Top

LED Lights

 

Definition: LED Lights or LED  Bulbs are actually Light Emitting Diodes (LEDs) possess all the properties of a conventional semiconductor  diode described in another section which passes current in one direction with the additional property of emitting light. The direction of current is from positive to negative. See sketch for diode symbol and picture of markings on a typical LED.  The top sketch is a schematic symbol for the LED. The arrow on the diode symbol indicate direction of current flow. The LED has a positive and a negative terminal. The positive LED lead is known as the Anode. The negative terminal is known as the Cathode. The small wiggly arrows above the main arrow represent light being emitted. The lower sketch represents what the LED physically looks like. It has two leads emanating from the base of the LED. The long lead is the positive lead (Anode) and the negative lead (Cathode). The LED resembles a miniature light bulb that comes in several colors red, green, yellow, orange, etc.

In the figure below (CASE 1) the LED lights up because the current is permitted to flow from plus to minus. In the second part of the figure (CASE 2) the LED is blocking the flow of current. Note that when using an LED it is necessary to always use it with a series resistor (R1) in order to limit the current flow to a safe value. For example, if the LED specification is rated at 2 vdc @ 20 ma, then the Battery voltage of +12 vdc will provide +2 vdc across the LED and the remaining voltage will be 10 vdc across R1 as per Kirchoff’s Law and the current thru the circuit will be calculated per Ohm’s Law as follows: R1 = Voltage/Current  which equals 10vdc/20ma =  500 ohms.  The Power value at R1 = I squared x R = 20ma squared x 500 = 0.2 watts. Therefore, R1 we should use a 500-ohm ½-watt resistor.

Typical LED Applications to Model Train Layouts

Track Signal Lighting– With the proper interface circuitry to provide the Track Turnout position, LEDs draw a fraction of the current of the average incandescent bulb with the advantage of long life on the order of 100,000 hours. For this application the following would be applicable: red led lights, green led lights, or yellow led lights. The size of the leds 1 mm,  3 mm, 5 mm, etc. would be a function of the specific application to the railroad model train scale being used.

Passenger Car Lighting– As a minimum it is possible to add LED lighting with some very simple circuitry which uses the track voltage as the source. It is also possible to install a regulator in each passenger car with an output capacitor to virtually eliminate interruptions caused by passenger car wheel pick-up bounce. Internal lighting is best accomplished with 5 mm LED lights (clear) while tail lighting is best accomplished with the smallest possible red LED lights. The Internet is a good source of schematic diagrams for these applications.

Model Building Lighting– One of the most important applications of LEDs is for Model Train Scenery associated the lighting up of the model buildings used in your layout. Again as with other applications, low current draw and long life provides model railroad LED lighting a real advantage.  Of course the LED must be surrounded by some sort of painted plastic shade to soften the generally bright LED light.

Additional Locomotive lighting can be added such as Ditch lights with associated circuitry. More about that in future articles.

Control Panel Lighting – The following figure depicts a model railroad control panel whereupon red and green LED’s are used to show model train turnout position. Another section discusses how to construct a model railroad control panel with LED lights that denote turnout control and position.

Control Panel Lighting For Railroad Yard– The following figure depicts a control panel whereupon green LEDs are used to show chosen path for train to enter. It is accomplished by wiring several LEDs in series to depict turnout positions as well as the track(s) being used.

How to wire LEDs in series – In the above Control Panel several Green Leds were wired in series to depict switch path that is to be by train. For example, we wish to illuminate 3 Leds in series and we are using a + 12 vdc battery or power supply. Just as before we have to calculate the value of resistor R1.

For example, if the LED specification is rated at 2 vdc @ 20 ma, then the Battery voltage of +12 vdc will provide +2 vdc across each LED and the remaining voltage will be (12-6) vdc across R1 as per Kirchoff’s Law and the current thru the circuit will be calculated per Ohm’s Law as follows: R1 = Voltage/Current  which equals 6vdc/20ma =  300 ohms.  The Power value at R1 = I squared x R = 20ma squared x 300 = 0.12 watts. Therefore, R1 we should use a 300-ohm 1/4-watt resistor.

The following figure shows how a small portion of the Yard Panel would appear using 3 LEDs for one track path and a single LED for an optional path. Notice that a third Power Supply, PS #3 is used because the Yard might not be physically located near the Main Panel.

 

LED Rope Lighting – This type of lighting which is comprised of a string of lights can be very handy to place under your layout table. This will be very handy to use when troubleshooting wiring problems.

In general, the following vendors are recommended for any of the above electronic components:

Radio Shack (Local Store or on-line at  www.radioshack.com)

Futurlec at  www.futurlec.com

DeMar Electronics at  www.demarelectronics.com/

For more detailed  information on Model Railroad Projects utilizing LEDs, navigate to the following links:

Turnout Indication LED Projects

Model Railroad Fire Engine LED Project

Model Railroad Crossing Flashing LED Project

If you wish to acquire a better understanding of Electronics Theory, I suggest you go to  the following  link: Electronics

Bottom