[volt-nuts] Finally got around to modifying my Fluke 845ab with LED's

[Mark Sims]

It should work fine.  The phototransistor does have some sensitivity in the blue region.  It uses a fast comparator circuit (with a little hysteresis).  One input is the average/filtered phototransistor output,  the other is the raw signal.  It can detect the PWM signal from a small flashlight (a couple of lumens) in a room lit by 10,000+ lumens of overhead LED lighting.  It can also detect the light output fluctuations caused by DC-DC converter switching.
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What would your PWM sensor circuit do if all of the AC was in the blue, and the
white light appeared as a DC bias on the phototransistor?