Building Your Electronic Gadgets

Converting a Light Fixture to LED

Introduction

Headboard Before
Several years ago I built a headboard for the master bedroom. This included several light fixtures, the most significant ones being the individually controlled reading lights. These are “eyeball” swivels with dimmers, allowing my wife or me to read while the other one went to sleep. Over time we accumulated several complaints: The bulbs burned out every six months or so, at a cost of about $5 apiece and the bulbs radiated a lot of heat. We’ve already been converting from incandescent bulbs to compact fluorescent elsewhere in the house, however, we couldn’t use these because we wanted to use a dimmer. I’ve been experimenting with high power LEDs for a while and decided it was time to move something from the workbench to the real world.

(You can click on any of the pictures to see a larger view.)

2014 Update:

I posted this originally in 2010. The lamps are still working perfectly. In fact, one gets used daily for at least 12 hours. There are a lot more LED bulbs available to replace incandescent and compact fluorescent bulbs, however, none of them have the full range of dimming yet.

Preparation

I researched manufactured LED light fixtures and found that they’re not what I would consider mainstream. That is, I can’t go to my local hardware store and buy a LED light fixture for general lighting. I found night lights, closet lights, flashlights, accent lights, under counter lights, and such, but nothing for general illumination.
I did find some bulb replacements but they were not compatible with dimmers and the brightest I found was a 30-watt equivalent, for $30. I needed something dimmable in the 50-watt equivalent range.

This conversion is focused on portable or cabinet lamps designed to plug into a wall outlet. The principles could probably be applied to permanent fixtures, however, building codes may require the conversion and installation be performed by a licensed electrician. For this project, mains power ends at a power supply with all sorts of certification stamps on it. All the construction is on the 12-volt, and presumably safe, side of the power supply.

During my experimenting I tested different power supplies, built some of my own, and tried out various emitters from a variety of sources. For this project I opted for easily available off-the-shelf parts requiring little more skill to assemble than rewiring an ordinary lamp. Here is a rundown of the parts needed to convert an existing incandescent lamp to LED:

All Parts

  1. Light Fixture – This could be a cabinet light like the one shown, or a reading lamp.
  2. 12-volt, 2.5 Amp Power Supply – and cord.
  3. Heat-Conducting Epoxy
  4. Driver Wiring Harness – with potentiometer for brightness control.
  5. LED Driver and Dimmer – LEDs need a driver to control the current supplied to the LED. This particular driver is a small epoxy cube that includes connections for attaching a potentiometer for brightness control. The wiring harness with attached potentiometer plugs onto the pins of the driver cube.
  6. LED Emitter – The brightest single LED emitter I could find is the 567 lumen Neutral White 3-up Endor Star. It actually has three LEDs mounted on a single holder and comes in several shades of white. You’ll need to decide which shade or “temperature” you want. See the section on Color Temperature below.
  7. LED Lens – This is needed to properly disperse the light. The lens is matched to the LED so be sure to get the right one. Part numbers are listed below.
  8. LED Power Lead – This is from a burned out power supply. I recycled the transformer but kept the cable, just in case…
  9. Control Box Lead – This lead goes from the driver/dimmer control to the LED power lead. Having these two leads with a connector in between makes it easier to thread the wires through the headboard.
  10. Heat Sink – High power LEDs get HOT! Even though the LED is mounted on an aluminum carrier, an additional heat sink is needed. I used a surplus 2-inch x 2 1/4-inch x 1-inch surplus CPU heat sink. This will get warm, but not so hot you will burn yourself. This size heat sink fit easily into the lamp. For my second lamp I had to use a slightly larger heat sink and I needed to round off the corners with a file to get it to fit. Check to be sure your heat sink fits into your fixture before you attach the LED to it. (Over the years I’ve had many computers and as they get retired I always kept the heat sink. You never know when it might come in useful, right?)
  11. Knob – for 1/8-inch shaft.
  12. 2.1mm Power Jack – The power supply plugs in here.
  13. Plastic Box – Used for mounting the driver and dimmer knob. This gets mounted conveniently so I can reach it easily while I’m lying or sitting in bed.
    This one is the Hammond 1551 in bone, sized 3.15″ x 1.57″ x 0.8″.

 
Suppliers, part numbers and prices are listed in the Resources section below.

Tools

Tools I used standard tools that every Maker probably has:

  • The file, drills and nibbler I used to prepare the plastic box.
  • Assorted glues are used throughout the assembly: Household Goop, Silicon RTV and cyanoacrylate super glue.
  • I used the multimeter to verify the polarity of the power connections and for troubleshooting.
  • LEDs are very bright when viewed closely, and can blind you for several seconds, even if you just glance at them directly. This is because all the light emanates from a small spot, for example the 50-watt equivalent light from the LED in this project comes from three tiny spots 1/4-inch apart. The original 50-watt bulb spread its output over a 2 1/4 inch circle. To protect the eyes during experimenting and testing, I lay the lens from an old pair of dark sunglasses over the LED.

Assembly

Step 1 – Solder wire to LED

LED SolderedStart by soldering a power lead to the LED. Use the cable with the female 2.1mm power jack.

  1. Check the polarity of the wires. Usually, the wire with the white stripe is the positive lead which is connected to the center pin. Check this with the multimeter.
  2. Place the LED emitter on a non-heat conducting surface.
  3. Solder the wire to the pads on the LED. In the picture you see the wire with the white stripe connected to the solder pad marked with a plus sign. Since the pads are on a, albeit small, heat sink, you will need a 35 or 40-watt soldering iron to do this. You should only take a few seconds to make the connection. From experience I know that if you take too long soldering, the entire aluminum holder will heat up and you risk frying the LED or melting the little lenses.
  4. Put some tension on the connections to be sure you have a solid joint.
  5. Lay the LED on a flat surface and place the lens over it. All three feet of the lens should touch the surface through the cutouts in the aluminum holder. Clip off or carefully file down any excess solder that prevents the lens from fitting properly.

Warning: Do not connect the powersupply directly into this lead. You will burn out the LED!

 

Step 2 – Attach LED to Heat Sink

LED Attached to Heatsink with Lens

  1. Select a heat sink from your collection, or buy one from a PC surplus store. Make sure that the heat sink will fit into the light fixture.
  2. Sand and clean the surface of the heat sink where you will glue the LED. If you plan to color the surface, sand and clean the entire surface. Do not color the surface before you glue on the LED, this may interfere with the epoxy bond.
  3. If you have not soldered the wires onto the LED STOP! and complete Step 1. It will be very hard to solder anything to the LED once it is mounted on the heat sink.
  4. Squeeze out small but equal amounts of the heat-conducting epoxy – just enough to be able to mix it well.
  5. Apply thin films to both the LED and the heat sink and press them together.
  6. Clean up any excess epoxy, especially in the cutouts so that it does not interfere with the lens mounting.
  7. Rest a small weight on the LED, like the handle from some pliers, to hold it in place.
  8. Set aside overnight.
  9. Attach the lens by placing small dabs of Goop in three of the cutouts. I used a toothpick. Make sure the lens is well seated.
  10. Let glue dry for an hour.
  11. If the heat sink is not already colored black, you can use a permanent marker to color it. While coloring the heat sink is not essential, it does make it blend with the fixture.

Lamp Fixture Disassembled
 

Step 3 – Prepare the Light Fixture

While the epoxy is setting or glue is drying you can dismantle the light fixture. The major pieces snap apart and the cord can be disconnected with a screwdriver. The cord and ceramic socket will definitely find a new use in another project.

 

Step 4 – Assembling the Control Box

Since it only took a few minutes to dismantle the lamp, we can now move on to preparing the control box while we wait for the epoxy to set.

Prepared Box

  1. Drill a 1/4-inch hole for the potentiometer on the top of the box.
  2. Drill a 3/16-inch hole for the power cable in the end of the box.
  3. Cut an opening for seating the 2.1mm power jack in the same end. I used the nibbler for making the rough opening and then filed it for a smooth fit.
  4. Use some cyanoacrylate super glue to glue the jack in place. If the opening got too large, use the super glue to tack the jack in place and reinforce it with Goop, being careful to not get any on the contacts.



Wiring Diagram
This diagram shows how the various connections should be soldered.
The dotted lines represent the positive leads, which connect to the center pin of the plugs and jacks. This lead will usually have a white stripe and I always verify this with the multimeter to be sure.
Wiring Connections
I didn’t use the wiring harness because there wasn’t enough room in the box I wanted to use.Instead, I bent the pins and soldered directly to them.
 

  1. Use some red and black wire from the wiring harness to connect the driver cube to the power jack you mounted in the box.
  2. Separate the potentiometer from the wiring harness, solder it to the driver and mount it in the box.
  3. Thread the male power lead through the hole next to the power jack, solder it to the driver, and apply some strain relief. This can be a dab of Goop or, in this case, a zip-tie.

Before you continue: Double check your soldering connections
to be sure they match the diagram and picture!
At least twice!

Step 5 – Glue Heat Sink into Fixture

LED Glued in Lamp FixtureOnce the epoxy has set and the lens is firmly attached, the assembly must be mounted inside the lamp fixture. This is a matter of mounting a cube inside a sphere.
Fortunately there are some convenient holes in the swivel ball.

  1. Thread the power lead from the LED through the back of the swivel ball.
  2. Set the heat sink, LED down, on a beaker or pill bottle with a cutout for the wire. This assures the heat sink is level.
  3. Settle the swivel on the heat sink, making sure it is also level. You should be able to see the heat sink through some of the ventilation holes.
  4. Holding the heat sink and swivel firmly with one hand, squeeze generous amounts of RTV silicon glue into four holes that are close to a heat sink edge. Unlike the Household Goop, the RTV will not flow in small spaces and is therefore a great gap filler. The result should look like the picture.
  5. Let the glue dry, preferably overnight.

Lamp Reassembled
 

Step 6 – Reassemble the Lamp

Thread the wires through the openings and snap the lamp parts together.

Plug Connections

Step 7 – Connecting and Testing

Connect the various leads as shown:

  1. The female connector from the lamp connects to the male connector from the control box.
  2. The male connector from the power supply plugs into the control box.
  3. Connect the power cord to the power supply and plug it into a switchable power strip that is turned off.
  4. Turn on the power strip.
  5. If the light comes on, Great! Test the dimmer control.
  6. Plug Connections

  7. If the light does not come on, turn the knob on the dimmer in the opposite direction and turn on the power strip.
  8. If the light still does not come on, leave on the power strip, twist the knob in both directions.
  9. If the light still does not turn on, go to the Troubleshooting section below.


Installation

Comparing Incandescent and LED
 
After I finished one of the conversions, I installed it to see how it would compare to the original incandescent lamp. You can see the result.
The LED is closer to daylight in color and seems to put out a bit more light. I tested both lights with a Watt-Minder and found that the original fixture draws 50 watts and the LED fixture draws only 13 watts, close to one fourth as much. In addition to saving on light bulbs, I will also be saving on electricity.

Economizing

The cost of converting the existing fixture was a little over $75.00, not including shipping charges. That does not include the original price of the lamp or scrounged parts like the heat sink and male power lead. You can get some additional cost savings in the following areas:

  • An alternative to using the heat-conducting epoxy ($12.99) is to use cyanoacrylate super glue. This requires that both surfaces are flat. The heat sink is already flat, but the aluminum “stars” are frequently rounded or have rough edges. Rub them on a flat file until you see file marks on the entire bottom of the star. Then, after soldering the wire, put one small drop of super glue on the star and press it firmly against the heat sink. Hold it for 30 seconds. One advantage to this is that you can later remove the LED from the heat sink by gently tapping a single-edged razor blade into the glue joint.
  • The wiring harness ($4.99) is a convenience for testing and providing the potentiometer. In this project we only used the potentiometer and some portion of the wires from the harness. If you have a 5000 ohm linear variable resistor in your inventory, you can use that instead, along with some scraps of hookup wire. I used a sliding resistor for another conversion as you can see from my ultra-portable reading light in the next section.

Another Conversion

Reading Lamp
My reading chair backs up to a bookshelf and there is not a convenient place for a reading lamp. I wanted an adjustable lamp that would not clutter the floor and would be easy for me to reach. I was experimenting with one of the heat sink, LED and lens assemblies and realized that I just needed a simple mounting and I would
have a powerful, ultra-portable lamp. I attached a small woodworking clamp to the heat sink with some compression washers and I had my reading lamp.
It has also come in handy as a trouble light.

If you look closely, you’ll notice that the LED star is fastened with some nylon screws. I drilled and threaded some holes in the heat sink for easy prototyping.
I also used some heat sink compound between the LED and the heat sink. There is a single cable going from the control box to the LED. Since I was not going to be threading any wires through a headboard, I didn’t need the plug and I could use a shorter cable. Power leads 8 and 9 are combined into a single connection, made from some scrap speaker cable. The control box shows a sliding resistor installed.

Troubleshooting

If the LED does not turn on you need to check the components, basically three pieces:

  • the power supply
  • the driver and
  • the LED itself

Debugging the LEDTroubleshooting consists of beginning at one end of the system and proceeding piece by piece until the trouble is found.

  • First, disconnect all the connectors.
  • Starting at the power supply, plug in the power supply and check the output with the multimeter. It should read 12 volts.
  • Plug in the control box into the power supply and check the output. It should read zero with the dimmer knob turned counter-clockwise, and 12 volts with the knob turned clockwise.
  • Test the LED with a 9-volt battery and a 470-ohm resister.
  • Recheck all the connections according to the diagram.

Color Temperature

Ordinary light actually comes in a variety of colors. As we move from one lighted area to another we have a general impression of being in plain “white” light. But as any photographer knows, this is not the case. A picture taken outdoors looks very different from a picture taken indoors with fluorescent lights, which is different from a picture taken under incandescent lights. Our eyes adjust naturally and normally don’t notice much difference. Actually, visible light has a characteristic referred to as “color temperature”. Until recently only photographers and interior designers paid much attention to it. With the advent of compact fluorescent lights (CFLs) and LED lighting, we now have a choice in the type of white light we use to illuminate our lives. You might use a certain color for accent lighting or for watching TV. Your plants will do best under a different color, and yet another color may be best to illuminate your drafting table.

Most CFLs and fluorescent tubes are marked with a color temperature, along with descriptions such as “Warm White”, “Daylight” and “Bright White”. If you want to match the warm glow of incandescent bulbs, it’s best to look for a color temperature of 3000K or less. I personally prefer lighting as close to daylight as possible, between 5500K and 6000K. For the bedroom lights I compromised at 4000K. This web page has a handy chart of temperatures and the type of lights that produce them. The following Wikipedia page goes into more detail.

Color Temperature Comparison
 
To help me decide which color LED I wanted for my lamps I set up a test. This consisted of taking one LED of each of the four colors I had at the time and comparing the light output. I mounted them in a white shelving unit backed with white photographic paper so each would illuminate a separate storage compartment. You can see the result. This is a single image taken with my electronic camera to ensure that color differences are not the result of different camera settings.

Resources

#
Description Supplier Model Number Price
1
Light Fixture Woodworker’s Supply 804-900 37.39
2
Power Supply LED Supply 12VDC2.5A 14.99
3
Silver Epoxy LED Supply ASTA-7G 12.99
4
Wiring Harness LED Supply 3021HEP 4.99
5
LED Driver LED Supply 3021-D-E-700 14.99
6
LED Emitter LED Supply 7007-PW740-N 16.36
7
LED Lens LED Supply DB-OPML-3-025 4.99
8
2.1mm Male Power Lead Surplus / Junk Box from old power supply
9
2.1mm Female Power Lead All Electronics CB-216 3.00
10
Heat Sink Surplus / Junk Box at least 2″ x 2″ x 1″
11
Knob for 1/8″ Shaft All Electronics KNB-224 1.00
12
2.1mm Power Jack All Electronics DCJ-1 .35
13
Plastic Box All Electronics 1551-KGY 2.10
Total: 113.15
  Posted: 2010.06.30       Updated: 2014.08.15