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Modified LED Night Light
designed by David A. Johnson, P.E.

 I bought some night lights a few months ago from Walmart here in Denver.  The devices were made by Elumina Lighting Technologies Inc. (www.eluminalighting.com), their model LED-6200A.  I think I paid about $15 for a pack of three lights.  I wanted a light which used LEDs instead of the usual 4 watt incandescent lamp.  The 4 watt lamps used in the typical night lights always seem to burn out after 3 months or so.
I use the lights to illuminate a dark hallway and provide some light into the master bathroom. In theory, a LED light should last for 10 years or so.  But, as I found out, the original Chinese made unit from Walmart didnít last but three months.

The Walmart night light looks a bit like a computer mouse.  It has a pushbutton switch, to switch between a dim and a bright setting.  It also has a CdS photocell, which turns off the device during the day. Inside were three white LEDs, wired in series.  When I first plugged the device in, it seemed to emit an acceptable amount of light.  But, after only three months, the light gradually faded until it was virtually useless.  This has happened to me several times before with other lights I have tried and results from the use of cheap inferior white LEDs, which have phosphors that quickly fade. I decided to open the thing up.  I traced out the circuit and determined it was the kind of circuit which I could easily modify.  I made some component value changes and pulled out the three dim LEDs.  I then replaced the three LEDs with 10 high quality super bright surface mounted units, which I soldered together into two strips of 5 LEDs each.  

The LEDs I chose were made by Osram Opto, their model LW-E67C-T1V2-5K8L-1-Z. I bought 30 LEDs through Digikey, so I could modify three night lights.  I paid about $1.00 each for the LEDs. The result was very pleasing.  The light now emits a nice bright white light and since I used quality white LEDs, it should last many years.  In the photo below, note the difference in the light output before and after I made the changes.  Also note the original and the modified schematics. 

The circuit uses the classic series capacitor method to produce a current limiting LED driver, powered from the AC line.  The series capacitor forms a constant current source and will route the same amount of current through the parts, even if 3 or 30 LEDs were used in series.  This type of circuit made it much easier to insert 10 series wired LEDs instead of the three original devices.  I also switched from a NPN darlington driver circuit to a single n-channel FET. This made it easier to select the right resistor value to turn off the light in the day.  I set the required CdS cell resistance at about 100K, so anything less than that in resistance should keep the LEDs turned off.  I did have to change the resistor R2 to a 39v zener diode, to limit the peak voltage across the filter capacitor C1, when the LEDs are turned off during daylight. 
Front View
 
Side View
 
Before Modification  --  After Modification
 
     
     
     

Click on Drawing Below to view PDF version of Schematic

Circuit Modified LED Night Light by David Johnson



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