Very simple LED question!

Brucebruce · Post by **Brucebruce** » Wed Apr 23, 2014 9:33 am

Hi all!

I have a question that’s probably laughably easy to answer!

I’ll try keep it short. I’d never modelling before and I’d never worked with any sort of electronics before. I got a USS enterprise from revel at Christmas and built it. It looks terrible. After that I read and read about all the modelling basics and paining techniques etc. I got another Enterprise model and I’m slowly putting it together with the hope that it will look a lot better than the first one! I am also planning on lighting it. I’ve read/watched a bunch about LEDs but I can’t seem to figure this one thing out. I bought a starter electronic kit that came with wires, breadboard and lots of other bits and pieces. I have my LEDs in place in the saucer section of the model. I also have the correct type of resistor attached to the positive leg of each LED. I’m running this off of a 9v battery at the moment but will change it to a 9v adaptor later.

I can’t figure out where my wires should go. Like I said a laughably easy question. I’m at work so I can’t take any pictures but I’ve drawn a really terrible layout of what I’m talking about. The two boxes at the bottom are the holes that the wiring will come out of. Should positive wire come in the hole, attach to the positive on LED 1, to the negative of LED 2 then from LED2 positive to LED3 negative and so forth until I have the negative wire leaving the positive of LED6, attaching to the negative of LED1 and back out the hole?

Hope someone can help me with this and/or understand the gibberish I just typed!

http://i.imgur.com/nvey1u7.jpg

Redag · Post by **Redag** » Wed Apr 23, 2014 11:10 am

First: you want to decide if you're building parallel or not. With series you'll do that one-to-the-next system you seem to be describing. I would advise a parallel plan. That way your circuit should work even if parts upstream drop out. You should google so you can look at circuit diagrams, that should be explanatory.

Rocketeer · Post by **Rocketeer** » Wed Apr 23, 2014 11:16 am

An LED takes about 3.3 volts, so your proposed series wiring scheme wouldn't work; all six LEDs together would require 6*3.3 = 19.8 volts.

But you can wire two LEDs in series; 2*3.3=6.6, which is less than your 9-volt battery.

LEDs need to have resistors in series with them to limit the current to at most 20 milliamps (mA). You can figure out what size resistor you need by subtracting the LED voltages from the battery voltage, then dividing by the current. In this case, that is: (9-6.6)/0.020 = 120 ohms. So you need a 120 ohm resistor (or the next largest standard size).

Then you need to solder up a "string" of 2 LEDs and the resistor: one end of resistor to the "plus" terminal of the battery; the other end of the resistor to the "plus" terminal of the first LED; the "minus" terminal of the first LED to the "plus" terminal of the second LED; the "minus" terminal of the second LED to the "minus" terminal of the battery. This is a so-called "series" circuit.

If you want to run six LEDs, you'll make three of these little circuits. Then hook the "plus" end of each circuit to the "plus" end of the battery, and the "minus" end of each to the "minus" end of the battery. That puts each 2-LED circuit in "parallel" with the others, so each circuit is fed 9 volts. Each little 2-LED circuit draws 0.020 amps (20 mA), so the total drain on the battery is 60 mA.

Good luck!

Brucebruce · Post by **Brucebruce** » Wed Apr 23, 2014 11:58 am

Thanks Guys!

Does this look about right, there the (R) is resistor and the blue pen is the wires?

http://i.imgur.com/mOYKQn5.jpg

I'll also be having LEDs in the hull and up in the warp nacelles..so basically there will be about 6 or 7 little circuits all coming out the bottom to hook up to the battery. How do I connect them all together at the bottom? Even if you tell me what the name of that would be so I could google it

Thanks again, you've basically repeated many things I've read/watched but in a much easier to understand way!

Brucebruce · Post by **Brucebruce** » Wed Apr 23, 2014 11:58 am

God,

Sorry about the size. Models, LEDs and, as it turns out, forums, are not my strong point!

Rocketeer · Post by **Rocketeer** » Wed Apr 23, 2014 12:52 pm

Your picture looks right.

You don't have to have a lot of wires coming out the bottom of your model; you can gather all the "plus" wires together inside the model and solder them to one larger wire, which would extend out the bottom of the model. Ditto for the "minus" wires.

Two things: Test your circuits before you glue the model together. And run a "burn-in" test; let the lights stay on for a considerable bit of time, an hour or two, before you glue the model together. That lets you smoke out any weak components or solder joints while you can still get to them.

One other thing you might want to consider is battery life. You talk about 6 or 7 little circuits. They'd each draw 20 mA, so you'd have a total current draw of 7*20 = 140 mA. A 9-volt battery (http://data.energizer.com/PDFs/522.pdf) has a total capacity of about 400 milliamp hours at that amperage, so your battery would last 400/140 = 2.8 hours. Fine for display at home, where you'd be turning it on for a few minutes to awe your friends. But at a contest, if you wanted the lights on all day, you'd end up having to replace the battery a couple times. In that case, you might want to consider using a wall wart instead.

Brucebruce · Post by **Brucebruce** » Wed Apr 23, 2014 1:04 pm

Hey,

Yeah, I'll be changing it to mains power at the end! Sadly I'm pretty sure none of my friends will be wowed by my Star Trek model. My girlfriend will at least pretend to be wowed though!

I have lots of the thin wire that's coated in rubber/plastic...I think it's maybe bell wire? Say I had 5 of them for the positive line, would I just strip all five ends, twist them together then solder them to another bit of bell wire? So that, like you said, there would only be two wires coming out of the bottom?

Thanks for the tip about leaving it on! I'll definitely be doing that before I glue it all up. Might give it a little shake too, to make sure nothing is going to come loose!

Rocketeer · Post by **Rocketeer** » Wed Apr 23, 2014 1:24 pm

Brucebruce wrote:...I have lots of the thin wire that's coated in rubber/plastic...I think it's maybe bell wire? Say I had 5 of them for the positive line, would I just strip all five ends, twist them together then solder them to another bit of bell wire? So that, like you said, there would only be two wires coming out of the bottom?...

Yep. Cover the joint with electrical tape or a bit of heat-shrink tubing to prevent any short circuits. Test each individual circuit first to make sure you get the polarity (plus/minus) right when you solder them together.

Another thing to consider is whether your wire is big enough; I believe that your bell wire will probably be large enough for the dinky amperages involved--but that's another reason to do a burn-in test.

And another thing: The LEDs will show through the plastic when they're turned on; you might consider painting the inside of your hull silver or black to prevent "light leaks".

Brucebruce · Post by **Brucebruce** » Wed Apr 23, 2014 2:13 pm

In total I will have 6 LEDs in the saucer section, 4 in the nacelles and another 4 in the hull. Do you think the bell wire will cope with that?

Yeah, when I did my primer coats I sprayed the insides too and I've since coated the insides in white. Hopefully this will be enough to keep the light in!

Rocketeer · Post by **Rocketeer** » Wed Apr 23, 2014 3:23 pm

Brucebruce wrote:...My girlfriend will at least pretend to be wowed though!

Sounds like a keeper!

Brucebruce · Post by **Brucebruce** » Wed Apr 23, 2014 5:44 pm

Rocketeer wrote:
Brucebruce wrote:...My girlfriend will at least pretend to be wowed though!
Sounds like a keeper!

Hehe, getting married in July, so she is!