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LED lights powered by a small Solar Cell???

Discussion in 'Electronic Basics' started by Codex23, Aug 14, 2003.

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  1. Codex23

    Codex23 Guest

    I am an art student with little to no knowledge of electronics, although I
    can figure just about anything out as long as i have someone
    to tell me where to start.

    I am looking to create an array of LEDs in any sort of pattern (creating an
    image), and have it powered by a small solar cell... preferably this solar
    panel would charge a small battery, and a small light sensor would tell the
    lights to become active (ie: the solar cell would charge the battery during
    the day, and at night the lights would come on, running off of the battery).
    simple enough maybe?

    here is the catch..... I cant have it be more than 1/3 of an inch thick,
    preferably 1/4 of an inch.... It can be as wide and as long as it needs to
    be, but it cant be any thicker than 1/3 of an inch. Any ideas???

    Thanks so much!
     
  2. Codex23

    Codex23 Guest

    well... ok. The apparatus would be placed outside in a sunny location. The
    number of LED lights could range anywhere from 50-150. I am also not opposed
    to using a battery that would just last a really long time, rather than
    using a solar cell to charge it back up, but still using a light sensor to
    only activate it at night.
     
  3. Codex23

    Codex23 Guest

    good thinking... ill give it a try.
    thanks.
     
  4. Rob

    Rob Guest

    Typical power consumption.

    Red LED 40mW
    Yellow LED 42mW
    Green LED 44mW
    Blue LED 80mW
    White LED 80mW

    Typical cell energy storage capacity.

    AAA alkaline 1650mW/Hr
    AA alkaline 3900mW/Hr
    C alkaline 11500mW/Hr
    D alkaline 27000mWHr

    AAA NiMH (750mAh) 900mW/Hr
    AA NIMH (1800mAh) 2100mW/Hr

    All of the above cells are too large for your application.

    Some cells with the required 1/3" thickness requirements:

    Varta VH600F6M (600mAh) 720mW/Hr
    Varta V600HR (600mAh) 720mW/Hr

    To calculate how long each cell type will last, divide the mW/Hr
    figure by the LED power consumption, for example red LED and a
    alkaline AAA, 1650/40 = 41.25 hours. If you used 50 LED's, the life
    would be 1650/(40*50) = 0.825Hr (49 minutes).
    If we take the number of LED's at 50 (Red) and use a light sensing
    circuit to limit operation to darkness only, say 9 hours, this is 50 *
    40mW = 2000mW * 9Hr = 18000mW/Hr. This is the required battery
    capacity to light 50 red LED's continuously for 9 hours. If we use AAA
    alkaline's as an example, you would need 12 of them. If we use the
    Varta rechargeable types, you would need 25 to give the same energy
    storage capacity.

    Now we need to charge the cells using a solar cell, which needs to put
    back during the day at least equal to that which was used during the
    night. As the sun does not shine every day, it should ideally have the
    capacity to generate several nights worth of energy during each sunny
    day to compensate for days when the sun does not shine.
    I will need some more time to calculate this, but a gut feeling
    suggests that it would need to be several Sq. feet in area. It will
    depend on location on the earth's surface on which the unit will
    operate. Where is this?
     
  5. Rob

    Rob Guest

    We have a energy demand of 18000mW/hr (18W/hr) each night.
    Let efficiency of solar cells be 10%.
    We thus need 0.18W/hr/day worth of solar cell area
    Assume an insolation value of 2.5kWhr/m^2/day.
    We thus need 0.18/2.5 = 0.072m^2 of solar cell area (approx. 10.6
    inches square). This is approx. equivalent to a '9W' solar panel.
    But we need some reserve capacity for cloudy day, say 3 days. So the
    cell needs to generate 3 days worth of operating power for each sunny
    day, so we need around 0.22m^2 of solar cell area (a '20W' solar
    panel).
    And of course, we need to be able to store three days worth of power
    as well, so we need to increase the cell capacity by a factor of
    three, around 54000mW/Hr. This is the equivalent energy storage
    capacity of a 12V, 4.5Ahr gel-cell battery, weight around 4 pounds and
    the size of a motorcycle battery.
     
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