# Soft circuits experts?

I’m exploring soft circuits wired in parallel with conductive thread. I’m having a heck of a time figuring out how to make custom designs with them and need someone to kinda help walk me through the math part I think I’m missing.

Current project when hooked up to a 9v battery makes a bit of a spark at the connection points but doesn’t light up all the way through the circuit. Hubs says a 12v/500mA power supply would just burn it down. He’s recommending a battery pack with some triple A batteries be used, but I feel like I need to understand why…or learn to make power supplies.

I’m certainly no expert, but a wiring diagram and/or pics would probably help. How many of what kind of devices separated by how much thread (@ 30 ohms/foot, the resistance on the thread isn’t as negligible as one might like), etc. In the quicky tutorial I gawked, they use a “watch battery” to light an LED with ~12" of thread per leg. That’s probably a CR2032 @ ~3.0v, 210mAh, for whatever that’s worth (i.e. probably supporting the idea that 12v, 500mA would be capable of smoking it, depending on just what “it” is).

I’ll post one. I’m pretty sure this is adafruit thread and they say 1.3 an inch on their website. I’m super new to electrical so sorry if I say terms wrong. Will post pic tomorrow.

If you’re throwing Sparks with a 9v battery something is really wrong. Are you using a step up transformer somewhere?

Take a resistance measurement on the battery plug terminals. If it’s near zero, you have a short.

Without a wiring diagram I can’t be of much help.

Yeah how you have it hooked up would help here. It does indeed sound like you have a short to GND somewhere which could cause an arc (sparks) when initializing. Now for your project as long as each LED in a parallel string gets an appropriate voltage drop (~0.6V) across it with some current (usually set with a resistor in series with the LEDs + the resistance of the thread) you should be dandy. I’d need to see the data sheet of your LED’s to see what ranges the LED is designed for but generally more current = brighter. But the trade off is your battery will run down faster, things will be a little hotter and your LED’s will reduce their lifespan. But I think for your project my primary concern would be battery life.

I’ll be around the space in the evenings this week maybe today and Wednesday/Friday. Maybe I can help you for a bit if you’re still having issues.

She could probably sew a few of these lines in parallel. At least that’d be my first idea if there are long runs of thread.

Ye olde circut

What’s happening when its lit. I suspect the single parallel circuit is too long but idk how to math and find out how many lights each circuit is limited to

From the looks of it, the LEDs are getting dimmer the farther away from the battery. This is an indication that they are not getting equal voltage to each LED, which would not be typical of a parallel circuit UNLESS the wire (thread, in this case) has a relatively high degree of resistance.

Do you have a multimeter? It might be interesting to check the resistance across one entire leg (+ or -) of the circuit (from battery to the last LED.) It would also be valuable to check the voltage across the legs of each diode to confirm my suspicion of voltage drop as you progress along the “ladder.”

EDIT: Also, what is the voltage rating of your LEDs? With no resistors, a simple parallel circuit would expect 9V (the battery voltage) across each LED… and I would expect your LEDs are probably rated for MUCH less.

1 Like

I got them from electronics and based on the part number it looks like they are 1.9V. If this is the adafruit thread their site said 1.3 Ohm per inch. My husband’s multimeter was reading 15 mega ohm not plugged in. I couldn’t test along the way because the thread kept getting super hot

If I stick two AA batteries together it lights up the LED connected directly to the batteries. One battery does nothing.

I made the bracelet from the learning sofa circuits but figured I should figure out the why on that before I begin playing with my Flora

Two thoughts:

1. Wire getting hot is an indication that there’s a high rate of current flowing through a measurable resistance. In a fabric “thing”, that heat is a potential fire hazard!
2. You should measure resistance with an OPEN circuit. Do not connect the battery. Just measure from the thread that connects to the battery terminal to the leg of the last LED along that same path. You could even check the resistance at each LED and see the difference from first to last.

EDIT: Maybe this means the circuit was open already?

I see no current limiting resistors. So essentially this means the only limiting resistance is the thread. The longer the thread the lower the current and thus the dimmer LED’s. Or since they’re is very little limiting resistance essentially you are maxing the current delivery capability of the battery.

One thing I can’t tell. Are you wiring the LED’s where each LED has a ground connection? Or are you stacking some LED’s and only the last LED in the chain connects to ground.

Edit: sorry to be clearer. Can you draw out on a piece of paper how you’ve sewn each end of the LED’s together with reference to the battery. Then I can understand the IR drops

I had assumed a true parallel arrangement of the LEDs (each has a connection to the - terminal), but given then potential for high resistance in the thread, even a proper parallel layout will not deliver a good “ground”.

I get the impression that @MrsMoose is not terribly comfortable drawing circuit diagrams. Based on what I see in the photos, I have assumed the circuit to look something like this:

Perhaps she can at least confirm that this is her intention.

1 Like

Inserting a resistor into the circuit as shown below should help normalize things… It’s then a challenge of calculating the right resistor voltage… and those calculations require knowing some more details about those LEDs (forward voltage drop and max power dissipation, primarily).

If you are going to stick with the 9v battery, I would create 2 series circuits of 4 LEDs and one series circuit of 5 LEDs and run the three series circuits in parallel across the battery. No resistors needed.

I’d be happy to stop by one night this this week to show you how this works and provide the math.

1 Like

You guess correctly. I have not the foggiest idea how to draw a circuit despite knowing how to draw in general. I knew of the fire hazard so I didn’t connect the battery. The resistance was tested by placing the pointy bits at the beginning and end of the circuit.

But I think your drawing is correct. I sewed a positive lead going around the design until I got to the last LED then knotted off. Repeated with a negative line and again knotted off at the last LED. Made sure not to cross them.

It worked swimmingly making the bracelet for the class prototype that only had 3 or 4 lights and a small coin cell battery. Once I tried to get fancy pants though it went south

@mstovenour’s suggestion above should be a workable solution (assuming the LEDs are ~2V Vf rated). The circuit diagram for that would look something like this:

This would of course require re-wiring (re-sewing) your circuit.

EDIT: One of the keys to keep in mind with LEDs is that they will typically exhibit a fixed voltage drop across the legs (in the range of 0.5 to 3V is common). In a series arrangement, you can add up the voltage drops until you get close to your power supply voltage and make a reasonable circuit. So, 4 or 5 LEDs at 2V each is 8-10V of load, and a 9V supply.

Alternatively, in parallel, you have to add some other element to the circuit to reduce the supply voltage down to the desired LED voltage. A resistor is the typical element for this purpose, but requires that you have additional details (such as current flow, which can be calculated based on LED max power and voltage) to calculate the required resistance.

And of course all of this assumes that the resistance of your wire/thread is negligible. 1.3 ohm/foot is reasonably negligible in a small circuit.

1 Like

Yes exactly what I was driving at (no pun intended). The LED’s should be stacked in chains which then can be put in parallel. Each line should then also have a current limiting resistor otherwise you still will have the heating issue. So for example if we want to run 1mA through each LED string (good starting point imho) we would do

Resistor per chain = (vbat-(2V*number_leds))/0.001

Now of course I’m neglecting wiring resistance now. But the advantage of having a resistor per chain is that you can adjust the resistor to make each line have equal brightness (pick smaller for more brightness or bigger for less brightness)

1 Like