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Realistic Lighting Using Light-emitting Diodes (LEDs)
I came to a mental roadblock on Dueling Shacks, and as I've looked at starting on either E J Whiley or Quincy's Salvage. I like having a plan as to how I will do something, and in each case, I've wimped out when it comes to lighting. So I figured that I needed to grab the bull by the horns and figure this out. One of the big stumbling points is that when a LED is hooked up it is unrealistically bright for most normal applications, and I was uncertain as to how to grapple with predictably reducing the intensity of the lighting. So I decided to turn to a knowledgable source of information to ask some questions.
I have done most of my research through information at ngineering.com. I talked with them and got some very good clarifying information that set me on this path. What I would like to share is some of the practical application information to hopefully demystify the application of LEDs in our models. I will also share how I have constructed an LED test and intensity-adjustment platform.
To get to the application end of this subject, it is necessary to wade through some theory to understand the application, especially when it comes to adjusting the brightness of the LEDs.
I hope you will let me know if this subject is of interest, or if you have any knowledge to add to the subject. I'll continue with the subject in another entry.
Mark
I have done most of my research through information at ngineering.com. I talked with them and got some very good clarifying information that set me on this path. What I would like to share is some of the practical application information to hopefully demystify the application of LEDs in our models. I will also share how I have constructed an LED test and intensity-adjustment platform.
To get to the application end of this subject, it is necessary to wade through some theory to understand the application, especially when it comes to adjusting the brightness of the LEDs.
I hope you will let me know if this subject is of interest, or if you have any knowledge to add to the subject. I'll continue with the subject in another entry.
Mark
Comments
I put a dimmer on a string if lights in my shadow box diorama. I really like the effect. It is a plug-in vs battery operated though.
I don't fully understand the theory either but when I want to do a project. I get the basics down of what I want to do then email Tim.
Post specific questions and I'll bet you can get some good answers...or at least a starting point.
Let's start with a discussion of some basic electrical concepts that will help us all understand some of the unique aspects of LEDs. A simple analogy may help the non-electrically minded modeler. Let's consider a water hose that has one sprinkler at the end of the hose that we are using to water our lawn. As we turn the water on at the faucet, water starts flowing, filling the hose, and we see some water coming out of the sprinkler. As we keep opening the faucet, the pressure in the line increases, more water flows, and the sprinkler puts out more water covering a greater area. There are direct parallels to this example and a circuit with an LED in it.
In our example, the water pressure in the hose is equivalent to the voltage at any point in the wire. The flow of water in the hose is analogous to the flow of electricity or current in the wire. The spray of water out of the sprinkler is equivalent to light coming out of the LED. I'll let you think about that for a minute.
Now let's add a wrinkle. The sprinkler is near my kid's sandbox and I don't want to get the sand wet, so I can only run the sprinkler to cover a certain area. If I kink the hose, It puts resistance in the line reducing the pressure and flow of water, so I can adjust it to just cover the area I want. A resistor in a circuit does the same thing as that kink, it reduces the voltage (pressure) and current (water flow) to reduce them to the level we need to operate an LED. This is exactly the concept we will use to reduce the intensity of the LEDs to a level that is realistic for our models.
Now as we will find out voltage and current (water pressure and flow) are directly related: more voltage -> more current, less voltage -> less current. Really, the whole crux of the issue with LEDs is that if we pass more current (water flow) through them than they are designed for, they fail.
Keep this simple illustration in mind as it will help you understand the technical details.
Terms we need to know:
Voltage - the electrical potential in our system, expressed in volts (V)
DC Volts - direct current voltage, think of a battery, with + and - terminals. The electricity only flows in one direction (VDC)
Current - a measure of the flow of electricity, expressed in amperes, or amps (A)
Milliampere, or Milliamp - 1/1000 of an amp (mA)
Resistance - opposition to the flow of electrical current, expressed in Ohms (Ω)
So here is the diagram of a simple circuit with the following components:
A power source (power supply or battery) shown at the bottom of the diagram
A simple switch, shown on the left side of the diagram
A resistor, shown by the zig-zag symbol
An LED (A = anode, C = cathode)
I want to pause a minute and give thanks to Tim Anderson at Ngineering.com who has been an amazing help. I have his permission to share information from ngineering.com. There are many sources of LEDs, but I use his products so those are the only ones of which I have any real knowledge.
Coming up: Some of the technical details. I hope this is proving interesting to some. Please let me know if I am missing the mark or if you have any questions as we go along.
Mark
I have a reserved seat in the lecture hall front and center. Looking forward to each installment.
Later, Dave S. Tucson, AZ
The test platform I laid out will let you test LEDs and even test them in the model for the desired effect. It will just take me a little time to wander down that path.
Thanks all, for coming on the journey.
Mark
I = V /R
where:
I = Current in Amps
V = Voltage in Volts
R = Resistance in Ohms
This is the equation we use when we know voltage and resistance and are looking for the current.
If we are looking for the voltage drop, and know the resistance and current, the equation is:
V = I x R
If we want to find the resistance across a device and know the voltage drop across it and the current flowing through it. then the equation is:
R = V / I
A practical example:
These specifications are typical for LEDs we might be using.
The key values we are looking for in these specs are the recommended forward voltage (3.2 V) and the recommended operating current (20 mA).
Let's use the examples that have been mentioned in this thread to identify the components on in our simple circuit.
Depending on our system, we will know our power supply voltage, but we need the value for the resistance to drop the voltage down to the level needed for the LED we are using. So we know the LED voltage (Vd) and operating current (I), and power supply voltage (Vps) so let's find the resistor we need for full intensity.
For a 12 VDC power supply
R = Voltage Drop Required/Current = (Vps-Vd)/I
R = (12 - 3.2) / 0.020 = 8.8 / 0.020 = 440 Ohms
The required power is determined by multiplying the current times the voltage:
Power = Vps x I = 12 x 0.020 = 0.24 watts
If there is not a resistor that exactly matches this, we want to use a standard resistor with the next highest available resistor.
For Ngineering, this would be a 453 Ohm, 1/4 watt resistor.
For an 18 VDC power supply
R = (18 - 3.2) / 0.020 = 14.8 / 0.020 = 740 Ohms
Power = 18 x 0.020 = 0.36 watts
For Ngineering, we would use a 750 Ohm resistor, 1/2 watt resistor
For a 9 VDS power supply
R = (9 - 3.2) / 0.020 = 5.8 / 0.020 = 290 Ohms
Power = 9 x 0.020 = 0.18 Watts
For Ngineering, we would use a 301 Ohm 1/4 watt resistor
So now we have seen how we can identify the resistor in a single LED circuit. These calculations will work with any manufacturer's LEDs and resistors, you simply need to know the recommended forward voltage and recommended operating current for the LED, and the voltage of the power supply you are using.
Please ask any questions you might have.
Next Installment: What if I have more than one LED in the circuit?
Take care,
Mark
The 3.2 Volts for the LED is what is required for it to turn on. That really is the voltage drop across it. So if we have less than that, it simply will not light. If we have more than that, we will push more current through it, and if it is above the maximum current, it will fail. (It doesn't really blow up, the light just goes out permanently.) So what we really need to look at is to provide the voltage necessary to supply the voltage drop across the LED, and calculate the resistance we need to provide 20mA to properly power the LED to full illumination.
So looking back at our example of a 9 V power supply, If we going to lose 3.2 V across the LED, then we need to add a resistor to dissipate the remaining voltage (9 - 3.2 = 5.8V) at 20mA (the desired operating current for the LED). By the way, 20mA = 0.020 A. This gives us the formula:
5.8/0.020 = 290 Ohms.
A standard resistor is not available in this size, so we choose a resistor with a higher resistance that is closest to this value. In the example above, that would be a 301 Ohm resistor.
I did some checking and found a listing of common resistor values, so these should be available pretty much through any source.
In my previous post, I said the next subject we would look at would be what we do if we need more than one LED. As I look at Quincy's I am looking at lights in the garage, maybe one in the shed, and then likely a couple of exterior lights for illumination after dark. In all likelihood, I would want these to be turned on at different times and certainly would want different intensities of illumination between the internal and external lighting.
So that brings us to a discussion of wiring architectures. As I mentioned before, I'll use Tim Anderson's drawings from Ngineering as he has given me permission to do that and they are really quite good.
If they are wired so that they branch from a common wire and all land on the same return wire, that is called a parallel circuit as can be seen in the drawing.
As you can see from the diagram, each branch has the same beginning and ending voltage, but as you add branches the current for each branch is added so that you are drawing the sum of the branch currents from the power supply. This also is important from the perspective of wire size.
If all multiple LEDs are wired one right after the other on the same wire, that is called a series circuit.
Reflecting on the example of water, you will get more flow through a garden hose than you will through a drinking straw. But if you are only drinking from a glass, the extra capacity of the hose is meaningless. Similarly, when you look at the really small amount of current being carried for the LEDs, you really only need a really small wire. Just for a comparison, 18 AWG is 0.04" diameter and can carry 10 amps. 38 AWG is about 0.004" in diameter (.19" O Scale, 0.35" in HO Scale )and can carry 31.4 mA which is easily enough to carry the current for a series circuit providing 20mA to the LEDs. So we can use really fine wire for wiring our lights. If you've seen some of the threads here, the wiring for LEDs can easily be hidden in the model.
There are some general rules for implementing these circuits:
1. In a parallel circuit, the voltage is the same through all components (LEDs), but the current is divided through each.
2. In a series circuit, the current is the same, but the voltage is divided.
3. In a series circuit, the sum of all LED voltages should not exceed 90% of the supply voltage to ensure stable LED light output.
4. In a series circuit, all LEDs should have the same voltage (Vd) and current (I) properties.
As we look at each of these two architectures, presuming we are using our same LED from before, each branch in the parallel circuit would have one 301 Ohm resistor and one LED with a forward voltage of 3.2 V.
But the series circuit presents a different issue as we have three devices all in a row, so we need to count the total of their voltages in determining what resistor we need to use.
I use a 12 regulated power supply, and LEDs with 3.2 V forward voltage. Our equation for resistance then becomes:
R= (Vps - (Vd1 + Vd2 + Vd3) / I = (12 - (3.2 + 3.2 + 3.2) / 0.020 = (12 - 9.6) / 0.020
R = 2.4 / 0.020 = 120 Ohms. So from the resistor listing above, we see that we would use a 121 Ohm resistor in this series circuit.
So that really is the basics that will underpin all wiring that we do.
I got everything up to this point pretty readily. If it is a bit heady for you, don't worry, I R an ingineer so I'm supposed to be able to figure these kinds of things out. But I don't want the LED's full-on intensity for a 1930s era railroad. I get that I will need to add a larger resistor to give less current to dim them, but how much?
When I asked Tim Anderson this question, he said, "It depends on how bright you want them, and only you can tell that." Tim also said one thing was really important, and that was to test, test, and test at every stage of construction. Because it is far better to find out that something doesn't work before you button everything up.
And from that, came the idea to construct a test stand for testing the wiring of individual LEDs and to be able to dynamically adjust the current to the LEDs to get the intensity of illumination I wanted.
And so that is where we will go next.
Mark
I’m looking forward to subsequent installments. Thank you, Mark.
My hope with this thread is to demystify this subject so everyone is willing to give it a try. Tim Anderson has prepared loads of documentation on his site with the intent of helping everyone. He really is a pioneer in this field and entered into it through model railroading. Just for a starter, if anyone wants to jump into a real treatise on this, you can see the document I have referred to for this at: https://ngineering.com/led_circuits.htm
My next post may be delayed a bit, as I have to either scan my hand drawings or create a CAD drawing of my plans for the test station. I have it partway assembled, but decided to start the thread midway so I could fairly convey some of my wonder, frustration, or successes as they happen.
I want to extend my deepest gratitude to all who contribute to this forum. It has been invaluable to me.
Mark
Wow, this is a great tutorial and hitting all of the right points. I add lights to my dioramas as well and thought I'd add a few things I have learned to add to your work. The last diorama I wired was Brett's HO Deer Creek Mine, which had about 12 Led's throughout, as well as a flickering burn barrel. There are pictures in an old thread of the build and lighting.
1) Managing the light colour/brightness: I use "Amber Gallery Glass" directly on top of the LED once I have wired and installed them into the shade. You can vary the amount of GG applied to get the desired colour/ brightness. It takes away the brightness and leaves an amber tint similar to incandescent bulbs.
2) Series vs Parallel: I run single LED circuits back to a perf board (usually 2" x 3" board) under each diorama. I never couple leds in series or parallel. Although I have done the math work for both scenarios I prefer to have a single resistor for each led so that I have the option of tinkering with voltage. It's also the simplest approach to both wiring and testing things for me.
3) Wire: 38GA magnet wire from Ngineering.
4) LED's of choice: Size: 0603, Colour: Warm White, Rated voltage: 3.0 - 3.2V
5) Resistors: I have found that using higher value resistors provides a closer/more accurate colour and brightness. Greater than 1K Ohm. On my current build I have used 4.7 K Ohm and 10 K Ohm.
6) Testing: I do many lighting tests throughout my builds to make sure brightness and colour are right for the setting/light type.
7) Terminal Blocks for Perf Board: I purchased about 100 small screw type terminal blocks that are invaluable to use as you build your diorama electrical circuit. You can find them anywhere but mine are "5.08mm Pitch Panel KF301-2P KF301-3P Screw Terminal Block PCB Connector". I mount these to the perf boards which allows for flexibility during your diorama build and testing.
I am really Looking forward to the rest of your thread Mark.
Mike
I really want a test stand that can be adapted to a number of different applications. So here are some of the criteria that I selected:
Able to be adapted to different power supplies
Easy to replace any component
Designed so as to protect the LEDs
Be able to vary the intensity of the LEDs
Be able to measure the resistance needed to achieve the desired intensity
Be able to measure current in the circuit at the desired intensity
Have the ability to test an LED installed in the model
It is always easier to sketch out plans ahead of time in order to modify something on paper before it is built, which is normally cheaper and faster as well. I had originally intended to use one of the Ngineering power distribution boards to experiment with wiring it, but as I was drawing things out, I realized it was unnecessary for the test stand, but I left provisions in the design to accommodate one in the future if desired.
Here is the schematic of the test platform now:
As you can see that the circuit has five fixed elements in the circuit: a power supply, protective resistor, potentiometer, current test switch, and on/off switch. The LED shown is the LED to be tested.
Power supply - use a well regulated, switching power supply, not just the cheap wall mount battery replacers. Any power supply between 9 and 18 VDC should work well. It makes sense to use the same power supply that you will use to power your LEDs, whether on a layout or diorama.
Resistor - This resistor should be sized to reduce the voltage to the recommended forward voltage of the LED being tested. This will protect the LED to ensure that we will never provide more voltage than what is needed for full intensity. In general, the sizing we discussed previously should work well. Remember, the resistance will be dependent on the voltage of the power supply used:
9 VDC - Use a 301 Ohm 1/4 watt resistor
12 VDC - Use a 453 Ohm 1/4 watt resistor
18 VDC - Use a 750 Ohm 1/2 watt resistor
Resistance Test Points - These two terminals allow you to easily measure the resistance across both the resistor and the potentiometer to determine the resistance needed for a desired level of intensity in the LED.
Potentiometer - This is a fancy term for a variable resistor, like a temperature control switch on a heating element. I would recommend using a 5K Ohm, wire-wound potentiometer. I have both multiturn and linear potentiometers that I will see which one works best. This will add a resistance above the base level for the LED and will reduce the current through the LED to lower the intensity.
Resistance Test Points - These two terminals allow you to easily measure the resistance across both the resistor and the potentiometer to determine the resistance needed for a desired level of intensity in the LED.
*Note: the On/Off switch should be turned off when measuring resistance, so as to get the correct reading.
Current Test Switch - This is a single-pole, double-throw (SPDT) switch that is used to switch the circuit so that the current in the circuit can be measured with a multimeter connected to the two test terminals.
On/Off Switch - This switch energizes the LED. It also is used to isolate the circuit from the LED to accurately measure the resistance across the resistor and potentiometer.
Connection to LED - I concluded that instead of hooking the LED to terminals, I would include test leads with alligator clips which would give me the ability to connect to a circuit on a model in case I wanted to test the lighting effect in place.
Please ask any questions you might have. The next stop will be the layout of the actual test stand and wiring schematic.
Mark
I am about 1/2 way through constructing the test stand, but will finish it up likely on Saturday, and will be able to show some pictures and results then.
My hope is to post a sketch of the actual wiring diagram I'm using for the test stand so it should make it easier for someone unfamiliar with electronics to wire it together.
Thanks again for the kind remarks.
Mark
Mark
Hopefully this approach makes it simple and flexible. Thanks for the ongoing input.
Mark.
Also, I started this off commenting on how this really was my first exposure to wiring LEDs. I've got to tell you, that if you simply follow the directions, it really is quite straight-forward. I can see where wiring your own gives you some level of flexibility and frankly, it really isn't hard. Here is a picture of the very first LED I've ever wired.
And guess what? It worked the first time! No problems.
So I actually completed the schematic and wired the test stand. Here is a picture of it as it looks.
The first knob adjusts the intensity of the LED, the switch that is next, allows you to switch the circuit over to measure current, the second switch is used to either turn the LED on or to turn it off. You measure the resistance in the circuity with the LED off.
The clothespin is what I chose to use to hold the LED. You can see the two leads for connecting to the LED. They are over 21" long each which should allow easy connection to a model sitting nearby. The four-terminal block on the front of the test stand is where you connect a multimeter to measure resistance or current. The left two are for resistance, the right two are for current.
It was really hard to effectively photograph the LED to show intensity when I took a photo of it directly, so I placed it inside of a wire spool to see the effect of the changes more indirectly. The next three shots show differing intensities of the LED.
Full Intensity
Medium Intensity
Low Intensity
I measured the resistance and current flow for each of the three intensities:
Intensity Resistance Current
High 453 Ohms 19.47 mA
Medium 1440 Ohms 6.4 mA
Low 2760 Ohms 3.37 mA
I'm happy that this works and should be really helpful. With this information, you can determine what you need to do to properly wired LEDs in series, such as a string of streetlights, or a series of lights illuminating the inside of a terminal or work area.
I'd like to continue on with that discussion in the next post. Also, I'm happy to share the schematic and give a list of materials if that will be of interest. I took some in-process photos showing how I mounted components, wired them individually, and then photos showing how to wire the components together on mounted terminal blocks.
With a little bit of time and some simple electrical wiring, you can do this too.
Thank for following along.
Mark
I learned some things and just followed the techniques that are on the ngineering website. It's kind of like reading Brett's construction manuals. You come out doing better than you thought you could do.
Mark