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

Chuck Harris cfharris at erols.com
Wed Sep 10 08:34:11 EDT 2014

Ok.  My point being that absent some way of telling what color light
your phototransistor is responding to, the modulation you are seeing
might not be from the white fluorescence output of the LED, but rather
from the UV/blue LED pump's output.  The white output might be
providing a DC bias to the detector, rather than providing an AC signal.

The best clue, I would think, would be to observe the output from a
white LED that is being switched fully on and off, and watch the depth
of the modulated signal coming out of your photo transistor.  It too
should be fully on and fully off.  I think it will never come even close
to off.... just fully on, and fully on - k, where k is a modulation
depth... which I think will be small if the modulation frequency gets
above 50Hz.

The white LED's I have worked with, though tiny, glow for a long time
after being run at full brightness.  That persistence should make them
poorly suited for use as a pulse modulated white light source.

The old GaAs LED's used to modulate well into the GHz region.  I doubt
that you can modulate the white light output of a white LED with much
depth into the 60Hz region.

-Chuck Harris

Mark Sims wrote:
> 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. ---------------------
> 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?
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