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Tuesday, November 29, 2011 08:14 PM
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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. |
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| 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 didn’t last but three months. |
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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. |
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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. |
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Front View
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Side View
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Before Modification -- After Modification
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Click on Drawing Below to view PDF version of Schematic |
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