Interesting observation. You said hum gets louder.
Implying hum is already there. What kind of phone?
Is it only that one telephone instrument?
Only occurs when LEDs are near the instrument, not near the
phone lines with the phone in another location?
Is the effect more pronounced at less than 3m?
Are the LEDs turned on/off while appear to be on? In other
words, LEDs are OFF then come ON OFF ON OFF ON OFF at some
high rate, perhaps near the 50Hz frequency?
Do the LED ckts affect an AM/FM radio held close by?
- Robert -
Robert A. Macy, PE .. m...@california.com
408 286 3985 . . . .. . . fx 408 297 9121
AJM International Electronics Consultants
101 E San Fernando, Suite 402
San Jose, CA 95112
On Sat, 26 Jul 2003 11:36:37 +0800
"Wan Juang Foo" <f...@np.edu.sg> wrote:
>
> Dear All,
> I observe recently that some self blinking (and color
> changing) LEDs 'are'
> what seem to be a substantial emitters of radiated
> emission/interference.
> These LEDs are rigged up by hobbyists as decorative
> illuminators and acts
> more or less like the blinking lights for Christmas
> trees. I observe that
> each of theses circuits can be made to hung like
> Christmas tree ornaments.
>
> I came across a situation where the telephone lines were
> 'substantially
> noisier' :-) when several sets of battery operated
> circuits was about 3 m
> away from the telephone. It is not a very scientific
> method but I did a
> quick check and found by the method of elimination ;-) or
> what some would
> call systematic trials to find the source of the problem.
> It took me by
> surprise that the LEDs had a substantial role to play in
> the interference.
> LED circuits gets connected (on), hum gets louder. LEDs
> circuits gets
> disconnected, hums gets quieter and so on and so forth...
>
> These are very simple circuits with a single resistor and
> the LED in
> series. The 2 AA sized NiCad battery with holder, single
> resistor and LED
> including wire, total length about 5", tip to tip. These
> circuits were
> found to (well at any rate, seems to) emit interference
> that cause a
> telephone to pick up (50Hz) hum! It looks like the mains
> hum was pick up
> and modulated by the 'device' and reradiated or
> broadcasted...
>
> I can see that the blinking action at about 1 Hz have a
> duty cycle and that
> may generate a lot of ringing but what is surprising is
> the interference
> finding its way into the a telephone handset! I find it
> hard to believe
> that how the 'carrier' of the mains hum can eventually
> gets demodulated a
> puzzle.
>
> One wonders what can be observed if I get the circuits to
> a OATS? There
> again, how do I recreate the 50 Hz environment to couple
> the mains into
> these LED circuit? I had the Helmholtz coil in mind but
> can that be
> 'treated' as part of a test setup?
>
> Did anyone here have similar observation? One wonders if
> there any 'product
> specific standards' for such battery operated device that
> uses LEDs as
> decorative illuminators.
>
> :-)
>
> One wonders what will happen if there are such gadgets
> line up to 'hit' the
> market this Christmas...
>
> Looks like there will be a lot of testing work coming for
> EMC engineers!
>
> Tim Foo
>
>
>
>
>
>
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