Using LEDs in models

A LED (short for Light Emitting Diode) is ideal for use in models:
it generates very little heat, doesn't wear or burn out and comes in many shapes, sizes and colors (infra-red, red, orange, yellow, green, blue and even white(ish)). There are even types available that can emit different colors or come with built-in blinking circuitry.

LED Basics

Schematic overview of a LED

In the picture, the lefthand lead (called Anode, usually the longest) connects to a tiny wire (blue in the picture) which is fused to the LED crystal (red in the picture).
The crystal is mounted on the righthand lead (called Cathode, usually thicker and shorter). Below that is the symbolic representation of a LED.
If you look at a LED held against a bright light source you should be able to see the shape of the leads inside the translucent body. The lead shaped like an upside-down 'L' is the cathode, that's always the one to connect to the '-' of the power source.
The cathode lead acts as a heat sink, so it is best not to cut the cathode lead off close to the body .

A LED is a diode, which means it only allows current to pass in one direction (hence the arrow-like symbol). If connected in reverse, current is blocked and the LED may even fail.
Another feature of a diode is that it needs a certain voltage before it starts conducting (and emitting light), this is usually called V_forward (or V_f for short). V_f ranges between 1.6V to 4.2V typically, depending on the color of the LED:

• typically 1.6V to 2.0V for red,
  
• 1.9V to 2.5V for green,
  
• 2.0V to 2.2V for yellow,
  
• 2.2V to 3.4V for blueish-green,
  
• 2.4V to 3.7V for blue,
  
• 2.9V to 4.2V for white.
  

An easy way to learn the exact value of V_f is to put the LED in a circuit with a resistor of at least 1kΩ and a power supply of at least 4.5V and measure the voltage across the leads of the LED when it lights up.

The most important figure you need to know about a LED is the current at which it emits light optimally, called I_forward (or I_f for short).
Brightness of a LED is not linearly related to I_f. However it does depend on how well the crystal can shed the little heat it generates: the cooler it is kept, the more light it emits.
Please note that with light emission optimal is not the same as maximal: most LEDs will emit more light at currents beyond I_f but it takes much more current for a smaller gain in light emission (for example a red 5mm LED will typically emit about 50% of its maximum amount of light at 5mA and about 95% of its maximum at 20mA).

Some do's and don'ts

• Don't connect a LED to a power source without a suitable resistor connected in series.
• Better not cut or sand down the top of the translucent body, it doubles as a lens
• When in doubt about polarity of a LED, take a big resistor (at least 1kΩ for each Volt the battery delivers) and try by quickly closing and opening the circuit again in which direction the LED lights up.

LEDs are not light bulbs, they're electronic components that work very different from light bulbs. Most important things to keep in mind:

• The clear body acts both as a lens focusing the light and as protection for the tiny gold wire loop on top of the LED crystal, so better choose the shape and type carefully, to avoid having to sand down the body.
• LEDs use far less current than light bulbs, so don't mix the two in a single circuit.

Apart from the do's and don'ts mentioned above, you're also looking to minimize total current, to stretch battery life.
Connecting LEDs in parallel increases total current . Inside models you have several reasons to keep the current (or rather the consumed power) as low as possible: power consumption generates heat and the less power a component has to consume, the smaller it can be. The latter is especially obvious with resistors where a resistor rated 1 Watt is much larger than one that is rated 0.25 Watt.
To adjust current and allow for putting the right voltage to the LEDs one or more resistors need to be added to the circuit, so it's a matter of applying Ohms law U = I x R to come up with the best circuit layout.

Some basic circuits

Let's start with a very simple circuit, typical for most aircraft and boats: a steady light on each wing tip (or on each side of the bridge) : red on the left side, green on the right.
The easy way is to connect the LEDs in series together with a resistor, as shown in the schematic on the left side.
What voltage do we need and what value should we pick for the resistor ?
Let's assume I_f for these LEDs is 15mA, V_f for the green LED is 2.2V, V_f for the red LED is 2.0V. Since we put the LEDs in series, we can add up these voltages, so that's 4.2V total. If we use 3 batteries of 1.5V, that leaves 0.3V for the resistor. We want 15mA of current flowing through the circuit, so 0.3V divided by 0.015A is 20 Ω (however 20 Ω is a value that isn't easy to find in shops so we'll take the next higher value that is available, which is 22 Ω. A table of commonly available resistor values is listed at the end of this page).

Putting LEDs in series is the safest bet but adding more LEDs causes a problem: you also need to increase the voltage: each LED requires about 2.0V, so with 6 LEDs you're already carrying a pile of batteries... You could also connect LEDs in parallel as shown in the next picture on the right.
The value for the resistor is determined by the sum of the currents flowing through each of the LEDs (15mA each makes 30mA total) and the voltage of the batteries (lets use 3V) minus the largest V_f of each of the LEDs (2.2V if one is green) making 27 Ω (actually 26.66 Ω but you won't find that value in a shop so we take the next higher one).
However, there's a snag: if the values for V_f differ between the LEDs, one of the LEDs will only light up dimly or won't light up at all.
To get it working you need to add a resistor in series with the LED that has the lowest V_f . This resistor has to balance the voltage over both LEDs. To continue the example with one green and one red LED, the red LED has the lowest V_f so we need to add a resistor to it. This resistor takes 0.2V (the difference between the V_f of both LEDs) and has 15mA flowing through it, so its value needs to be 15 Ω (actually 13.3 Ω).

You can put more LEDs in parallel (add a resistor to all LEDs with a lower V_f than the highest one in the parallel group) or put groups of parallel LEDs in series, whatever suits your power supply best.
Add a bypass resistor (shunt) across a parallel group (or a single component) if the total current through that parallel group needs to be higher than the LEDs can handle. For example in the picture on the right the total current through the lefthand parallel group is 45mA (3x 15mA) whilst the LEDs in the righthand parallel group can only handle 30mA.
The value for the shunt is determined by taking the largest V_f of the LEDs in the parallel group (2.2V if we assume a green LED in the group) and divide that by the current we need to reroute (15mA in this case), making 150 Ω (actually 146.6 Ω).

Basic LED circuit designer

I found a website that will design a basic circuit based on several parameters such as the number of LEDs, their V_forward, the input voltage, etc. : LED Circuit Designer (external link).

An application: 1/144 Jaguar GR.1

This tiny model (fuselage length 11cm, wing span 6cm) was fitted with flashing wing tip lights and two oscillating fuselage anti-collision beacons.
All details can be found on a separate page.

Common resistor values

Resistor values come in a number of series, most commonly found is the E12 series, usually color coded with 4 colored rings.
More accurate resistor values usually have either 5 colored rings (3 for value, 1 for multiplier, 1 for tolerance) or its value and tolerance printed in digits on the resistor body.
The E12 series has a tolerance (accuracy) of 5% (indicated by a gold colored 4th ring) or 10% (silver ring) and consists of the following values:

An easy way to check and decode resistor values is by using this Resistor Color Code Calculator (external link).

Disclaimer: all values quoted are typical values only, refer to manufacturer specifications for exact values.
Always double-check the completed circuit before installing it into a model.
I don't accept any responsibility for misuse of the information I supplied !