Add led lighting to your scale R/C aircraft

Authentic scale lighting adds an important element of realism you your scale R/C airplane. This article will show you step by step how to add authentic led lighting to your scale airplane.

Led lights make a terrific addition to a scale R/C aircraft for many reasons. First, they allow you to add lighting that is correct in size, shape and color. Second, led lights consume very little electrical power for the amount of light that they produce. Third, they are inexpensive and easy to work with. Coupled with one of Lake Microsystem’s LED controllers (Flash Magic, Light Magic, Jet Magic) your led lights are fully controllable with any spare toggle switch on your transmitter and an unused channel.

While there are many different kinds of leds available in the marketplace, we have found that T1-3/4, 5mm super bright leds are the best choice for lighting a scale R/C airplane. These are available in many different colors, but white, red and green are ideal for duplicating the look of full size aircraft navigation (position) lights, flashing beacons and landing lights. We have chosen the following leds:

Navigation (position) lights right wingtip (green):  Green T1-3/4 5mm 1600 mcd with 140 degree viewing angle. These leds consume 99mW of power and have a forward voltage of 3.3v.

Navigation (position) lights left wingtip and beacons(red):     Red T1-3/4  5mm 1200mcd with a 120 degree viewing angle. These consume approximately 85mW

Navigation(position) lights tail(white): White T1-3/4 5mm  1500 mcd with a 120 degree viewing angle

Landing Lights(white): White T1-3/4 5mm 18,000 mcd with a 30 degree viewing angle. They consume about 100mW.

It is probably worthwhile at this point to say something about what the numbers cited above mean. The “luminous intensity” of a light source is measured in “candella”. It is a measure of the intensity of the light in a particular direction. A milli-candella (mcd) is one thousandth of a candella. The term comes from the word “candle” and a common candle has a luminous intensity of 1 candle or 1000 mcd. Applying this to the lights described above, we see that the nav lights are brighter than a candle over a cone 120 degrees wide. The landing lights are as bright as 20 candles, but only over a cone 30 degrees wide. Both types of led consume about the same amount of energy, but that energy is focused over a much smaller area in the landing lights so they are much brighter in that small area.

As a practical matter, the leds we have chosen for the navigation lights and the beacons are plainly visible at dusk all around the traffic pattern. In the dark, they stand out brightly anywhere in the traffic pattern. The leds we have chosen for the landing lights focus the light into a tight beam, and they are very bright indeed. In the dark, they will actually light up the runway on final approach. They are so bright that you are advised not to look into them directly because they will cause discomfort and the instant loss of night vision.

One of the things you will immediately notice about the super bright leds is that, unlike their smaller cousins, they are all clear, regardless of color. The color is produced by the wavelength of light, not by the color of the glass. This makes them efficient, but is has an inconvenient side effect – they are impossible to tell apart.

diodes 1

They have a long lead(+) and a short lead(-)

 

They have a long lead(+) and a short lead (-)

So, here is a handy tip: get or make yourself an led tester. This is a simple device consisting of a 9v battery, a battery clip and a 1000 ohm resistor.

diode tester

Don’t forget the resistor

 

Touch the red lead (+) of the tester to the long lead of the led and the black (-) lead of the tested to the short lead of the led. You will find that this simple device makes your job ever so much easier.

diode test

If you connect it backwards, it will not work

A word of warning – do not leave out the resistor! If you connect a battery to an led without a current limiting resistor you will be rewarded with a brief flash of light while the led burns up. Next, you will be looking for a new led.

What is forward voltage?

The next thing you need to know is a bit more complex, but its is critical knowledge if you intend to make up your own led wiring harness. Leds are semiconductor devices. Unlike a light bulb which has a heated fillament, an led produces light in response to electrons crossing a junction between to different semiconductor materials. This is a one way trip, the electrons can only cross the junction in one direction, they cannot go backwards. This “one way” action is called a diode, which gives the led its name “light emitting diode.” Getting the electrons across the junction requires a push, and the push comes from voltage, which you can think of as pressure in a water pipe. The amount of push, or forward voltage, depends on the energy of the light produced – more for blue light and less for red light. White light, being a combination of all the colors, needs a big push, or a high forward voltage. The forward voltage for leds range from about 3.5v for white or blue to about 1.7 for red.

When the electrons cross the junction, they give off light, or energy. The loss of energy results in a voltage drop across the led equal to the forward voltage. This means that every time that electrons in a circuit cross the junction of an led, the voltage in the circuit on the other side of the led drops by the amount of the forward voltage. Figure. If the voltage source driving a series circuit (the battery) does not have sufficient voltage to make it across all of the junctions, no current will flow and none of the leds will light up! Yikes!

One way to overcome this limitation is to wire the leds in parallel. That way, all of the leds have the same voltage across them, and as long as it is above the forward voltage, the current divides equally between them, and they will all light up. However, this scheme has some serious flaws.  The electrons in the circuit are lazy. They always take the easiest pathway. If the leds in the circuit are different colors, they have different forward voltages. Since it is easier to climb a low fence that a high fence, most the electrons will go through the diode with the lowest forward voltage, and few (or none) will go through the leds with higher forward voltage. The red leds light up but the white ones won’t.

PARALLEL

There is another problem, too. The light that an led produces is a function of the current through it – more current, more brightness. In a parallel circuit with mixed color leds, even if they all light up, they will be different brightnesses. For these reasons, engineers design circuits with mixed leds as series circuits. This way, all of the leds have the same current, and all have the same brightness.

This is why you should wire your navigation lights, landing lights and flashing beacons in series, not in parallel. Lake Microsystems’ led controllers (Flash Magic, Light Magic and Jet Magic) use constant current controllers to regulate the current through the leds to give just the right current and brightness to all the leds. These controllers are intended for use in series circuits. However, you must have high enough battery voltage to exceed the sum of the forward voltage drops in that series circuit.

What are current limiting resistors and constant current controllers?

As mentioned in the discussion about the led tester, leds are sensitive to the amount of current that flows through them- more current, more brightness. But, every led has an absolute maximum amount of current that it can tolerate. If you exceed the maximum, poof! No more led. So, how do you get the right amount of current?

The best way is to use a constant current controller. It makes sure that the leds get exactly the right current all the time, regardless of the voltage. If you cannot use a constant current controller, then you must use a current limiting resistor. A resistor is a passive circuit element that makes is harder for current to flow in a circuit. They come in a wide variety of different resistances. The higher the resistance, the more the current is limited. This relationship is known as Ohm’s Law, which says that the current in a circuit is proportional to the voltage divided by the resistance or I = V/R. If you know the current you want, say 20 milliamps (0.020 amps), and you know your voltage, say 12 volts, then, the resistor you need is 12/.020 = 600 ohms. This is about right for our super bright leds.  The 600 ohm resistor is wired in series with the leds and will limit the current to a maximum of 20 milliamps. However, if you are using constant current controllers (e.g. Lake Microsystems’ led controllers) you do no need and should not use current limiting resistors! They will confuse the current controllers and your leds will not achieve full brightness.

Making your wiring Harness

Basic Wiring Rules:

Since your lighting systems are independently controllable, you will need a separate wiring harness for each of your three lighting systems. The most complex of the three is the navigation lights because they are located in both the fuselage and the wing. We recommend that you follow these basic rules:

1. Use 26-28 gauge stranded wire for your lights – Stranded wire is more flexible and less likely to crack with repeated bending.

2. Be consistent in your wire color scheme – All positive leads are red and all negative leads are black. Note that in the series circuit you will have to connect red wires to black wires. If the wire leads to the anode of an led (+ or long lead) or to the positive terminal of the controller , make the predominate wire color red. If a wire leads to the cathode of an led (- or short lead) make the predominate lead black.

3. Solder connections –  Soldered connections are far stronger, more reliable, and much better electrically. Strip the end of each wire about 1/2″ from the end with a wire strippers. Slip a piece of 1/8″ shrink tubing about 1″ long over one wire. Twist the bare stripped wires together and solder them with rosin core solder. Fold the soldered connection along the side of one end, cover it with the shrink tube, and shrink it with a heat gun till it is tight and completely covers the soldered connection.

4. Solder the leds – Strip both ends of the wire that will connect to the led about 1/4″. Slip two pieces of shrink tube over the ends of the wire, the first one on both wires and the one on the red wire.

solder_001

solder_002

Hold the led by one lead while you solder the red lead to the long wire then snip off the excess lead and slip the shrink up tube to cover the entire lead from the base of the led to the insulation on the wires and shrink it.

solder_003

 

 

 

Solder the black wire to the other lead and repeat the shrink tube process.

solder_004

solder_005

Finally, you need to add your wiring harnesses to your R/C airplane. We will cover this topic in detail in another post.

Tagged with:
This post was written by

Leave Your Comment