David Butcher: Solar Powered Capacitor Storage Automatic Outdoor Lighting

The Summer of 2005 was interesting, with soaring energy prices, and some close calls for blackouts in California. I had decided to install solar powered lighting in the rim of our in-ground pond (which is kept full with waste water from our reverse osmosis system, but that is another story).

With all the energy news in mind, I decided the lights had to be:

Well, I can assure you that such a thing, either assembled or in the form of circuit diagrams, does not exist. Until now.

I spent a long time studying everything I could find on the Web (thank you, Google) and I eventually figured everything out. As I write this, the LED's have turned on automatically at dusk for the first time. Here are the parts I used and the techniques I used to make it all work together:

  • Solar Panel - The solar panel I used has a long, long history. It is about 25 years old. It is the generation component of a Solar Attic Fan from William Lamb company, circa 1980. The panel is aluminum-backed, and it is essentially pure silicone rubber poured around 12 2 inch diameter Solarex solar cells. In full sun it puts out about 5.7 volts. I have not measured the amps for a while, but I think it's about 350 milliamps. It's nice that such an old device can still serve a useful purpose.
  • Wiring - Somehow, I had to get electricity to LEDs spaced around the outside of the pond. The length of the cable was almost 80 feet! With that length, heavy wire was called for. I used Carol 12 gauge "Super Heavy Duty" outdoor low voltage outdoor cable. With a run that long, you can't wire like this: 
    Plus  -----------------------------
      |   |   |   |   |   |   |   |
     LED LED LED LED LED LED LED LED
      |   |   |   |   |   |   |   |
Minus -----------------------------
    The last LED will see a much lower voltage than the first. Using what I learned from a detailed website describing how to wire multiple batteries in parallel, I wired the LEDs up like this instead: 
    Plus  -----------------------------
      |   |   |   |   |   |   |   |
     LED LED LED LED LED LED LED LED
      |   |   |   |   |   |   |   |
      -----------------------------  Minus
    What a clever idea. The positive and negative are at opposite ends of the cable. The cable forms a huge loop around the pool. Here is another view, imagine that the "=" signs are two conductor cable: 
    Plus
v
========================
                       ||
Minus         The Pond ||
v                      ||
========================

    The result is that the length of wire to each LED is the same, regardless of where the LED is in the string. And THAT means the LEDs all see the same voltage. And THAT means they all seem to be equally bright. That was a tough one to figure out, but there it is.
  • LEDs - I knew I needed bright LEDs, and I preferred white LEDs. I searched many, many sites for the brightest I could find AND afford, and settled on 18,000 MCD white LEDs. Let me tell you, that is BRIGHT for LEDs. I have 7 LED's in the circuit right now, and they run all night. They do gradually get dimmer as the capacitors discharge, but they never go out.
  • Wiring Brackets - somehow, the wiring had to be attached to the back side of the glass blocks that made up the side of the pond. I could not find any kind of clip that would work, so I had to invent one. Using silicone rubber aquarium sealer, I glued two pieces of aluminum channel together to make a unique new shape. It's called "Handi Metal" and all hardware stored carry it. Here is what is looked like, looking at the end of the metal pieces: 
    First Piece
| |
|_|

Second Piece

|   |
|___|

Glued Together

| ||   |
|_||___|
    I then used a hacksaw to cut the new shape into pieces about three quarters of an inch long. I glued each piece to the back of a glass block with silicone rubber. The result was a "trough" held out from the back of the glass block by the width of the wider channel. The wire was placed in the thinner channel, like this, as seen from the side of the glass block: 
             ------------------------------------------
        |                                          |
        |                                          |
        |                                          |
|0||   ||<-- Back of the glass block               |
|0||___||       Front of the glass block --->      |
        |                                          |
        |                                          |
        |                                          |
	 -------------------------------------------
    The Zeros in the channel at the back of the block represent the cable conductors. I carefully slit the cable insulation, pried the stranded conductors apart a bit, and the inserted the two legs of the LED into the conductors, and then soldered a few strands of each conductor to the leg of the LED. I finished each light position by sealing everything back up with a generous application of silicone rubber.

    You may wonder why the wires could not just be glued to the back of the glass blocks. The LEDs stick out about a quarter of an inch from the cable. By mounting the cable half an inch back from the glass block surface, there is clearance for the LED to aim AT and THROUGH the glass block.

  • Power Storage - I wanted something that would last a LONG time, and not be toxic, and that would help support an industry that I believe is going to change our lives in the future. Instead of batteries, I decided to use Ultracapacitors. I bought me four 100 Farad Elna Ultracapacitors for my birthday. They are rated at 100 Farads and 2.5 volts maximum each. I wired them up in series/parallel like this: 
    
------------------ Plus
   +         +
 Cap 1     Cap 2
   -         -
------------------
   +         +
 Cap 3     Cap 4
   -         -
------------------ Minus
    That arrangement give a storage system that is rated at 5 volts maximum, and 100 Farads. Not shown are the overvoltage diodes I added to each capacitor to limit its voltage to about 2.4 volts.
  • Automatic Dusk/Dawn Sunset/Sunrise Controller - I looked everywhere on the Web for this, and nothing seemed to be close. I was forced to design the circuit myself. I saw a few circuits that used Op Amps, and some used CDS cells to detect light levels, but I wanted something simpler. I decided to use two IRF511 N-Channel Hexfets that I had left over from a previous experiment.

    I am not an electronic engineer. I do know a thing or two about electricity, and I studied circuits examples from many sites until I figured out how to hook these MOSFET devices up. The resulting circuit is very simple, and I think it's pretty elegant. Someone will probably write me and tell me I need to update the circuit, but here is the schematic - and as of tonight, it's working! 

         Plus                              Plus
      --------------------|<----------------
     |             |   |         |    | o o | S P
     |            LEDS R1        R2   | o o | o a
Capacitors         |D  |         |G   | o o | l n
     |             *G------------*D   | o o | a e
     |             |S            |S   | o o | r l
     |             F             |    | o o |
     |             R3            |    | o o |
      --------------------------------------
     Minus                             Minus
    It's a little hard to read, but not that hard. Operations is simple. R1 "wants" to turn the first MOSFET "on." That means the LEDs would be on all the time if that was the entire circuit. If the voltage introduced to Gate of the first MOSFET were removed somehow, it would shut off. That's where the second MOSFET comes in.

    The second MOSFET is "on" whenever there is sufficient voltage present on it's Gate. That happens whenever the solar panel is producing power (during the day). If the second MOSFET turns on, it conducts the voltage provided by R1 to ground, turning off the first MOSFET. The second MOSFET is only on when the solar panel is making power. Presto. Solar panel off, LEDs on. Solar panel on, LEDs off.

    So, add it all up: LEDs are extremely efficient, Ultracapacitors can be charged and discharged tens (maybe hundreds) of thousands of times, the cable is routed to provide consistent voltage to each LED, the LEDs become even more efficient as the voltage drops, and will last for one hundred thousand hours or more (much of the time they are working at less than maximum power as the diminishing charge on the Ultracapacitors gradually tapers the voltage down...), and the solar panel is very small - about 6 inches wide and 20 inches long.

    It is a maintenance free, automatic, alternate energy system using all state-of-the-art technology, and best of all:

    The Pond looks GREAT at night.

    David "Photons, not Neutrons" Butcher

    Check out my other whacky experiments as well!

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