I bought a couple of inexpensive 20 LED strings that are battery-operated. I should have paid more attention a while back when some were writing about
their own experiences using this sort of thing, but I didn't know where to find any then so I lost track of which threads may or may not have explained in detail how to do this.

I know I can fit them in as-is on some of the models I have, but I would like to use them in some other kits as well but don't want to ruin the light strings and have to toss them out.

1. If I want to use them do I have to use the string lights all-or nothing, or can I cut off lights at the end of the string I do not need?

2. Can I add wire to extend the wiring of individual LED's in the string?

3. Can I add wire to extend the distance from the batteries to the beginning of the lights?

I am thinking I have to use the whole light string as-is to prevent screwing up the voltage/amps/whatnot and thus causing the lights to burn out or not work at all. I thought I would ask and see what info I can get about doing this.

Thanks!

Great question, I don't know the answer but i was going to ask it myself. at least the leds run cooler and are brighter than those other battery lights. how bright are yours ?

Answers...

1. If the LEDs have individual resistors on them, you can cut the string. If they don't, you would probably have to adjust the voltage to prevent a shortened string from burning out. If the resistors are there, you should be able to see them.

2. Yes. Just keep the polarity correct

3. Yes. Again, keep the polarity correct.

Good luck.

Pat's right.

My guess is that they have no resistors as this would raise the manufacturing cost. Instead they are probably designed to use the battery voltage as is.

Each LED needs about 3V. In series this is cumulative, i.e. 2 LEDs wired end to end need 6V, 3 need 9V and so on...

If the string of LED's is powered by a 9V battery it probably is 6 or 7 separate circuits each with wires going to the supply so each circuit is no more than 3 LEDs in series. If it's a 6 volt supply (4, 1.5 volt batteries in series) this would allow 2 LEDs per circuit. This latter sounds more likely as 20 is evenly divisible by 2. What kind of battery supply (how many, what type) does it use?

If you cut it up you'll need to power 3 LEDs on a 9V source or use a resistor to lower the voltage. Alternatively you could use a 6V source or a 3V source for 2 LEDs or 1 LED respectively.

Thank you gentlemen! I should have given this other information initially but I wasn't thinking, obviously.

The Leds are nice and bright, certainly what I would want to use in my own models. They had white, blue, and multi-colored strings of 20 bulbs each.

The package gives little information. Light length = 6.3 feet, total length = 8.3 feet.

The are called 'LED Micro Lights'. Looking head-on at the top of the bulb, the size ranges from 1/8 inch to just a small fraction over this in diameter. Viewed from the side, the top of the LED to the bottom (right where the wires enter/exit the bulb) is very close to or slightly over 3/16 inches in length.

The LED's are done in series - at least there are two wires into each LED, then two wires out from each LED going to the next light in the string.

There are no resistors apparent, and I don't think there are any.

The batteries used are three 1.5 V AA batteries. I'm guessing they are being used to add up to 4.5 V. They are loaded into a little holder, with on/off switch, and the two wires going to the first LED in the string come from the battery box.

If I am looking at the battery box correctly, I think the batteries are also set in series (top of one to bottom of the next, etc.). The two on both ends face one way while the one in the middle faces the opposite direction.

If I therefore can safely extend individual LED wires, then I would just extend the ones at the end that I want to use, and just leave the rest of the wires outside the model hidden in the model's base.

I have no idea what resistor values would be needed to cut up the string and use it that way, so I would rather err on the side of caution, even if it is bulkier and less elegant solution.

2 wires in and 2 wires out with a 4.5 volt supply means they're wired in parallel.

There might be a single resistor in the battery box as 4.5V is high to drive LEDs that are 3V. This calculator (http://www.hebeiltd.com.cn/?p=zz.led.resistor.calculator) suggests it might be 3.9ohms (see attached diagram).

You can safely separate them but I'd use a resistor with each one on more than a 3V power supply. I'd go with the 4.5V battery holder. If you do that each LED needs a 75ohm resistor. Play with the calculator I linked above to get ideas. Using a forward voltage of 3 (voltage drop) and desired LED current of 20 is close enough to what it probably is to be OK.

Thanks for the help! :thumbsup: I couldn't remember which way series and parallel were wired, obviously.

There's a little blue plastic plug of some kind between the in/out wires on the LED's.

I thought it was just a separator to keep the wires apart, but maybe I should take a better look. I'll get my magnifier headband and some good lighting and see.

There IS a single resistor in the battery box. It has 3 colored bands close to each other at one end, Yellow, Orange, Gold. Then way off at the other end there is another Gold band.

yellow orange gold is 4.3 ohms, the last gold is +/-5% using this calculator (http://www.hamradio.cc/electronics/resistor_calculator.php). Pretty close to 3.9 ohms. This stuff is a lot like cooking...

Thanks! :) I know the colored bands tell about the resistor, but it's been 40 years since I last knew what they meant!

I think I am beginning to understand the calculator a little bit. But mostly I am deeply confused due to lack of understanding what I am doing and how this stuff works.

If I want to buy a resistor, do I specify the OHMS and the WATTS both?

If I want to use 4 of the leds from the string, and use the box that already has a resistor in it, do I need another resistor in addition to the one in the box, or remove the one in the box and use the values below?

Supply Voltage VOLTS = 4.5
Voltage Drop Across LED VOLTS = 3
Desired LED Current MILLIAMPS = 20
How many leds connected = 4

Calculated Limiting Resistor OHMS = 18.75
Nearest higher rated 10% resistor = 22 Ohm

Calculated Resistor Wattage WATTS = 0.12
Safe pick is a resistor with power rating of WATTS = 0.2

Thanks again. I really hope this stuff will make sense to me soon......

Remove the old one and use the value specified by the calculator.

The wattage is an indication of the maximum power the resistor can handle without burning out. You can get as high a value as you want, but too low is bad. I'd probably shop for 1/2 watt resistors (0.5). You can get an assortment but they usually include too many useless values. 22ohm 1/2 watt from rat shack (http://www.radioshack.com/product/index.jsp?productId=2062311&cp=2032058&pg=3&f=Taxonomy%2FRSK%2F2032058&kw=resistor&categoryId=2032058&kwCatId=2032058&parentPage=search)

I usually run 9V source with 1 resistor per LED. 330ohms (http://www.radioshack.com/product/index.jsp?productId=2062319&cp=&sr=1&kw=resistor+330&origkw=resistor+330&parentPage=search) is correct for this application and cuts down on the number of different type of resistors you need.

THANK YOU STEVE244! :)

I do have a couple more questions now though. You said "I usually run 9V source with 1 resistor per LED. 330ohms is correct for this application and cuts down on the number of different type of resistors you need."

Would this be only using a single LED with the battery, or multiple LED's, each with it's own 330ohms resistor?

If multiple LED's, would they be wired in series or parallel?

Also, I seem to recall from standard old Christmas lights that parallel wiring is good to use because if one light goes out, the rest will still continue to function. Is this true of the LED lights I described in these little strings I bought? I.E. 2 wires going in and 2 wires going out of each LED. I ask because the LED's I have seem to have the in/out wires as an integral internal piece of the lightbulb itself, not a light socket that a bulb plugs into like I am used to.


Anyway, I think that this stuff is finally being absorbed into my brain. I'm going to go pick up some resistors and a couple other things and give it a test. I have the calculators bookmarked so I can do my calculations and then print the page to take with me.

Depending on how this goes, I might even eventually get up the courage to try blinking lights, or flashing patterns of lights. I know that this is more complicated and requires some extra materials, but once I get my confidence level up who knows?

Thanks again!

Multiple leds, each with it's own resistor. They would each have a direct path pack to the power source: a separate circuit.

The advantage of this is if one circuit fails they don't all fail. Another advantage is it doesn't matter what the characteristic of each LED is, they will all light independently of the others.

There are lots of ways to implement this; you could run a set of wires back to the battery box terminals for each circuit, or you could conserve on wiring and run a positive and negative wire around where you have lights, and connect the led/resistor combo between these two wires.

Here's an example from this site (http://www.theledlight.com/ledcircuits.html):
http://www.theledlight.com/img-tech/parallel2.jpg


Depending on space and effect, you can also make some circuits parallel, some series. You might not have room to fit a resistor for each LED. Just remember these circuits should all have the same type LEDs or some may not light.

Get a solderless breadboard (http://www.radioshack.com/product/index.jsp?productId=2734155&cp=&sr=1&kw=breadboard&origkw=breadboard&parentPage=search) and some jumper wires (http://www.radioshack.com/product/index.jsp?productId=2103801&cp=&sr=1&kw=breadboard&origkw=breadboard&parentPage=search) to experiment with. This site explains the breadboard. (http://eecs.vanderbilt.edu/courses/ee213/Breadboard.htm)