A Curtain of Light

It's dark outside this time of the year here in Sweden and the plants in my window crave additional light. This was when I came up with the idea to create a narrow curtain of light from above which would just light up the plants but not be blinding into the room, nor waste too much light and energy by shining through the window.

Essentially this is a follow-up project of my Remote controlled LED ribbon on another wall in my living room.

A quick sketch of the window area on the back of an envelope gave me the basic geometry. The ceiling height of my living room is 240 cm, my plants are at the level of the window sill, elevated by about 30 cm off the floor and I estimated the best location of the lamp to be about 20 cm below the ceiling. The plants are standing on the window sill and on wooden stands extending about 50 cm into the room.

The geometry gave me a half angle of the necessary light cone of 10 degrees.

I knew already that there are lenses with different opening angles available for standard power LEDs on so-called star-mount baseplates. However, it is not clear from the general description of these lenses, whether the angle is given as the total opening angle of the emitted light cone, or the half angle. But since this is not an extremely critical application I decided to buy lenses with a specified angle of 15 degrees.

The next choice is the color of the LEDs. Plants essentially only need red and blue light for photosynthesis, but these purple looking plants lights might not be something you want to have in your living room window. Instead white is probably the color of choice, but which white: cold white, warm white, neutral white,...

I finally placed my order:

• 50pcs 15 degree led Lens for 1W 3W High Power LED with screw 20mm Black holder
• 50pcs 3W neutral white 4000-4500k LED + 20mm star PCB

...and the waiting began. Delivery time from China to Sweden is normally between 2 and 4 weeks, and it has happened in the past, that I had forgotten for which project I had ordered the parts. There was little risk this time, because the increasing darkness of November reminded me daily of this project.

Awaiting the shipment from China I did some more back-of-the envelope calculations. My living room window is 250 cm wide. I knew from previous projects that I could easily get different aluminum profiles in lengths of 100 cm and 200 cm and decided that a total length of 200 cm would enough for this design.

So how many of the 50 LEDs would I need? I made sketches with LEDs spaced 7 cm, 8 cm and 10 cm apart. In commercial LED strips and even many LED light bulbs you will find groups of 3 series connected LEDs with an additional dropper resistor giving a system voltage of 12 V.

So my options were:

1. 7 cm spacing: 9 groups of 3 LEDs or 27 LEDs
2. 8 cm spacing: 8 groups of 3 LEDs or 24 LEDs
3. 10 cm spacing: 6 groups of 3 LEDs or 18 LEDs

I decided to go for solution 2 with a total of 24 LEDs. There will certainly be use for the remaining 26 LEDs and lenses in a future project.

Finally my LEDs have arrived. I have used star-based power LEDs before, but I have never seen them on this kind of framework - directly off the production line. The soldering certainly looked ok, but there was a lot of flux residue and heatsink compound around the chips on the background of the black solder mask on the boards.

My first plan was to construct an individual driver circuit for groups of three LEDs in series. But then on the other hand, I wanted to finish this project before the darkest time of the year really set in. Therefore I decided on the much simpler circuit which is also used on the regular type of LED stripes and in many LED bulbs: three LEDs in series, connected to a 12 V power supply with a additional dropper resistor in series.

The general idea is that individual variations between LEDs will cancel each other out when connecting a couple of them in series. The dropper resistor will then stabilize the current through the string of the LEDs. However, when you buy your LEDs from an unbranded Chinese supplier, you will not get any reliable data for the LEDs. Even the power rating might be more wishful thinking than reality...

In order to properly design the circuit I hooked up three LEDs while these were still connected to the cake of 25 LEDs and with my bench power supply I measured the "typical" characteristics of these three LEDs. You can see the current-over-voltage and power-over-voltage curve in the graph above. The LEDs were actually sold at a rated power of 3 W each, however at 3 W the current-voltage-curve already looks as if it was dominated by the internal series resistance of the LED chips.

As you can see, with a 2 Ω resistor and connected to 12 V the LEDs would end up at a working point with a current of slightly below 500 mA and a power of 1.8 W per LED. A reasonable value I believe and that's also what I went for.

What about the resistor and its power rating? At 500 mA there will be a voltage drop of 1 V over the resistor and hence it will dissipate 1/2 W.

Having decided on the electrical design I went to the local hardware store Bauhaus and browsed through their supply of aluminum and plastic profiles. With me I had one of the LEDs and lenses in order to test the physical design by experiment rather than just a drawing.

I found a nice aluminum profile which came in 1 m and 2 m lengths:

• Gust. Alberts GmbH & Co. KG
  a x b x c 30mm x 20mm x 2mm
  

  4004338474744
  2m

I also found an almost matching black PVC profile which will be used as a cover

• Gust. Alberts GmbH & Co. KG
  a x b x c 25mm x 20mm x 2mm
  

  4004338479145
  2m

Having purchased the profiles it was time to prepare the mounting of the LEDs and lenses. I used the previous CAD design to print out a template and then pre-punch the 48 positions for the mounting holes.

Using a drill press I first drilled the holes for the matching 2 mm screws to attach the lens holders to the aluminum profile. The star-pcb LED bases will then be fastened between the aluminum profile and the lens holder.

After all 48 holes were drilled I counter-drilled them from the reverse side to be able to later hide the screw heads and get a flat backside of the whole LED rail.

The drilling continued with the black PCV. Here I drilled 20 mm diameter holes at the positions of the LEDs. Since the width of the PVC profile only was 25 mm minus the 2 mm thickness of the material, there was not much place to spare.

Now we come to the LEDs themselves.

I prepared 24 pieces, each 5 cm long, of red and black wire by stripping off the insulation and tinning the ends. In order to solder the LEDs together I then soldered a red wire to the positive contact (anode) and a black wire to the negative contact (cathode) for all 24 LEDs.

Having prepared all LEDs it is now time to mount them onto the aluminum profile.

I found that (metric) 2x10 screws were the ideal fit for the thickness of the aluminum profile and the mounting studs on the lens holders. They also neatly fitted into the countersunk holes I had drilled before.

Each LED base gets a dab of thermally conductive heat sink compound on the back, before it was clamped between the aluminum profile and the lens holder by screwing the lens holders from the backside onto the aluminum profile.

Repeat 24 times...

Now it is time to connect the LEDs together. Just to remind you of the general schematics:

• each group of three LEDs is connected together by soldering the red wire of e.g. the left diode to the black wire of the right diode
• make sure to isolate the solder joints with heat-shrink tubing
• all open black wires at the end of the groups are connected together to a common ground (-) rail
• a second common rail supplies the +12 V to all groups
• the LED groups are connected to the +12 V rail through a 2 Ω resistor

As explained before, the power dissipation in the resistor will be 1/2 W - there will be a current of 500 mA causing a voltage drop of 1 V. The resistors I used are rated for 2 W power dissipation and I soldered them onto small pieces of copper-clad circuit board as isolated standoffs off the aluminum profile.

Before putting the black PVC cover over the aluminum profile to hide away the wiring, it is time for a first test. Using a switched mode power supply with 12 V rated at 5 A, I connected the full LED string to the power supply.

And it works!

Then it was time to actually mount the LED light above my plants in the window. I used an IKEA curtain rod to give the construction more stability and currently everything is held in place with cable ties.

• 24 power LEDs (EUR 0.30/piece)
• 24 LED lenses with holder (EUR 0.20/piece)
• 2m aluminum profile (EUR 10.-)
• 2m PVC profile (EUR 4.-)
• 48 metric 2x10 screws (EUR 0.05/piece)
• 4m red isolated copper wire 1.5 sqmm/16AWG
• 4m black isolated copper wire 1.5 sqmm/16AWG
• heatshrink tubing
• thermal heatsink compound
• switch-mode power supply 12 V at 5 A (EUR 10.-)