[Laser] PWM of LED in QST

[Tom Becker]

  OK, I've read the QST article (thanks, Raymond) - and agree with you, 
James, that the visually-average LED intensity should not vary using 
this method for voice frequencies.

I see no assertion of what Vcc the author used (although he writes "If 
Vcc=12v..." when discussing LED current limiting) but he does suggest a 
modulation value, 1VRMS (~2.8VPP); he writes that he used audio from a 
radio and measured it with a DVM.  A DVM is a poor choice to measure 
audio levels, particularly peaks, so he might well have been providing 
even more than 2.8VPP.

Regardless, the CV input of 1VRMS is probably too large.  Even at 
Vcc=15v the LM555 spec shows a max CV of +/-1v; it would be less for a 
lower Vcc.  Too high a CV will cause the 555's Threshold comparator to 
misfire and produce an unbalanced PWM output.  You're right, he likely 
inadvertently over-drove the method.

So, bamboo under his nails?

Tom