POWER & LEDs

 This page is part of my Model Remodel series of articles.

DISCLAIMER: If you choose to attempt any of these modifications, you assume all risks thereof. I just wanted to share my experiences here. Neither Eaglemoss, nor myself, are responsible for any damages that may occur.


The Power

In stock form, Eaglemoss designed our U.S.S. Enterprise D partwork model to use a bunch of AAA and button-cell batteries to power all the lights. There are at least two battery packs in the saucer, one in each nacelle, one in the neck, and one in the battle section. There is no way I wanted to keep changing these batteries all the time, so I will power my entire starship with an external power supply. Using external power also simplifies the installation, wiring, and electronics, and at the same time frees up more room to fit the things I need inside the hull.

The electronics/lights supplied with the model are designed for about five (5) Volts of Direct Current (VDC). I had to make a choice going forward on whether to keep using 5V or go with something else. The considerations mainly revolved around the LED Strip I wanted to use for backlighting the new resin windows. While a 12V LED strip would be brighter, it would also create a lot more heat inside the model. Since all of my electronics run on 5V, I decided to just keep it simple and move forward using 5V to power my model. This also meant I could reuse any of the stock electronics I might need.

It did cross my mind that I might be able to power my entire setup using a standard USB cable. However, when I first tested the lighting and sound system in DEMO mode to check the maximum current draw, my bench power supply peaked at about 2.3 Amps (A) @ 5 VDC. This was even before I connected the battle section and the second warp nacelle lights. Since standard USB can only provide between 0.5A and 0.9A, it was clear that it would unsuitable to power my setup. My tentative solution will be to buy a AC-to-5 VDC wall adapter and incorporate it into the display base later. This adapter will likely need to provide 4 or 5 Amps at 5 VDC.


The Connectors

Just in case I wanted to use any of the stock components, I bought a bunch of pre-wired plugs (male) and sockets (female) on eBay. The wiring plugs/sockets used by Eaglemoss are common JST micro 1.25 mm pitch connectors. These connectors are also available online in many other places such as Amazon and DigiKey. Since our Enterprise (and most other Eaglemoss die-cast builds) use these connectors in 2-pin, 3-pin, and 4-pin arrangements for various connections, I bought about 30 pairs of each arrangement:


The LEDs

All of the lights inside our Enterprise model are LEDs. An LED or Light-Emitting Diode is a semiconductor component that emits light when charged with an electrical current. LEDs are very power efficient and run cooler than most other light sources. As LEDs are powered by Direct Current (DC), they have specific polarity – an anode (positive +) side and a cathode (negative -) side. If an LED is hooked up backwards, the diode inside will restrict current flow and they will not light up. On most LEDs, the leads/legs are different lengths to tell which is which, with the anode (+) being longer than than the cathode (-):

On my Enterprise model, there is only one stock LED component I plan on re-using during my ‘Model Remodel’. That is the small PCB for the Main Impulse Engine. I will use one of the pre-wired socket connectors I mentioned above to connect this PCB to my electronics setup.

To light the starship’s Windows, I bought this two-pack of LED Strips. These 1-meter (3.3 ft) flexible LED strips are powered by 5V DC, can be cut shorter as needed, are dimmable, and are quite bright. They are available in a few different colors, however, since the studio production models used white neon to light their windows I ordered mine in the Daylight White (6000-6500K) color. These strips also include a pre-connected USB cable and dimmer, but I cut those off so I could just attach the wires directly to my electronics.

Daylight White 5V LED Strips for the Hull Windows

For the new Beacon Lights I am adding to my model, I am using pre-wired micro 0402 3V LEDs. Details on how I wired and installed the first of these LEDs can be found on my Bridge/Deck Two page. These are very small which allowed me to attach them to fiber optic and get the light where I needed it to go.

Pre-wired micro 0402 3V LEDs for the Beacon Lights

For the Warp Nacelles, I used some standard steady/breathing 3mm round LEDs in the Bussard Collectors and a different 5V LED Strip for the Warp Grilles (this strip is cool blue and has the LEDs very close to each other). Details on these can be found on my Warp Nacelles page.

Cool Blue LED Strip for the Warp Grilles

For most of the remaining 2x3x4mm rectangular LEDs, I jumped on eBay and found some pre-wired 5V 3mm ‘flat top’ LEDs in different colors, such as: red, green, orange, blue, warm white, and normal white. I picked these pre-wired LEDs because they already have the correct resistor installed so I can connect 5 VDC directly to them without any concerns. Unfortunately, these LEDs are round, not rectangular like the Eaglemoss ones, so I might have to sand them down to fit.

Pre-wired 5V flat-top 3mm LEDs in various colors with resistors pre-installed

My current intention is to use these LEDs all around the model for various purposes:

  • RED – Port (left) Formation Light, possibly the Saucer Impulse Engines
  • GREEN – Starboard (right) Formation Light
  • ORANGE – Possibly the Saucer Impulse Engines
  • BLUEDeflector Dish
  • WARM WHITE – Forward Formation Light
  • WHITEBridge/Deck Two

Powering an LED

To power an LED correctly, we need to know two important values for that particular LED: Forward Voltage and Operating Current. For example, there is a label inside the box of my Assorted 3mm round LED Set where these values are clearly listed.

NOTE: The third value shown here for each LED is the light output in millicandela (mcd) and is not relevant to this discussion.

If we wanted to light one of these Red LEDs, we can see that it has a Forward Voltage of 2.0 – 2.2 Volts (V) and an Operating Current of 20 milliamps (mA). Therefore, if we had a power supply that provided exactly 2.0 – 2.2 Volts DC  AND could limit the current to only 20mA, it would light up just fine. Unfortunately, real world applications are usually not so perfect.

Where it starts to become tricky is when the supply voltage and/or maximum current does not match the LED. For example, I am powering my Arduino Uno with a 5 VDC supply. Every output pin on my Arduino can then provide this 5V at up to 40mA of current. If we powered our Red LED directly off one of these pins, it would likely burn out very quickly. It is just too much current at too high a voltage for the tiny diode inside the LED to handle. This is where Resistors come into play.

Using Resistors with LEDs

Resistors are current-limiting devices and lower the current of a circuit by converting excess energy into heat.

Using a simple resistor in series (inline) with our LED circuit, we can bring the voltage AND current of the 5 VDC power supply down to the values required by the LED. In this case, we want it to be around 2V (Forward Voltage) at 20 mA (Operating Current).

But what kind of resistor should we use? The primary measurement of a typical resistor is Resistance measured in Ohms (depicted by the Omega symbol Ω) and indicated on the outside of the Resistor using color-coded stripes.

To calculate the resistance needed, we have to use a formula from Ohm’s Law and a little basic math. Without getting too deep into the law itself, consider this formula:

Resistance (Ohms) =Voltage (Volts)
Current (Amps)

If we could provide exactly 2.0-2.2 Volts DC to our Red LED and we wanted the LED circuit limited to the LED’s Operating Current of 20 mA, this formula will provide the Resistance needed.

There are two things of note here: First, I used the lower end of our Forward Voltage value (2 Volts) to make the math easier to follow. Second, Ohm’s Law uses full Amps (A), so we need to convert the Operating Current milliAmps (mA = thousandths of an Amp) to full Amps by dividing the 20 mA value by 1000:

Resistance (Ohms) =2 (Volts)= 100Ω
0.02 (Amps)

Doing the math here, we divide the 2 Volts by 0.02 Amps and the resulting Resistance value is 100Ω (Ohms). Therefore, by placing a 100Ω Resistor inline with our LED, we could use a 2 VDC power supply that supplies much higher Amps and the LED would be safely current-limited to its optimal 20 mA Operating Current.

Now, in my setup I am using a 5 VDC power supply. So, can we still power this LED with it? Yes!

LEDs are diodes and all diodes create a Voltage Drop as current passes through them. With LEDs, this Voltage Drop value is the same as the Forward Voltage value. To adjust our formula in this situation, we need to subtract the Forward Voltage of the LED from the Supply Voltage first, then divide by Amps:

Resistance (Ohms) =Supply Voltage – LED Forward Voltage
Operating Current (Amps)

Inserting the values, our formula looks like this:

Resistance (Ohms) =5 – 2= 150Ω
0.02

Subtracting the LED Forward Voltage (2V) from the Supply Voltage (5V) leaves us with 3V, which we then divide by 0.02 Amps. This gives us a Resistance value of 150Ω (Ohms). This is how I knew to use 150Ω Resistors inline with each Red LED on my breadboard while testing my electronics. Remember, each type/color of LED can have a different Forward Voltage and Operating Current, so you may need different Resistors throughout your project to safely maximize their light output.

TIP: If the Resistance value does not match a Resistor you have, use the next larger Resistor. Say the formula results in 400 Ohms and you don’t have a 400Ω Resistor, you may need to use a 470Ω on that circuit. I have run into this situation on my build many times, but a few more Ohms won’t noticeably affect the light output. It is better to be safer with a lower current, than sorry with an overcurrent.

ANOTHER TIP: If you oversize the Resistor connected to an LED, it will put out less light and eventually not light at all. However, this may come in handy if an LED is too bright compared to others. I recommend going up in Resistor value one step at a time until you achieve the effect you are looking for.

Resistor Power Limits

There is second value that is often overlooked when using Resistors: their Power rating/limit. This value is typically expressed in Watts (W).

Since a Resistor limits current by converting excess current to heat, we need to make sure our Resistors can handle the load without burning up. If Resistors are overloaded, they can fail, overheat, emit smoke, and possibly catch fire. We do not want any of that at all.

To calculate the power passing through our Resistor, we can begin with the following formula:

Power (Watts) = Voltage x Amps

Since the LED in our circuit is a diode with a Voltage Drop, the Resistor itself only needs to handle the excess power above this Voltage Drop value. For example, if our power supply is 5V, and the Voltage Drop of our Red LED is 2V, the excess power of the circuit is the remaining 3V. We also know the Operating Current Amps from the previous calculations, so putting these values in to the formula looks like this:

Power (Watts) = 3V x 0.02A = 0.06

Doing the math results in a power of 0.06 Watts through our Resistor. The resistors I am using on each LED are rated for 1/4 Watt (0.25 W) so they are more than capable of handling this load. This is another reason I did not bump up to a 9V or 12V power system – my Resistors will stay cooler dealing with 5V versus the higher voltages.


Powering Multiple LEDs

Powering many LEDs at the same time means we have to decide whether to use series or parallel connections, as well as one or many resistors. Without getting into the pros and cons of each option, I will just say we will likely have a lot less headache if we just power each LED individually and put a correctly-sized Resistor in series (inline) with that LED. Turns out, this is the also most common method of creating LED circuits in electronics.

You can actually see this type of configuration in practice on the stock Eaglemoss PCBs. There are individual tiny SMD (Surface Mounted Device) Resistors connected to the ground pins of every single LED socket on the board. I would not normally use the same value resistor for every LED (or pair of LEDs), but Eaglemoss likely did this to keep costs down and simplify production.

Fun Fact: The tiny number printed on these SMD Resistors is the Resistance value in a standard 3-Digit EIA coded format. With this format, the first two digits are the significant value (51) and the third digit is the how many zeros to add to the end of this value. Therefore, these Resistors marked ‘510’ have a resistance of 51Ω – 51 with no zeroes. If these were marked ‘511’, they would be 510Ω, a ‘512’ marking would be 5100Ω, and so on.


In Conclusion

It took a bit of learning, but working with LEDs has been satisfying and a lot of fun! I hope my explanations here were not too confusing. In the next section, I will explain how I started hooking up my LEDs to an Arduino and controlled them with an Infrared Remote Control.

Next Section


ARDUINO – Arduinos, Programming, Timing, and Remote Control