# led plant light

Discussion in 'The Projects Forum' started by CoreyDondero, Mar 13, 2010.

1. ### CoreyDondero Thread Starter New Member

Mar 13, 2010
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So I have a cactus that never gets any light and I want to build a 100 LED circuit that I can plug into my wall outlet.

Here are the specs for my LED's

Reverse voltage: 5 V
DC foward voltage: 3.2 typical 3.6 max
DC foward current: 20mA

I have done some research and I think that I can connect four bulbs in series and just make 25 of these four bulb series in parallel. Will this work? What can I use to convert my AC power to DC 12V?

I know almost nothing about LED's so any help is appreciated, thanks.

2. ### Audioguru Expert

Dec 20, 2007
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You must limit the current with a resistor to less than 20mA for each series string.

Nobody knows what is the exact voltage and each LED is different. They might be 2.8V then 4 in series need 11.2V and will burn out if you feed them 12V.
They might be 3.2V then 4 in series need 12.8V and will not light if you feed them only 12V.

Use 3 in series and in series with a current-limiting resistor. Then you can feed them 12V safely. Use Ohms Law to calculate the resistor value.

3. ### Wendy Moderator

Mar 24, 2008
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Last edited: Mar 13, 2010
4. ### CoreyDondero Thread Starter New Member

Mar 13, 2010
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Thanks for all your input, now I just need a cheap way to convert my 120V ac to 12V dc. Any ideas?

5. ### retched AAC Fanatic!

Dec 5, 2009
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Wall wart. You can get them EVERYWHERE, cheap. You just need to be sure of the current.

Like a cordless phone plug, or a laptop charger.

6. ### Wendy Moderator

Mar 24, 2008
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The higher the voltage of the wall wart, the better, 24VDC being a practical max.

7. ### CoreyDondero Thread Starter New Member

Mar 13, 2010
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Ok thanks. So how can I figure out the current that I need?

8. ### retched AAC Fanatic!

Dec 5, 2009
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Once you decide on a circuit, you can calculate the draw. Then you choose a current above that.

9. ### Wendy Moderator

Mar 24, 2008
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Basically you use the Vf of the LED to put the LEDs in series. You use as many of them as you can, but leave it one LED short. If you have LEDs that drop 3.4V, and a 24V power supply, then you can have 6 LEDs in the chain (6 X 3.4V = 20.4). Subtract the total drop from the power supply, then divide by the current (lets assume 20ma). So it would be 24V - 20.4V = 3.6V, and 3.6V / 0.02A = 180Ω, which happens to be a standard resistor.

There are circuits that regulate current, so it doesn't matter how many LEDs you have in the chain. This is what I was showing earlier. At 20ma you probably won't need much in the way of heatsinking, but then again, you will need some.

Have you decided on a color mix yet?

If you haven't read the article, I strongly suggest you do so. It explains a lot of LED basics.

10. ### CoreyDondero Thread Starter New Member

Mar 13, 2010
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Thanks for that article, it was very helpful. I'm not sure what lights im gonna use yet though because I need to research what color would be good for a plant, but im thinking some bright white lights.

11. ### Audioguru Expert

Dec 20, 2007
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Pure white contains all colours.
But white LEDs are not white. They are made to appear white. They are actually a blue LED covered in a translucent yellowish phosphor. I don't think plants will like the missing reddish and blueish colours.

The web is full of Plants Lights circuits that use red, green (not much) and blue LEDs that can be added to make any colour and pure white.

12. ### retched AAC Fanatic!

Dec 5, 2009
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Brightness has little to do with photosyntheses. Brightness is a 'human eye' response to combinations and intensity of certian wavelengths. The wavelengths that are actively involved in photosynthesis are PRIMARILY ~460 and ~650/ Human eye is greatest between 500 and 600. So bright 1000w Sodium halides, are better for reading then plant growing. The reason they make them SO BRIGHT is because they have poor plant loving wavelengths, so they boost brightness to try to "fill the gaps"

The rest of the spectrum are usually chosen for display. If you are growing orchids(or other display types) and you want to see the beautiful colors in the pedals, you need a full spectrum array.

You have a BIG LEAF. basically. You can but LEDs with pretty near 460 and 650nm for little money. And because 1w leds run pretty cool(temp not color) you can put them VERY close to the plant without fear of burning the plants up.

If you need any more info than that, this is a area I am very experienced in.. In greenhouse and indoor.

[ed]
Green does 'zero' for photosynthesis. it is reflected by the plant, not being used. Remember, what WE see are the colors that are NOT absorbed by the objects.
[/ed]

Last edited: Mar 20, 2010
13. ### Wendy Moderator

Mar 24, 2008
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Don't know if it is true or not, but I remember reading in a scifi book that plants use near infrared. Any idea if this is true?

My hypothesis doesn't match this chart (from Wikipedia) at 460nm and 650nm. Are both wavelengths critical?

14. ### retched AAC Fanatic!

Dec 5, 2009
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Plants USE almost all of the visible spectrum. BUT only ~460nm and ~650mn for photosynthesis. The others are for patterns and colors to attract pollen transporters and the such. SO if you only had 460 and 650 on an orchid, it would grow, then never reproduce because nothing would bee (PUN) attracted to it to pollinate it.

You must understand how lighting filters work. When you see green, It is because it IS NOT being absorbed by the plant. Therefore, green does nothing for the plant (for photosynthesis) It can attract pollinators, however.

The reason the flowers are different colors/shapes/patterns is for reproduction. The GREEN where photosynthesis occurs.

Bill_Marsden, yes both wavelengths are critical, but not always the same amounts at the same time.

Plants use the naturally occurring changes in sun color throughout the year to know when to start vegetation and flowering cycles.

Last edited: Mar 22, 2010