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<RJ>

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Jun 4, 2007, 8:16:15 PM6/4/07
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I've got a LED flashlight. ( 3 LEDs, 2 X AA batterys )

If it's ON, and I hold it near an AM radio,
there's a definite "whistling" in the radio.

I had assumed it was battery direct to LED.
What's oscillating ...enough to affect the radio ?

<rj>

David L. Wilson

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Jun 4, 2007, 8:54:49 PM6/4/07
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Are you sure you are not blocking the antenna from receiving the intended
signal and then it is hearing an internal noise? Try bringing other (metal)
object close the same way and see what happens.


dog...@mindless.com

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Jun 4, 2007, 9:15:53 PM6/4/07
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I'm going to hazard a guess that these are "white light" LEDs.

Those require about 3.6 volts to operate. 2 x "AA" = 2.4 (NiMH/NiCAD)
or 3.0 (alkaline).

You'll notice that almost all of the el-cheapo plastic LED flashlights
with white LEDs use either two 3 volt lithium coin cells (for the
keychain type) or three AAA or AA cells.

But that's a bit long and inconvenient, so a small DC-to-DC converter
is used to step up the voltage from 3 volts to 3.6 or so. To make the
transformers very tiny and efficient, the frequencies used are a lot
higher than your 60 Hz AC power. What you're almost certainly hearing
on your AM radio is either the ON/OFF switching of power into the
transformer, or a harmonic of that ON/OFF switching.

In the early days of computing, before sound cards (or even enough CPU
horsepower to think about making anything fancier than an text
display), hobbyists would write software programs that would
indirectly cause music on AM radios by running different patterns of
use over the lines running out of the CPU. Same idea, just they were
doing it deliberately and your flashlight does it as a side effect. :)


HFguy

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Jun 7, 2007, 3:57:55 AM6/7/07
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dog...@mindless.com wrote:
>
> In the early days of computing, before sound cards (or even enough CPU
> horsepower to think about making anything fancier than an text
> display), hobbyists would write software programs that would
> indirectly cause music on AM radios by running different patterns of
> use over the lines running out of the CPU. Same idea, just they were
> doing it deliberately and your flashlight does it as a side effect. :)

This was done on an 'Altair' kit computer in 1975. The Altair is
considered to be the first personal computer.

http://www.digibarn.com/collections/movies/digibarn-tv/erik-klein-altair-8800-playing/index.html

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