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LEDS,LEDS and more LEDS. Options
darbyvet
#1 Posted : 14 December 2020 05:09:18

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A Light emitting diode (LED) is an electrical component that emits light when a current is passed through it.LEDs will only pass current in one direction (that is what a diode does) so you need to make sure you wire them up properly or they wont work.LEDs are perfect for mighting models. In the days before LEDs filming models were lit with regular bulbs and/or neon bulbs.These generated significant heat and required high voltage pwoer supplies. The filiming model of the Millenium Falcon had real fans installed under the grills on the top of the hull to keep the model cool when the engine lights were lit. The filming model of the Enterprise D made for Star Trek The Next Generation had neon lighting for the nacelles and apparently could give you a nasty shock due to the high voltage required to power the neon lights.

LEDs offer many benefits for lighting;

They are very bright

They have a low voltage requirement

They draw very little current

They last a long time

They dont generate heat

They are small and come in many different colors


There are two kinds of LED.Through the hole LEDS have 2 wires that are used to solder the LED to a circuit board with holes predrilled. SMD (Surface mount diodes) LEDS are much smaller and are designed to be mounted directly to the surface of a circuit board.You can buy SMD leds that have wires soldered on to them already so you dont need to attach them directly to a circuit board.You can also buy SMD LEDs that are soldered to a backing strip creating a long strip of LEDs.


LEDs have some characteristics that you need to understand so you choose the correct LED for your project.These are the main things you need to look at;

Forward voltage

Forward current

Luminescence

Viewing angle


The forward voltage is a measure of how much the voltage drops across the LED.We need to know this number because if we provide less than the forward voltage the LED will not light.if we are connecting LEDs in series we also need to know the total foward voltage of all of the LEDs because that number must be less than the voltage we are providing to the LEDs.

The forward current rating is the maximum current that the LED can handle.IF you provide too much current across the LED it will burn out instantly.If we know the forward voltage, the voltage of the power and the current rating of the LED we can calculate the resistor we need to use to protect the LED.

Forward voltages are different for different colored LEDS.Forward current is based on the size of the LED, not the color of the LED.

Luminescence is a measure of the bright the LED is at its rated current.It is measured in millicandela (mcd). The highe the mcd the brighter the led is.You can also vary the brightness of an LED by lowering the current across the LED.

Viewing angle.LEDS have a lens above the semiconductor that focuses the light.This is encased in the plastic covering.Some LEDs have a very narrow viewing angle to concentrate the light into a narrower beam.These LEDs are used for flahslights or spotlights. Other LEDS produce more diffuse light.There are generally called "diffused" LEDs in the description and they are in many cases what we want to use to light a large area inside a model.


Unless you have a very simple set up with a 3v battery and a couple of LEDs you will need to add resistors to your LEDs to limit the current that goes across the resistor. if we look at a 3mm LED is has a current rating of 20 ma. So we must make sure that the current coming from the power supply (which will be several hundred ma) is reduced to a maximum of 20ma before it reaches the LED.


You can buy prewired LEDS that have everything already done for you.You just have to select them based on the volatge you are planning to use.I will briefly discuss how to figure out which resistor you need if you want to buy bulk LEDs and resistors.



darbyvet
#2 Posted : 15 December 2020 03:24:06

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Through the hole LEDS


These are the commonest form of LED and the easiest to use.They come in a variety of sizes from 1.8 mm up to 10mm in diameter.The commonest shape is cylindrical, but there are other shapes such as rectangular and lighthouse. They come with a clear casing for non diffused LEDS and colored casing for diffused LEDs.For single color LEDs there are 2 wires coming out of ther LED.The longer wire is usually the anode (positive) and the shorter wire is the cathode (negative).
LEDs are current resisted which means you need to add a resistor to the anode of the LED to reduce the current flowing through the LED.The forward resistance of the LED is determined by thew size of the LED.The larger the LED the larger a forward current it can handle.The forward voltage of the LED is determined by the color of the led, not the size.Red LEDs have a low forward voltage (around 2v) and blue LEDS have a higher forward voltage (3.4v).

So every RED led has a forward voltage of 1.8-2.2V, but a 5mm red LED may have a forward current on 150ma wheras a 3mm red LED may have a forward current of 20ma.

If you plan on using several LEDs you can either connect them in parallel or series.There are advantages and disadvantages of each type of circuit which I will get to shortly.

darbyvet attached the following image(s):
single led.jpg
types of leds.jpg
darbyvet
#3 Posted : 15 December 2020 04:17:06

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SMD (surface mount diode) LEDs



SMD LEDS were designed to be soldered directly onto printed circuit boards.They are very very small and bright.Thye are perfect for lighting models where you have very limited space. You can buy them in bulk with no wires , but be warned soldering wires onto SMD LEDs is quite challenging. The best opiton is to buy prewired SMD LEDS.

SMD LEDs some in a variety of sizes, but be warned it is VERY confusing with 2 different kinds of number classifications.Again the ones that are prewired are preferable and many vendors classify them according to size ranging from 0.6 mm to 5mm. Most of the prewired SMD LEDs have resistors already attached-You just have to select the ones with the correct voltage rating for your project.

SMD LEDS are also used to make LED strips.LED strips are a bunch of SMD LEDS wired in parallel on to a backing board.

darbyvet attached the following image(s):
SMDLED single.jpg
SMDLEDS on a PCB.jpg
smdledsizes3.jpg
smdledsizes2.JPG
smdledsizes.JPG
darbyvet
#4 Posted : 16 December 2020 05:29:55

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LED strips



LED strips are basically a bunch of SMD LEDS wired together in parallel with resistors. They are great for lighting large areas and they are very efficient because you can connect many LEDs with just 2 wires. They usually require 12V DC.

A good use of LED strips would be to light the engine of the Millenium Falcon or light the interior of the secondary hull of the USS Enterprise.



There are tons of kinds of LED strips available.Some are designed to put on cars and some are for accent lighting.You want the ones that use DC current, not AC.They come in a variety of colors and brightness levels.As we already discussed SMD LED come in different sizes.If the LED strip is made from small SMD LEDs the strip wont be very bright.If they are made with large SMD LEDS they will be much brighter.

LED strips also come in different denisty of LEDs. They are usually labelled as the number of SMD LEDS per meter. The standard LED strips have 30 SMD LEDS per metre.High denisty LED strips have 60 SMD LEDS per meter.Obivously the high density LED strips provide far more light because you have twice as many SMD LEDs per metre length.


LED strips usually come in 1 metre lengths, but they can be cut to size .However, there are only certain areas where you can cut the strip .Each LED strip is split into groups of 3 SMD LEDS with copper connectors to connect to the groups together.When you cut the strip you must cut at the copper connectors (black lines on the photo).This way you can cut the 1 metre strip into smaller parts.All you then need to do is solder a positive and negative wire to the copper tabs.


The major disadvantage of LED strips is power consumption.IF you have a long strip of SMD LEDs they will need quite a bit of current and may drain a battery (especially a 9v battery) very quickly. Thye also require 12V so you may need a voltage regulator to step up the incoming voltage to power.

The third photo shows a picture of a LED strip that has a diffuser incorporated into the backing strip.These are meant to simulate NEON lights and are really good if you need a diffused light source. When you light up an LED strip you will see the individual SMD LEDs as ppoints of light.If you then put this into a model like a nacelle on a USS Enterprise you wont see a uniform glow, but several hot spots of light.There are techniques to diffuse the light as Mark already showed and I will show some more when I build a Star trek Deep Sapce Nine Runabout.However, if you use the LED strips with the diffuser cover you will get a smooth uniform light source you wont need to diffuse the strip.





Finally I want to mention NEOPIXELS.There are a special form of LED strip.With a regular LED strip when you power up the strip all of the leds light up.With a neopixel strip you can light each LED individually and even control the color and brightness of each LED.The individual SMD LED are addressible.This means that with a microcontroller you can turn individual LEDs off and on and make sophisticaed lighting effects. A good use for neopixels would be to make the light on the fornt of the KITT car from Knightrider, or the Cylon raider helmet.
NEOPIXELS are quite expensive and they are complicated to program and they require a LOT of current


darbyvet attached the following image(s):
ledstrip1.jpg
ledstrip2.jpg
diffusedled strip.jpg
neopixel.jpg
darbyvet
#5 Posted : 17 December 2020 03:11:38

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"Special effects" LEDs



I would briefly like to talk about some special LEDs before we move on to how to wire them.

The way of the LEDs we are talking about are either off or on.If you provide power to them they turn on and when you turn the power off they turn off. For some models this is fine.But what happens if you want to add blinking or flashing or flickering lights such as navigation or strobes on a USS Enterprise build or pulsing engines? There are 2 ways to achieve these special effects.You can either use circutry to control the power supplied to the LEDs or you can use prewired LEDs that have the circutry already installed either on the LED itself or in one of the wires of the LED.

You can buy prewired LEDs that blink off and on.You can buy prewired LEDs that will flicker to mimic a flame and you can buy breathing/fading LEDs that pulse.These are all very useful in making a more dynamic lighting for your model and they dont require and special circuitry or microcontrollers.

RGB LEDS


These are basically 3LEDs in one casing .They have 4 wires.One for each of the red,green and blue cathodes and a common anode.You can vary the brightness of each individual color to make any color you want.RGB LEDs usually require a microcontroller to control them.
darbyvet attached the following image(s):
rgb led.jpg
darbyvet
#6 Posted : 17 December 2020 03:45:51

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WIRING LEDS


So you have planned out how many LEDs you will be using in your model and where theyt will go.The next step is figuring out how to wire them up to the pwoer supply.

LEDs can be wired up in 2 different ways-Series or Parallel.

There are advantages and disadvantages of each type of circuit.

Series circuits are easy to wire up because you connect the cathode of one LED to the anode of the next one. The biggest disadvantage of series circuits is if one LED burns out none of them will light.Series circuits are also very efficent and dont drain batteries as quickly as parallel circuits.Series circuits are alos not good if you need a lot of LEDS because the voltage of the pwoer supply must be larger than the total forward voltage of all of the LEDS in the circuit.

Parallel circuits are harder to wire up because each LED needs to be connected to the positive and negative and each LED needs its own resistor.IN this set up if one LED fails the rest will still light.Parallel circuits use a bit more current and if you have a lot of LEDs you may need a DC power supply.




Current limiting resistors


Regardless of which type of circuit you use you will also need to put current limiting resistors into your circuit to drop the current output of the power supply to the forward resistance of the LED. You can simply buy LEDS that have resistors prewired.IF you want to do it yourself you will need to use a calculator to determine the resistor value .There are tons of calculators online. The equation you use is based on Ohms law. here is a link to the calculation;

https://ohmslawcalculato...led-resistor-calculator

Ohms law states that the value of the resistor is the voltage of the power supply- the forward voltage of the LED divided by the forward resistance of the LED.


darbyvet attached the following image(s):
ohms law.png
darbyvet
#7 Posted : 17 December 2020 04:27:16

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Wiring LEDS in series



Wiring LEDs in series is very simple.You connect the anode of the first led to the positive from the power supply.You then connect the cathode from the first LED to the anode of the second LED and so on.... LEDS in series are great for very tight spaces where you dont have much room for wires.You need to add the current resisting resitor to the circuit.It doesnt matter if you put the resistor on the anode side or cathode side of the circuit.

In series ciruits each LED gets the same current so they will all be the same brightness.The voltage is divided between the LEDS. The practical upshot of this is that the power supply voltage MUST be larger than the sum of the forward voltages of the LEDs.This is a significant problem in many cases because generally we use fairly low voltage pwoer supplies for models.Even if you use a 12V DC pwoer supply you cannot use more than 6 red LEDS in the series circuit because when we add the forward voltage of the 6 leds we get 12V.

darbyvet attached the following image(s):
LEDS in series.jpg
darbyvet
#8 Posted : 17 December 2020 04:58:59

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LEDS in parallel


Wiring LEDS in parallel is a bit more complicated and you need more wires because each LED has to be connected to the positive and negative. In parallel circuits all of the LEDs receive the same voltage and the current is divided between the LEDs.This the opposite of series circuits.
You also still need current limiting resistors.In parallel circuits each LED should have its own resistor .You may be tempted to just put one current limiting resistor like in the series circuit.It may work, but you will get current fluctuations between the LED and varying brightness of the individual LEDs.

The main benefit of the parallel circuit is you can have many more LEDs than with a series circuit. Since we are dividing current and each LED only draws 20ma we can potentially add far more LEDS. Even if you use a 9V battery with 750maH current rating you could power 37 LEDs for 1 one hour (750mah/20ma).

In some large models you may have several dozen LEDs and/or LED strips.You may not have enough current for all of these LEDs.In this situation you can use a circuit called an LED driver board.LED driver boards can manage hundreds of LEDs and they draw much less current than parallel circuits.

I almost always use LEDs is parallel for my lighting circuits.

I will be posting a video link to show some LEDs in action in the next few days



So that is enough theory.The next step is to start building some models with LED lighting.

The first model will be a very simple lighting kit for an MPC 22 inch Eagle II cockpit.

The next model will be a runabout from Star Trek:Deep Space 9. This model will have more LEDs, but there will only be LEDs, no circuitry

After that I will be building DS9 USS Defiant.This will have evne more LEDs and include some circuit boards for lighting effects.

Finally I will be building a half scale ET spaceship.This will have several dozen LED,LED strip, neopixels ,a motorized special effect and a sound board.This build will also include using microcontrollers to control the LEDs and LED driver boards for some much more complex sound and light effects.

darbyvet attached the following image(s):
LEDs in parallel.jpg
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