Designing Soft Circuit using Ohms Law

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Materials specs:

Conductive Thread 27 ohms m

Battery 3V CR2032

White LilyPad LED Forward Voltage 3.3V

Spark Fun conductive thread DEV-31814 (is that the correct item number?..can’t look it up at the moment…30 ft on a bobbin)

As best I can tell From my research so far, to light 11 white LilyPad LEDs, not all can be placed in series as one circuit.

If I understand correctly, the LilyPad battery holder will accommodate 2 parallel circuits.

Would 2 parallel circuits, one with 5 LEDs and one with 6 LEDs, produce enough voltage to power efficiently?

Let me know if my questions do not make sense or if I am missing any information. Thank you.

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Hello cdh263,

Are you referring to this [Bobbin?

The forward current of the rated LEDs is 30mA maximum. You’ll most likely see 15 - 35 mA of current draw. The CR2032 is rated for a 250 mAh, so if you connected them all in parallel you’ll draw around 220 mA.

The problem with running 2 parallel circuits it that those will be in series and that will, sort of, halve the voltage from 3V to 1.5V. 1.5V may not be nearly enough to light the LEDs correctly. They could light up but they’ll be really dim.

Also, keep in mind that any length of thread you use will also lower that voltage.](Conductive Thread Bobbin - 12m (Smooth, Stainless Steel) - DEV-13814 - SparkFun Electronics)

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Yes, Brandon…that bobbin of thread.

Thanks for the explanations. A few follow-up questions…

If I understand your explanation correctly, to get the most efficient circuits (brightest LED and longest LED lifetime), if I use the CR2032 as power source in a LilyPad holder, I should run 11 parallel circuits with one LED each. This would take 6 CR2032 battery holders. This would also minimize length of thread to any one LED…which is an important consideration along with others.

  1. What are pros and cons of using 9V battery instead? Would I be able to maximize # of LEDs per circuit and accommodate slightly longer length of thread between components (than with CR2932) without compromising design otherwise?

  2. Compared to my project design, why does the…

Illuminated Mask circuit design In SparkFun’s Sewable Electronics Kit Guide sufficiently power 3 LEDs in series with CR2032?

Light-Up Plush circuit design sufficiently power a circuit with 2 LEDs in series?

Night Light Pennant sufficiently power what appears to be 3 LEDs in series?

I feel like I need a refresher on everything to do with designing soft circuits using Ohms Law and other factors. I simply do not want to completely, from the outset, trouble-shoot my way into an ideal circuit design by iterating prototypes using multimeter.

Let me know if any of my thinking or questions is flawed. Thank you. Cindy Hilmoe

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Hello cdh263,

I want to make sure that I’m not getting confused. I’m suggesting a circuit like the following:

coincell_bw_1.png
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1.) If you use a 9V battery, you’ll have to add additional resistors or more thread to make a large enough voltage drop so that 9V doesn’t blow the LEDs.

2.) Those projects all use the LilyPad LED boards in parallel, not series. So that’s why they work.

After reviewing your earlier post, the following circuit may be a bit more feasible:

coincell_bw_2.png
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This configuration will ease up the total resistance on the “ends” of the LED rows. This circuit is electrically equivalent to the earlier one but will help the end LEDs because you’ll be dropping more voltage with longer leads of thread in the earlier configuration.

It may help to visit our [Engineering Essentials if you feel that you need to brush up on Ohms law and other electronics topics.](https://www.sparkfun.com/engineering_essentials)

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Thanks for all the great suggestions and guidance…!!!

Cindy