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I have a project that I want to build but I'm not sure how to do it. I want to take 30 5mm white LED's and wire them up so that when it gets dark enough to see them they all turn on. I know it must be fairly simple but I can't find any instructions anywhere. What I am trying to find is:

1. How to power the LEDs? (the battery must not be too large because it needs to fit on a garden sculpture)
2. What circuit can I use to turn on the LEDs when its dark and turn them off when it bright?

Thank you if you can help
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You could use a light dependent resistor to check the light: have a look here. Then have a look at this multi LED tutorial.
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Assuming you want them all to come on together, with a higher than 5V supply, you can use a series-parallel combination.   

For example, let's say you have a 12V power supply and your LEDs are rated at 3.5V.   If you put two in series, that's 7V across the LEDs and 5V across the resistors.   With 30 LEDs, you'd wire-up 15 sets of those.   (The more you wire in series, the more efficient your system with less power wasted in the resistors.)

With a resistor current limiter, it's generally best (best for current/brightness control, but not for efficiency) if you drop about half the voltage (or more) across the resistor.  So if you want to go with 3 in series (10 sets), I'd say use at least a 15V power supply.

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1. How to power the LEDs? (the battery must not be too large because it needs to fit on a garden sculpture)
You might want to look-up some Amp-hour ratingsfor various batteries and do some Amp-hour calculations, based on the current required for the LEDs plus the Arduino and other circuitry.   

If you're not up-to-speed on Ohm's Law and Kirckhoff's Laws... Briefly, the same current flows through series components.  So, two LEDs in series share the same current, and use half the current of two LEDs in parallel (at the same brightness).

And, a constant-current switching LED power supply doesn't waste power heating-up current-limiting resistors.  That means your battery would last longer (at the cost of a more-expensive, or more complex, power supply).
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Thanks, That helps clear up some of the fog but is still leaving me with some questions. If I have the Series-parallel combination, and each pair of leds in series needs a total of 7V's then does that mean I need a 105 V = (15 rows x 7 volts each) battery? I also know I will need something to control or complete the LED circuit but what would that be? A relay? Transistor? MOSFET? And lastly what sort of circuit can be used to detect light level (I am hoping to not have to use an arduino for this to make it cheap to produce many)? I am assuming it will use a photo-transistor or photocell of some sort. And to make things easier these are the leds which I will be using. http://www.hacktronics.com/LED/White-LED-5mm-diffused/flypage.tpl.html
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Do you want them to turn on full or gradually brighten? I'd think that on full (while its still light out) then gradually dim as it gets dark, then possibly twinkle would be neat. Easy to do using the PWM Outputs.
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Thanks, That helps clear up some of the fog but is still leaving me with some questions. If I have the Series-parallel combination, and each pair of leds in series needs a total of 7V's then does that mean I need a 105 V = (15 rows x 7 volts each) battery?

No, just 7V. Try the LED Wizard and associated documentation for more help.

 I also know I will need something to control or complete the LED circuit but what would that be? A relay? Transistor? MOSFET? And lastly what sort of circuit can be used to detect light level (I am hoping to not have to use an arduino for this to make it cheap to produce many)? I am assuming it will use a photo-transistor or photocell of some sort. And to make things easier these are the leds which I will be using. http://www.hacktronics.com/LED/White-LED-5mm-diffused/flypage.tpl.html
[/quote]

Any of a relay, transistor, or mosfet would work. The mosfet would be the preferred method though (with the relay being relatively expensive and the transistor inferior to a mosfet in terms of efficiency).

The white LEDs you're looking at offer no datasheet that tells you how "good" they are and they're really quite overpriced. Over the past ~5 years LEDs have advanced quite a bit; you might be getting older technology here. You'd be better off digging through a professional site like Digikey/Mouser/Newark where there's a wider selection, specificiations available, and probably a cheaper price.
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How do I determine what MOSFET to use in my project. I know you have to check the maximum collector current (Ic) but then there is an Ib which is not on the datasheet. This is the MOSFET I have, will it work or do I need to go buy another one? http://inst.eecs.berkeley.edu/~ee40/su10/labs/Booster/IRF510.pdf

-thanks
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