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Thread: Cutting LED A/C Strings to make singing faces

  1. #1
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    Default Cutting LED A/C Strings to make singing faces

    I've been searching and can't quite find the info I'm looking for. I want to make a singing tree face for next year. I bought Red, Green and Blue LED AC lights on sale. I want to use my Arduino and SSR Relays to control the lights. One of the strings will be only 19 Lights for the top lip. If I put 120V across them they will each have 6.3Volts, which I believe will make them really bright or blow them up . Is there a standard resistance for LEDs that I can use to calculate the resister I will need to drop the voltage? V=IR So I guess I need the voltage and current to calculate what I'll need to put in for a resister. I'm wondering if anyone has that Info? Maybe they are all different and I'll have to measure it manually?

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    Default Re: Cutting LED A/C Strings to make singing faces

    No standard. You need to find an online calculator that will help you determine the resistor you need. Understand that if you only need 19 but have a 50 count string, you need to drop from 120 volts to roughly 45 volts. You also need to know the total apps and watts of the string.

  3. #3
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    Default Re: Cutting LED A/C Strings to make singing faces

    http://www.gtsparkplugs.com/Dropping_Resistor_Calc.html

    From 50 to 19, 1.8k 3 watt resistor

  4. #4
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    Default Re: Cutting LED A/C Strings to make singing faces

    You might also want to consider that different color LEDs will have different current requirements. I use this worksheet to calculate the resistors for my custom strings. This is NOT my work so take it for what it is worth but it has worked great for me over the years!

    Typically you use a 20ma forward current and the average voltage for whatever color Led you are using to calculate your resistors. Here is a link to a nice article on the forward voltage for different colors and calculating resistors to consider when making strings.

    https://www.petervis.com/electronics...alculator.html
    Attached Files Attached Files

  5. #5
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    Default Re: Cutting LED A/C Strings to make singing faces

    I put my extra into a silver static bag and closed it using Duct Tape. Works great.


    2019 - Going to visit my Daughter in New Zealand (again). I will be dark for the 2nd year in a row. Sigh..

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  7. #6
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    Default Re: Cutting LED A/C Strings to make singing faces

    You can shorten your LED strings, the pdf beeiill shared gives you the basic information you need. The exact calculations (and resistors) depends on the color of the LED - they each have their own forward voltage drop to consider.

    One thing not mentioned is that the resultant strings of different lengths... will dim differently. Maybe not enough to matter, but they will dim differently. I've been trying to develop a model and simulation for the dimming curves in the visual space to predict the appearance. Not ready yet.

    I've done a LOT of string shortening (and sometimes lengthening) while building 9 angel-bells of different sizes and colors. I've also done a lot of converting multi-color strings to solid color strings: the LED count and/or resistance must be adjusted depending on the color. Most of my work has been done on half-wave strings, but some full-wave and the principle is the same.
    Note: While each segment of each bell is a different count, and hence string(s) lengths, that I have not observed significant dimming differences across the bells. Or rather, when I create a test to look for it, it is visible. But in normal sequencing of chases with fades, etc, I don't see it.

    For Walmart Holiday Time LED strings, here are the voltage drops I measured (and used in my calculations) at 20ma
    Red: 1.94v
    Yellow: 1.95v
    Green: 2.89v
    Blue: 2.86v
    White: 2.82v

    Remember that most strings are made up of smaller sections. For example, the 50ct Walmart strings are actually two 25 count sub-strings. Many 100ct strings are 33/33/34. You can usually add or remove 1 or 2 LEDs to the 25ct (or 33ct) sub-string without needing to change the resistance. More than that and you have to adjust to maintain brightness or to avoid burn out). I have found it more useful to think in term as of the effective resistance of each LED and adding/subtracting to the original resistance accordingly, rather than trying to calculate the resistance for a new number of LEDs. Thus, to remove one Red LED 1.94v at 2ma you can put in one v/V/i = R or (1.94v)/(0.020 Amp) = 97 ohm or 100 ohm). A single 100 ohm resistor would only be i^2R = W = (0.02)(0.02)(100) = 0.04 Watts so almost any size could work. Always be sure to use twice the wattage rating you expect to see.

    You could remove, say, 6 Red LEDs and replace them with 6* 100 ohms /REd LED = 600 ohms , and 0.25W (so use 1/2 watt). To lengthen strings, you would have to replace the original resistor with one of smaller value. You'll find the resistor (for half wave strings) in the larger socket/blob. It can usually be removed, if not you'll have to disassemble the string.

    The shortest strings I have done have been 12 LEDs long. This usually requires around 4K total resistance at 1.6W, but I split that into 2 2K-ish resistors of 2W each for safety margin. It's hard to find higher wattage resistors in various R values for cheap.

    I've typed this quickly: if anything is unclear, please ask.

    Jimboha
    Last edited by jimboha; 01-04-2019 at 01:14 AM. Reason: Added fading visibility clarification
    Springville, Utah

  8. #7
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    Default Re: Cutting LED A/C Strings to make singing faces

    Wow! Thanks so much for your help. I had no idea the different color needed different voltages. I have tons of info here. I'll let you know how the testing goes.

  9. #8
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    Default Re: Cutting LED A/C Strings to make singing faces

    Don't shorten the strings, just put the extra bulbs behind the face, maybe cover with a anti-static bag or a simple Ziploc bag like Martin suggested, no muss, no fuss and save you a headache. Now you do want to try to get a string as close to the # needed as you can, would not be smart to use a 70 or 100 count string for 19.

    But even those larger count strings are made up in sections, just like incans, look for where there are only 2 wires between 2 bulbs. you can cut them there and add a plug to the remaining section(s) for another string
    Matt

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  11. #9
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    Default Re: Cutting LED A/C Strings to make singing faces

    Quote Originally Posted by Ruppro View Post
    But even those larger count strings are made up in sections, just like incans, look for where there are only 2 wires between 2 bulbs. you can cut them there and add a plug to the remaining section(s) for another string
    Unless they are full-wave. Cutting apart full-wave strings into the smaller segments is possible, but not trivial.

    Jimboha
    Springville, Utah

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