Lots of amazing information here. I am a newbie to wiring, soldering and not able to process what everyone is saying, as most of it sounds like a foreign language. So, I am going to read all the posts a few times and put together a wiring plan and then post it here for your critique. I will try to have it posted by late next week. A BIG THANK YOU to everyone who's posted. I've learned so much! SpaceGrab is gonna be glowing this year for sure.
For clarification: the LEDs in the 35 LED strings are strung with the LED sitting between the two wires. Each LED is encased in plastic on top of the soldered connections. It looks like a ladder with the LEDs as the steps and the wire as the legs. Or, you could also say it looks like a bunch of letter H stacked on top of each other. I believe this is parallel, right? I hooked a 4.5v source to a strand without the resistor and the first two LEDs fried, but the rest of the string kept working (one I put the resistor in!) so it must be parallel? Everyone is saying this is not possible, but it seems to have worked at BM all week long last year! Maybe ignorance really is bliss?
Ah, that's good information. It sounds like it's using the bad--but inexpensive--method of distributing the current. If any of those LEDs fail, more current will go through the others and shorten the lifespan. At that price though, it's probably expected. At least it's not those incandescent Christmas bulbs that really start to burn up.
If they're red LEDs, they'll probably drop 2.0V, so the strand is using:
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Total current = (4.5V - 2.0V) / 2.7 Ohm = 0.93Amp
Current per LED = 0.93A / 35 = 0.026Amp
Watts per strand = 4.5V * 0.93Amp = 4.18W
The earlier replies' ~20mA estimates above should be roughly correct. Each strand will use about an amp of power, which is actually bad for Alkaline battery lifespan. You'll actually get more lifetime from your deep cycle batteries because they can tolerate higher discharge rates. Car batteries are not good though.Wasted power
If you use a 12V power supply and simply replace the resistor, to get the same amount of current (1Amp) you would need to replace that 2.7Ohm resistor with a higher valued resistor:
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Resistor = (12V - 2.0V) / 0.93A = 10.75 ohm
The problem? Now you'll be using about 12V * 0.93A = 11W per strand
--and 9W worth being just useless heat! That sucks! At about 1 Amp per strand, and 3 strands, your 220AH batteries will only last 220AH / 0.93A = 73 hours if you completely drain them to death (bad to do!), and for only three strands. Even if you use a 6V battery, you'll still be wasting energy as heat.
A better solution is to buy one of those step-down converters One Seeing Eye suggested, and set them for a voltage just above the forward voltage of your LED strands (if they're red, that is 2.0V, for other colors it'll be different). For example, get one and set it for 3V, and replace that 2.7 ohm resistor with a 1.1 ohm resistor
(1.075 is the exact value, but that'd be hard to obtain). If you're going to buy resistors anyway, why not save some power? The math:
You could also try connecting the strands together end to end.
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(3V-2V) / 1.1ohm = 0.90Amp
Current per LED = 0.90 / 35 = 25mA = probably longer lifetime for your LEDs anyway.
However, if the way the current regulation is done now
is a bad idea, it's a worse idea to put those parallel LEDs in series with each other. The different voltages, currents, LEDs manufacturing, etc. will all affect each other and if some of the lights burn out, all your strands could die.Multimeters
Something you should also obtain is a multimeter
. Here's a cheap one
at Sparkfun (though before you buy it or something else, see if there's anything else to buy to combine shipping). The reason you'll want one is so that you can check voltages, and also troubleshoot things if things go wrong. It is also for safety reasons: you want to make sure that you have no shorts (metal connections) between the battery power and ground. You can literally burn metal if you accidentally connect the terminals, especially on those deep cycle batteries. Use the multimeter to make sure that your grounding block and all the other electronics are properly wired.
If you get a multimeter you should also check the forward voltage on those LEDs. Reds are usually 2.0V, but it might be different. If you get other strands and colors you might as well check those too.Sequencing and control
But wait, before you buy anything, see if you want to do some sequencing with your lights. For example, you can easily hook a transistor (MOSFET) in series with the strands and switch them on and off
. They're pretty cheap
and you can use kits
for them too if you pay a little more ($4).
How to sequence them? Connect the control (gate) pin of the MOSFET to a switch (see top figure of this
) or to something like the output of a microcontroller (like an Arduino board). If the voltage is high the light will turn on, if low, the light will be off. Also note that the MOSFET drops some voltage too (1.5V in the above kit's case), so that needs to be factored into your chosen resistor value and power supply. Example:
MOSFET source-to-drain voltage = 1.5V, LED forward voltage = 2.0V, choose a cheap 0.5ohm resistor
($0.14) for current regulation
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(PowerSupplyVoltage - 1.5V - 2.0V) / 0.9A = 0.5 ohm
PowerSupplyVoltage = 0.9A * 0.5ohm + 1.5V + 2.0V = 3.45V or about ~ 3.5V
Even easier is to just measure the voltage across the resistor to determine the current, and adjust it until you get what you want. The setup above uses 0.9A * 3.5V = 3.15W, which is less than the original strand--and you can make them blink.
project. You're not obligated to use any of the above, but it'll give you an idea of what can be done if you want to.