I wonder if the self blinking LEDs might have an oscillator running at roughly 50 Hz. And, the signal pick up could be magnetic field coupling to the telephone.
Dave Cuthbert Micron Technology From: Wan Juang Foo [mailto:f...@np.edu.sg] Sent: Friday, July 25, 2003 9:37 PM To: emc-p...@majordomo.ieee.org Subject: self blinking LEDs as EMI sources WAS: LED lamps 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 This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: emc_p...@symbol.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: emc_p...@symbol.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
