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DiscoverCircuits.com -- Hobby Corner
"Hobby Circuits for
your Hobby Projects"
Last Updated on:
06/19/2009 02:29:16 PM
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Modified LED Night Light
designed by David A. Johnson, P.E. |
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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 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. 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. |
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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 |
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Before Modification -- After
Modification |
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Click on Schematic below to view PDF version of this
Circuit |
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