[Laser] PWM of LED in QST

[n5gui at cox.net]

I would like to get comments from the group about the Hands On Radio column in the October issue of QST.

It seems to me like a reasonable effort to encourage some interest in light communications, and amateur radio experimentation with LEDs.  I also like the author.  That said, I find the article technically flawed.

The idea is a PWM using dual timer circuits, as in two 555s or a 556.  The first is an oscilator, the second is a modulator driving an LED, or it could have been a laser.  It appears to be a pretty straight forward setup, intended to show concepts with a simple circuit.  It is not intended to be a practical and effective component in a useful light comm system.

The problem that I have is the text describes flickering observed when audio is fed into the circuits input.  It goes on to explain the that the flickering is the result of the modulation, which is a type of amplitude modulation.

Where I have a problem is that flickering should not be apparent unless there is something wrong with the system.  The human eye cannot distinguish the changes in brightness at the pulse rate or at audio frequencies.  Classic amplitude modulation ( by that I mean like you get from plate modulation of a tube transmitter, DSB with full carrier, not DSBSC and definitely not SSB ), and specifically PWM should not have any variation of the intensity of the LED that is apparent to the human eye.  I can full well imagine that an over modulated AM signal, corresponding to PWM where the duty cycle exceeds 100 percent, or drops below 0 percent, will cause flicker.....  But that is the result of asymetric clipping so that the average LED brightness changes over a time period long enough for the eye to proccess it.  I am not sure of the typical eye response, but I suspect it is more than 100 milliseconds, less than 10 Hz.

I am quite sure the modulator was heavily overdriven.  If  it had been used in a a simple light comm demonstrator, that should have been obvious.

I really would like to see light communication encouraged.  I don't want to fault the writer for a mistake that I can attribute because he has never seen a working light comm system.  Can we suggest a better way to get our message out?  Better yet, can we help writers in amateur radio magazines to provide technically sound and interesting articles?


James
 n5gui