HOME Schematics Discover Forum Hobby Corner Dave's Circuits Electronic Resources Book Corner Contact Info


  Circuit Diagnsis

  Previous Issues

Modulated Light Interference

I got a call from a music listener.  He was begging me to help him with a problem. A few years ago he had purchased a very nice cordless audio headset.  The system worked well and he used it often.  Then, he moved to a new house.  Once he got settled, he tried using his headset but it would not work. Or, more precisely, it would not work very well.  It made some terrible noises on top of the music. I asked him to describe the thing.  Based on his description it was clear that it was an infrared type and not an RF device.  It used an array of infrared LEDs to transmit the audio signals to the portable headset.  I asked him a few more questions about how he used it, what kind of room, the nature of the walls and such. 
I had my suspicions about the cause of his problems so I asked him what kind of lamps were in use in the room where his audio player was located.  He was puzzled by the question but laid the phone down while he checked.  About a minute later he came back to the phone and said that all the lamps were compact fluorescent lamps (CFLs).  Bingo!  I asked him if he ever used such lamps in his old home.  No, he used standard 60w incandescent lamps at his old house.  I told him, I was fairly certain that I knew the problem.  As proof, I asked him to perform a simple experiment.  I told him to try his headset with all the lights turned off in his room.  Since this was in the middle of the day, he should have enough light in the room to see what he was doing without the use of any of the lamps.  Again, he laid the phone down and was gone for a couple minutes.  When he came back on line, he was excited.  He said with the lights off, the headset work perfectly.  He then asked me what was causing the problem.
Compact fluorescent lamps (CFLs) are terrible modulated light sources.  Some CFL devices are worse than others but nearly all splatter out all kinds of ugly light pulses (see oscilloscope photos below).  In some cases, their modulated light can influence how infrared remotes work on TVs and DVD players.  I have heard cases where some nearby CFL lamps completely killed remote TV operation.  What is the fix?  Donít use CFLs in situations where their light can cause problems.
The infrared remotes on TVs and his cordless headset use light detectors with visible light blocking filters but they can still be overwhelmed by some nearby CFLs.  As more and more of these lamps become standard hardware, the light spectrum will continue to get noisier.  Lights using LEDs should be much cleaner but their use has not yet overshadowed CFL lamps.
Wireless IR Headset CFL Lamp
Modulated Light from CFL 50KHz and 140KHz Optical Noise from CFL

Issue 2 -2011     February

Page 1 Back
Envelope
Circuit
Diagnosis
Experimenter's
Corner
Good Idea
gone Badly
New Products Rants &
Raves
What the World
needs Now
Wily Widget


HOME Schematics Index Hobby Corner Dave's Circuits Electronic Resources Contact Info
Imagineering Ezine    Discover Solar Energy Dave Johnson & Associates Faraday Touch Switches


 About Us   |  Advertise on DiscoverCircuits.com   |   Report Broken Links  |    Link to DiscoverCircuits.com  |    Privacy Policy

Copyright  2002 - 2015 David A. Johnson & Associates.  All Rights reserved. 
 Linking is ALLOWED but COPYING any content or graphics to your web site is EXPRESSLY PROHIBITED.