Hey everyone, brand new here in the forums and a noob when it comes to good LED procedure. I'm hoping for some sound advice on my project. Here's what I've got going on:

My nine year old daughter is a Star Wars nut (like her Dad) and is going as the Jedi Stass Allie for halloween. I'm building her a light saber hilt (school won't allow the blade). I'd like to light up the hilt with (8) 5mm LEDs. Here's the breakdown:

1) Red illuminated push button switch: (1.7v) https://www.radioshack.com/products/push-button-switch-red-led?variant=20332092037
4) Green 5mm LEDs (at the base of the blade emitter) (3-3.4v) https://www.amazon.com/gp/product/B006G6TKQW/ref=oh_aui_detailpage_o01_s00?ie=UTF8&psc=1
1) 7 color blinking 5mm LED (5v) https://www.radioshack.com/products/radioshack-5mm-7-color-blinking-led?variant=20332052293
1) Orange 5mm LED (1.9v) https://www.amazon.com/gp/product/B004UZ3FNU/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1
1) Green 5mm LED (same as the green LEDs used in the blade emitter).
1) 9v battery as a power supply.

I'd like to know if I'd be better off wiring these in parallel or series. Also, please help me figure out the different resistor(s) I'll need because I'm mixing different LED types. Thanks in advance from the Republic and the Jedi order!

 

Hey everyone, brand new here in the forums and a noob when it comes to good LED procedure. I'm hoping for some sound advice on my project. Here's what I've got going on:

My nine year old daughter is a Star Wars nut (like her Dad) and is going as the Jedi Stass Allie for halloween. I'm building her a light saber hilt (school won't allow the blade). I'd like to light up the hilt with (8) 5mm LEDs. Here's the breakdown:

1) Red illuminated push button switch: (1.7v) https://www.radioshack.com/products/push-button-switch-red-led?variant=20332092037
4) Green 5mm LEDs (at the base of the blade emitter) (3-3.4v) https://www.amazon.com/gp/product/B006G6TKQW/ref=oh_aui_detailpage_o01_s00?ie=UTF8&psc=1
1) 7 color blinking 5mm LED (5v) https://www.radioshack.com/products/radioshack-5mm-7-color-blinking-led?variant=20332052293
1) Orange 5mm LED (1.9v) https://www.amazon.com/gp/product/B004UZ3FNU/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1
1) Green 5mm LED (same as the green LEDs used in the blade emitter).
1) 9v battery as a power supply.

I'd like to know if I'd be better off wiring these in parallel or series. Also, please help me figure out the different resistor(s) I'll need because I'm mixing different LED types. Thanks in advance from the Republic and the Jedi order!

A light sabre doesn't have a blade.

In the imaginary world of the movie; its "hard light" - perhaps the same thing as the "hard light drive" for the Rimmer hologram in red dwarf.

Someone told me that the original movie before CGI and digital remastering; the light sabre was a fluorescent tube with a strip of foil along its length, and spinning for the visual effect.

You could do that with a transistor inverter to power the tube - but if the glass gets broken, its just as dangerous as a blade.

There are a couple of options - a rechargeable LED worklight with coloured cellophane pushed inside the clear plastic tube. Or if you've lots of time on your hands - you can get LED candle bulbs with 3 or 4 LED stripes inside. Its lots of fun breaking them open to get at the LEDs, and they probably won't be single Vf LED units - you'll still need an inverter to generate enough voltage.

The clear tube you build it into obviously cant be glass - maybe someone here knows where to get clear plastic. If you use a rigid former to mount the LED strips - you might get away with transparent flexible hose.

 

Thanks for your reply Ian. I'm not sure I understand it however. I'm just looking for some advice about how to power the LED lights on the hilt. I'm not building the glowing blade part. Much appreciated.

 

Thanks for your reply Ian. I'm not sure I understand it however. I'm just looking for some advice about how to power the LED lights on the hilt. I'm not building the glowing blade part. Much appreciated.

An LED, like any diode; has a sharp knee curve - at the knee point, a small increase in voltage produces a large increase in current - which can destroy the LED.

There are plenty of constant current circuits online - the circuit will impose a volt drop; your power source must cover that in addition to the Vf of the LED. Vf is about 1 3/4V for red, about 2V for green and about 3.4V for white or blue.

You could run the LED(s) off a single alkaline cell if you use a blocking oscillator "Joule thief" circuit.

Many LED keyfobs use 2x 2016 coin cells for blue or white and a single 2032 for any other colour. Current is only limited by the cell internal resistance - it doesn't do the LED any good and the battery is short lived for continuous use.

Frankly - I'd just go for an off the shelf LED flashlight and stick a clear plastic rod on the front to serve as a light pipe.

A 3D flashlight is like the keyfobs and relies on cell resistance to limit the current - a white 2D flashlight has to have some circuitry to boost the voltage.

The innards from a solar LED garden light are yet another option.

 

Here's the deal: 9V batteries are not excellent current sources.
You want to build series strings whenever you can get away with it because 5 of those use less current than 8 single circuits.

A pair of greens at probably 3.2 volts each maths out like this:
9V -3.2V-3.2V = 2.6V leftover in the 9V battery.
2.6V leftover/0.02 amps is 130 ohms.
You can use 100 ohms or 120 ohms, and thus overdrive the LEDs because this is not a long lasting purpose. If the LEDs die in 5,000 hours instead of 10,000 hours, nobody is going to know. Or you can fudge in the other direction with 150 ohms. That isn't going to affect the brightness much...nobody will know.

An orange at 1.9V and a green at 3.2V?
9V -1.9V-3.2V = 3.9V leftover.
3.9V/.02A = 195 ohms.
standard sizes...180 ohms, 220 ohms, two of the 100 ohm resistors in the pack you bought for green LEDs...whatever.

and, no, you can't add up LEDs to get as close to 9V as possible. They will go out quickly as the battery weakens. You have to allow at least 10% to 15% of the 9 volts for the regulating resistor, and 30% sounds better for using a 9V battery.

A 7 color blinking LED with 3 legs???
There is not enough information on the RS website to tell me what to do with those, but you have the math examples.
Return with more info or more questions, PRN.

 

#12,

Thanks for your reply. Why are 9v not great current sources? I choose the 9v because it fits nicely into the hilt of the light saber.

The 7 color LEDs is 5.0 volt max So I assume: 9V - 5v = 4v left over / .02 = 200 ohm resistor. Is this correct?

Thanks.

 

Why are 9v not great current sources?

Too much internal resistance. The last time I checked a 9V battery contains 440 ma-hours. If you use 44ma, it will be dead in 10 hours. You're looking at vaguely 100 ma, for a lifetime of 4.4 hours, and I promise, you're pushing the limits on how much that battery can deliver.

So I assume: 9V - 5v = 4v left over / .02 = 200 ohm resistor. Is this correct?

I would try that, at the risk of ruining one LED. There is a risk. If the LED blinks off, there will be no current through the resistor and the voltage on the LED will instantly be 9V. I'm thinking, place a 4.7 volt zener diode in series with the LED, then 1000 ohms in parallel with the LED so it never gets more than 5 volts, even when the battery is fresh and has a little bit more than 9 volts.

The problem I suspect is that the blinking LED has internal circuits so it is NOT looking for you to supply 20 ma. It is looking for you to provide 4 or 5 volts and the internal circuits handle the current limiting. Then again, I could be wrong because I can't see the specsheet. I'm just trying to get you in business. If I'm wrong, don't burn up a dozen LEDs insisting I couldn't be wrong. Just blow one or two up and check back here.

 

Here's the deal: 9V batteries are not excellent current sources.
You want to build series strings whenever you can get away with it because 5 of those use less current than 8 single circuits.

N.

They're better current sources than cylindrical cells with low internal resistance - but they don't last long.

Most LED keyfobs rely on the internal resistance of 2016 or 2032 coin cells - I even have a 3D LED flashlight that relies on cell resistance!!!

 

I'm building her a light saber hilt (school won't allow the blade). I'd like to light up the hilt with (8) 5mm LEDs.

Just a thought...
What about using a strip of tissue paper and a very small fan for the "blade"? I think it might be an interesting effect with the LED's illuminating the flapping paper.