

My way of saying it may be rough, so I will try to describe a sample similar to what the QST article describes.  Assume the first 555 is an astable multivibrater, oscillating at 10,000 Hz, which is a period of 100 microseconds.  The second 555 acts as a voltage controled one shot adjusted to produce a 50 microsecond pulse with no external input.  The zero input duty cycle is then 50/50.

The pulse rate is too fast for the eye to detect the changes.  10KHz.

Modulate the system with a 1000 Hz square wave.  For 500 microseconds, the duty cycle is 75/25, and for 500 microseconds the duty cycle is 25/75.  If you then average for 1000 microseconds, the math goes .75 * 500 + .25 * 500 for the ON time and .25 * 500 + .75 * 500 for the OFF time.  The end result is still a 50/50 duty cycle on average.

The pulse rate is too fast for the eye to detect the variation.   1KHz groups of 10KHz pulses.

Modulate the system with a 1000 Hz sine wave.  The analysis is a lot more complex, but the bottom line is that the net duty cycle will average 50/50.  True, some times it may be 80/20, but there will be another time 20/80 to balance.  Same for any other value.

Modulate with any audio frequency below 3KHz ( the design point for voice, and the reason 10KHz was chosen as for the pulse rate ).  You will still get shifts with high duty cycle, with offsetting low duty cycle.  Same with complex multiple waveforms.  There will be offsetting highs and lows.

And since the modulation will be at audio frequencies, that is fast compared to the ability of the eye to see the flicker, the eye cannot see the variations, only the overall average.

PWM is used for controlling motors, like a slot car.  Most of the time we think of using the controller to go from zero to max speed.  For an audio modulator, we have to run the slot car at half speed, then speed it up and slow it down, but the time to run the circuit around the track does not change even if the amplitude of the speed variations change.

To carry the analogy of the slot car, you can set the average speed of the car above or below half speed.  As long as the speed variations stay within the "real" range, that is they push beyond 100 percent or below 0 percent, the average can stay in "linear" range.  On the other hand, if you overdrive the variations, you hit the limits of reality.  That in turn will skew the average.

In terms of the PWM driving an LED, if you keep the input within the linear input range, the variations in duty cycle will cancel out at audio frequencies.  The eye does not run fast enough to follow the variations.  If you overdrive the system into non-linear range, you can get "flicker", but it will also severly distort the audio received.


I hope this explains the situation.  It is directly comparable to an oscilloscope trace of a carrier that is amplitude modulated at three conditions:  zero, less than 100 percent modulation, and more than 100 percent modulation.  



James
 n5gui



--- Tom Becker <GTBecker at RighTime.com> wrote: 
>   Why wouldn't PWM cause flickering?  I don't get or have access to QST 
> so I can't comment on the schematic (although using a 555 as a 
> "modulator" intrigues me; is it using the 555 comparators to switch its 
> output at the oscillator rate?), but PWM is often used to produce lower 
> average currents, mimicing analog changes, by digitally switching full 
> currents for variable periods.  The effective average current change 
> should be visible as the modulation intensity changes, no?
> 
> Tom
> 
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