Realistic Lighting Using Light-emitting Diodes (LEDs)

MarkG

So I mentioned the results of my test earlier. I'd like to go back to that and show how it can be really useful, instead of simply more stunning math?

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

So if I was just wiring up one LED, I now know the resistor values that I need to have to obtain that level of intensity. No guessing. So from this point forward, I have a good place to start with the different lighting intensities.

However, my original challenge, was how do I know what resistance I need if I wire three LEDs in series for the interior lighting on my building. For my layout, I intend on having a different circuit for each of the areas of lighting so that I can turn them off independently for different effects, or time-of-day lighting.

So if I want to have low-intensity lighting using three of these LEDs in series I'd use the formula we used earlier:
R= (Vps - (Vd1 + Vd2 + Vd3) / I = (12 - (3.2 + 3.2 + 3.2) / 0.00337 = (12 - 9.6) / 0.00337 = 2.4/0.00337 = 712.2 Ohms

So I now know that I will need to use a 715 Ohm resistor using the charts shown earlier. Once I identify the levels I like to use, following the testing method above, I have all the information I need to calculate the resistances I need for different applications without going needing to do the retesting.

With the extended test leads on the test stand, I can even validate this solution by hooking up to leads in the model and seeing the results before wiring in the resistor.

It took me a bit to draw up the schematic in a simple way that I could share, so here is the schematic. A file is available that I am happy to send to anyone who wants it.

One of the characteristics that I wanted to incorporate is the ability to replace any device without having to undo any other wiring. That is why you will note that all the terminals associated with a potentiometer or switch are connected. This gives me flexibility to add something if I want to later.

The value for the resistor shown should be selected to match the DC voltage of your power supply as shown below:
Power Supply Resistor
9 VDC 301 Ohm, 1/4 watt
12 VDC 453 Ohm, 1/4 watt
18 VDC 750 Ohm, 1/2 watt

The potentiometer should be a 5 KOhm, multiturn, wire-wound, linear(optional) potentiometer.

I'm happy to provide a bill of materials for the test stand I built if you have an interest.

I'll add pictures in my next post that I took during construction.

Mark

MarkG

Here are pictures of the build of the test stand, with comments on what I used or why I did something. This first photo is of the wooden frame components after they have been painted. The truth is that these were just scraps I had around: 1/4" plywood for the sides and front, and 3/4" plywood is used for the base.


These are the materials I started out intending to use. I didn't use the small distribution PC board but made allowances for using one in the future if I wanted to. I think that getting a good power supply that will provide well-regulated power is worth it.


In retrospect, I don't think I would have used the small surface mount resistor, but it is what I had for future work. Although it is not really that hard to work with, I would likely have just ordered one at the same time I ordered the potentiometer.


Here all the components assembled and mounted. You can see that I have wired in the resistor, power supply, as well as the test leads for connecting to the LED.


This shows the devices from the side. I referred to the terminals on the switches as 1, 2, and 3 starting at the top. A little electrical fact that may be of no interest, but when you flip the switch up, terminal 2 is connected to terminal 3. When you flip it down, terminal 2 is connected to terminal 1.


This photo shows the test terminal strip on the front of the test stand with holes drilled under each of the screw terminals. The left two terminals are where the resistance is measured; the right two are where you measure the current.


This shows the power supply, resistor, test leads, and devices connected.
(The power supply and resistor connections are different than in the previous photo as I simplified the wiring schematic.)


Next, I added the wiring for the test points. Just a note, I wired the front panel terminal points first, then terminated them to the main terminal strips. This made terminations and determining the correct length easier.


This shows the remaining jumpers added. Wiring is now complete.


When I mounted the knob on the potentiometer, I turned the potentiometer all the way clockwise until it stopped. I then mounted the knob with the index line pointing straight up. This accomplishes two things: it gives an easy recognition of when the potentiometer is at the end of travel and makes adjustment more logical as you turn the knob counterclockwise to decrease the intensity of the LED.

The left switch toggles between run mode and test mode
The right switch turns the LED on and off.

To test an LED
1. Start with both switches in the down position
2. Measure the resistance across the first left two test terminals to ensure that it is showing the resistor is properly in the circuit.
3. Flip both switches to the up position, the LED should light.
4. Take an initial measurement for the full current reading.
5. Flip the left switch to the lower position
6. Measure the resistance on the left two test terminals
7. Measure the current on the right two test terminals (the LED will turn on when you are testing the current)
8. Flip the left switch to the upper position.
9. Adjust the intensity of the LED to the desired position
10 Repeat steps 5-9 for as many points as you desire.

When you are done, you will have a "performance curve" if you will for the LED, that you can use in the future.

I am sure I left something out, and welcome any questions and comments. Take care in this crazy time.

Mark

Robert.G

Mark, we are all grateful for this tutorial. At first it sounds like Chinese, but after a second look at it, it's not that hard to get.
Thanks a lot.

MarkG

Thanks Robert. I stand in rapt amazement at the skills of those in this forum and very much appreciate your skills with landscaping and creating a window of reality.

I ventured into this, frankly, intimidated by the whole topic. My hope was to debunk this, and I'm glad I ventured into this. It really is quite straight forward.

I suppose that I could offer to build a test stand for someone if they didn't have the knowledge to do it, but it is really simple at its core. I'm just happy to be able to accurately model lights now, as most I see seem to brightly illuminated for my taste.

Mark

Mountaingoat

Mark's jig is the ultimate for finding the perfect illumination and his discussion very accurate. Thank you Mark. As an electricaI engineer, I've done more math and test jigs than I can remember, but I like simple.

If you want very simple, use at least a 470 ohm resistor in line to start. That will let you go up to 12V without damaging the LED, but at full brightness. I keep only 3 values in ready supply. 470, 1000 and 2400 ohms. I typically start with the 2400 ohm because I dim my LED's way down. Then I add more in series if needed until I get the desired results.

I do use the resistors with leads (about $.05 each or less) as it is much easier to solder to than the tiny chip resistors. I hide them in the rafters or underneath the structure, wherever they can't be seen. Just insure that open wire joints are isolated from each other to prevent shorts. Heat shrink tubing works wonders. Rick

EricMG

So, if I want simple lights with less wiring, who do I buy from?

Mountaingoat

Both Ngineering and Evans Designs have what you need. Evans has a little better selection of prewired LED's for simplicity. They come in both Warm White and Cool White. Warm White is very close to incandescent lights, especially if you dim them down like I did in both my truck repair shop and current office warehouse build for reference. The cool whites come close to fluorescent lights if anyone needs that color light. Rick

EricMG

Thank you.

MuddyCreekRR

The cool white also work good for outside floods...just regulate to between 2 and 2.5 volts....