Using LED's 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). There are even types available that can emit different colors or come with built-in blinking circuitry.

LED Basics

LED cross-section and symbol (1.6KB)

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 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.

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, this is usually called Vforward (or Vf for short). Vf ranges between 1.8V to 2.5V typically, depending on the color of the LED (1.8V to 2.0V for red, 2.0V to 2.2V for yellow, 2.2V to 2.5V for green).
The most important figure you need to know about a LED is the current at which it emits light optimally, called Iforward (or If for short). Brightness of a LED is not linearly related to If. 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. The cathode lead acts as a heat sink, so it is best not to cut the cathode lead off close to the body .

Some do's and don'ts

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

Apart from the do's and don'ts mentioned above, you're also looking to minimize total current, to stretch battery life.
Connecting LED's 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 LED's 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

two LED's in series with a resistor (0.79kB) 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 LED's 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 If for these LED's is 15mA, Vf for the green LED is 2.2V, Vf for the red LED is 2.0V. Since we put the LED's 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 Ohms (however 20 Ohms 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 Ohms. A table of commonly available resistor values is listed at the end of this page).

two LED's in parallel with a single resistor added (1KB) Putting LED's in series is the safest bet but adding more LED's causes a problem: you also need to increase the voltage: each LED requires about 2.0V, so with 6 LED's you're already carrying a pile of batteries... You could also connect LED's 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 LED's (15mA each makes 30mA total) and the voltage of the batteries (lets use 3V) minus the largest Vf of each of the LED's (2.2V if one is green) making 27 Ohms (actually 26.66 Ohms but you won't find that value in a shop so we take the next higher one).
two LED's in parallel with two resistors added (1KB) However, there's a snag: if the values for Vf differ between the LED's, one of the LED's 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 Vf . This resistor has to balance the voltage over both LED's. To continue the example with one green and one red LED, the red LED has the lowest Vf so we need to add a resistor to it. This resistor takes 0.2V (the difference between the Vf of both LED's) and has 15mA flowing through it, so its value needs to be 15 Ohms (actually 13.3 Ohms).

several LED's in parallel and in series (1.9KB) You can put more LED's in parallel (add a resistor to all LED's with a lower Vf than the highest one in the parallel group) or put groups of parallel LED's 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 LED's can handle. For example in the picture on the right the total current through the lefthand parallel group is 45mA (3x 15mA) whilst the LED's in the righthand parallel group can only handle 30mA.
The value for the shunt is determined by taking the largest Vf of the LED's 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 Ohms (actually 146.6 Ohms).

to be continued...

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.

The electronics for this model consist of two parts: an oscillator circuit with power supply fitted into a display base and a small circuit with four LEDs, some resistors and a capacitor fitted inside the model. To light the wing tip lights, fibre optics were routed through the wings.

to be continued...

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 multipier, 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:

E12 resistor series  Electronic Component color codes
color codevaluemultiplier
black  01
brown  110
red  2100
green  5100k
blue  61M
The values listed above are encoded in the two leftmost rings on the resistor body, a third ring indicates the decimal multiplier (black = 10^0 (= 1), brown = 10^1 (= 10), and so on).
The values for the color codes are always the same and are also used on other electronic components such as capacitors and coils.

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 !

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