Light emitting diodes

by Charles Darley

Light emitting diodes or LED's as they are more commonly known in the electronics world, may be the wonder of technology after the transistor.

How can something so simple, as we see it now, out last the filament light bulb, be run on low voltage and low current, have versions that are in many colours without the use of filters, of differing sizes and differing intensities of light, some can be flashing, some need a current limit resistor whilst other do not and now even very good white ones exist.

So what does an LED look like.

You can see that one leg or connection is longer that the other. The shorter one indicates the side of the LED which has a flat on the side.

This shorter leg / flat size is the side that is connected to the - (NEGATIVE) side of the voltage supply.

5 volt or 12 volt LED

Assuming we are using a LED which does NOT require the current limit resistor (one of the 5 volt or 12 volt versions) the other leg can be connected to a + POSITIVE voltage source, not exceeding that required by the LED, when the LED will light as there is a "built in " limit resistor. However connecting such an LED round the wrong way is likely to lead to failure as they do not like what is called a reverse voltage.

Standard LED

A standard LED requires a limit resistor. Without having to go into the maths you can assume that a 1000 ohm resistor (1K) will be ok for most voltage up to a maximum or 22 volts and not less than 12 volts. On 22 volts a 1k resistor gives you a current flowing of 22mA or for a 12 volt supply 12mA.

As a generalization 22mA is considered a maximum that the LED likes but you can go into things more technically and look up the data sheet for the LED in question.

There is a minimum voltage required else the LED will not light which is about 3 volts.

The current limit resistor can be fitted in series with either lead of the LED which should then be insulate with sleeving to prevent a short circuit in use.

Flashing LED

The flashing Led has not only a built in resistor but also the circuitry to make it flash at about 1.5 flashes per second. Minimum voltage about 5 volts and a current capability of about 22mA is required to operate the flashing LED.

Well they make very good light signals on the track side in place of the semaphore signals but can also be used with semaphore signals when using a white Led to illuminate the red and green filters in the arm of the signal.

If you make up a signal light with the GREEN LED above the RED then a simple change over switch can be used to change it from GREEN to RED.

The end of your set of coaches or trucks needs a red tail light. You can easily make this by fitting a single red led, battery, switch and resistor if required by the type of LED --- or if you wish pick up the power from the track but include a BRIDGE RECTIFIER so that it will show both forwards and in reverse BUT it will go out when the trains stops!