Well, since I still had the circuit diagram open… here’s your suggestion:
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Thanks for that. I’m at work and have no access to any tools. I’m not an EE by trade anymore so I don’t have all my toys anymore. 
I would love that. I feel like I need to understand the math behind the solution if I’m going to be able to continue with drafting my own designs and explain to others why Option A is safe but Option B is not.
1.3 ohm an inch according to Adafruit. I imagine the final will definitely be resewn and I’ll probably keep this one as an example of Don’t.
Is it bad I’m surprised there isn’t some kind of app for designing these where it can tell you that your plan sucks lol?
EDIT…I’m also sure there has to be limits on how long a threaded system can be and how much juice can go through it? Back to needing to math I suppose lol
So power dissipation P = IV where
I - current
V - voltage
Suppose you have a voltage of 9V. If you draw 500mA of current (a lot of LED’s or resistive short to ground) from the battery that’s 4.5W of energy. In the above proposal assuming 1mA per branch and 3 branches we get 9V * .003mA = 0.027W. The 3mA comes from an electrical law that currents basically sum together when they come back together (think water coming from a tower into multiple pipes downstream).
As you can see it’s many times smaller and thus less heating. I think low mW range is safe for most costuming applications.
So rule of thumb is shoot for 1-10mA of current per branch when using LED’s. LED current is set by a resistor in series. You can use the formula Resistor = ((Vbat-VLED*numLEDPerBranch)/current (in Amps)
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Don’t feel bad! Actually in electronics we do have an app called LTSpice where you can simulate circuits. But it think it’s overkill. I’m actually interested in doing some LED costuming for Halloween this year (I bought a adafruit flora). Maybe I can put together a crash course in basic electric calculations to support this.
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Yes the limit will be the voltage drops due to the wire. The voltage drop across the wire takes away from the voltage drop across the LED. Eventually the voltage drop gets so low they turn off. But with a 1mA or so bias current the wire resistance isn’t a big deal I think. 1Ohm * 0.001 is 1mV.
What I mean is I cannot take a spare 12V wall wart and wire it in without issues if a 9V is causing problems…I think.
You should totally do a crash course class. I’m also able to provide snacks to anyone tutoring if they tell me what they like lol. Apparently there is a lot more to this than just knowing Ohms Law. I’ve tried reading web pages and such but I’m audio and haptic. I need someone to explain and plug in the numbers a few times to understand.
You can but it’s not super safe to directly tap off of. You’d want something in between to protect you with a current limit feature. Adafruit and Sparkfun sell boards that’s will take a cord in and give you voltage rails with that kind of protection I think. I’ll see if I can find one.
The Flora has USB power. I’m just trying to save it for a special occasion lol
how has Ohm’s Law not been mentioned…
I’m not saying it’s needed, but I’ve never heard a discussion about electrical calculations NOT start there. 
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How about tomorrow after 6pm or so? Any time after that up to about midnight also fine. Just let me know and I’ll meet you in the elab.
I’ll make it work! Thank you. Lemme know snack requests and anything I need to bring
Yah, I hate to start regurgitating laws and formulas without understanding the persons base knowledge. My favorite for someone that would rather avoid algebra is this wheel:
https://www.electronics-tutorials.ws/dccircuits/dcp_2.html
But if the person knows a little algebra then it is much simpler to mention two equations that define everything on that wheel and are very easy to memorize.
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I’ll meet you at 6pm unless you say later. Thanks for the offer to bring snacks, but not really necessary. Bring the circuit and your favorite note taking tool. If you like reading ahead I’m basically going to explain KVL. https://www.electronics-tutorials.ws/dccircuits/kirchhoffs-voltage-law.html But it is very simple with LEDs and resistive thread.
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10 4. I’ll definitely read ahead. At some point I want to work in fancy doo dads like sensors, buttons, boards etc. and definitely have it plug in or solar charge instead of relying on battery…bur one must walk before they run lol.
You guys are neglecting the battery internal resistance. Those little 9v battery’s have a lot. I suspect the Vled is more like 3v for yellow and will help measure it. If it really is 2v then we will add another LED in the strings to get below 20ma. If you are not worried about the exact LED current then the most efficient way to design these bling LED designs is to omit the resistor. It just eats your battery and has nothing to show for it. An efficient approach is to string enough LEDs until you can get into the safe operating current range of the LEDs. This always means the LEDs are running less than full brightness but there is nothing wasted as heat in the resistors. This works well for cases where bling vs. runtime on an unregulated battery is important.
I admit I have ZERO experience with these wearable circuits, so I’ll happily defer to you. However, if she’s experiencing “super hot” thread-wires, it’s a pretty good bet that she has too much current. Of course, putting LEDs in series rather than parallel should help mitigate that issue. In any case, if these things are semi-disposable, optimizing the current/brightness is probably not high on the priority list…
At 20mA P= .02*9V = 180mW
At 1mA P= .001 * 9V = 9mW
Power across resistor at 1mA (assume LED consume 7V of battery rail)
R = (9-7)/.001 = 2kOhm
Pres = I^2R = (.001)^2*2k = 2mW
Ptot = Pled + Pres = 9mW + 2mW = 11mW
So resistor actually doesn’t burn much power at all.
It might be a small number but it is still 22% of the battery that will be wasted in the resistor. (There is also a small mistake in your last equation. Total power is 9mA). Why waste any power when lighting more LEDs is better in this application? What is the benefit of the resistor vs adding another bling inspiring LED? What is magic about 20mA? Just that it is the rated current. What is wrong with 18mA or 15mA in this application? You might assume the LED has a fixed voltage drop but the reality is that they behave like resistors also. Just look at the voltage drop vs. current graph from any LED datasheet. You will find a nice linear trace. So, rather than adding a resistor we can use the internal resistance of the battery and LEDs to limit the current to something safe. Of course this is only a good solution when more LEDs are also good and you are not trying to hit some peak luminous intensity.