I have started working with LEDs in my structures and need some general "education" on them, best wire gauges to use, appropriate resistors, power sources, etc. where some of my questions have already been researched by others who are proficient. Does anyone have a list of several on-line sites that could give me solid backround knowledge in this area?
Thanks,
Cedarwoodron
Search the DCC section here for LEDs and you'll probably find a dozen versions of my standard LED spiel which one of these days I will get smart and copy it to my own web site and just post a link when someone asks instead of going through the whole thing over and over. Except for calculating the resistor, using LEDs to light up buildings is about as uncritical as it gets - current is low so telephone wire is more then sufficient. Search LED calculator on Google and you'll find any number that can tell you what resistor to use. They key is knowing what the ratings mean on the package. LED voltage is absolute - whatever the package says, use that as the LED voltage in any calculator,. The current is a MAX. Use about HALF that value in any calculator. That's really all there is to it. If you wire it up and the LED doesn;t work - it's probably backwards. It won't hurt anything. The Wiki entry for LEDs will tall you more than you ever wanted to know: http://en.wikipedia.org/wiki/Light-emitting_diode
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
Thanks Randy for your help and advice- just can't beat the quality of the folks on this forum!
Happy Holidays!
-Cedarwoodron/
Chief Engineer and "novice electrician" Minnesota, Sioux Lake & Western Ry- serving the good people of the Upper Midwest
Cedarwoodron: This is how I connect mutiple LEDs within a structure. I am using .020 phosphor bronze wire for the buses(I am sure there are cheaper alternatives but I had a bunch to use so I did). The phosphor bronze wire is stiff enough that it will pretty much stay where it is put but I use Testors tube glue to hold the ends in place against the styrene. I glue the bus bars in place first and then solder the LED/resistor assemblies to them. Sometimes when soldering close to a glued end the heat will ruin the glue so I recommend using a heat sink like a pair of needle nose pliers with an elastic around the handles to form a clamp to disipate the heat. This repair house/machine shop is not intended to be viewed with the roof off so I am not worried about the resistors being in plain sight. The LEDs are white with a coat of beige paint to soften the colour. The resistors are 470 ohm. If anyone has any comments or suggestions, please do so. Sorry - the last picture does not show the LEDs with power on. I have disconnected parts of the bus system to be able to work on the interior and I was too lazy to solder it back together.
Thanks
Dave
I'm just a dude with a bad back having a lot of fun with model trains, and finally building a layout!
Wow that is pretty amazing detail with your lighting. Any special tips about painting the LEDs like what type of paint, etc.?
Tim Fahey
Musconetcong Branch of the Lehigh Valley RR
Here is a link to a new manufacturer of LED lighting kits specifically for scale structures:
http://www.microlumina.com/
Their name is Microlumina.
In general LEDs have a voltage drop across them from 0.7 to 1.2 Volts(typically closer to the 1.2Volts). And in general the LED draws 10 to 20 mAmps(milliamps). That is very, very low current so just about any wire will do. You will need a dropping resistor to regulate the amount of current to the LED. Your equation will be the following:
Dropping Resistor=(Input Voltage-LED Voltage)/Desired Current
So let's say our voltage drop across the LED is 1.2V and our desired current is 15 mA and input voltage is 12V(DC Voltage):
(12-1.2)/0.015=720 Ohms
Resistors only come in certain values(fewer values in 5% tolerance vs what is available for 1% tolerance). 720 is a standard 5% value.
Now to complicate things we can hook LEDs up in series or in parallel. If you hook the LEDs up in series the voltage drop across them is addititive. So let's say we hook up three LEDs in series, then to calculate our dropping resistor:
(12-(1.2+1.2+1.2))/0.015=560 Ohms
If we hook them up in parallel, the voltage drop stays the same but the current is addititive. So if we have 3 LEDs in parallel our calculation is the following:
(12-1.2)/(0.015+0.015+0.015)=240 Ohms
Now there is one more complication here which is power dissipation by the dropping resistor. Typical resistor is a 1/4W power dissipation. Power is calculated as the following:
Power=current*current*resistance
So in the case of the three LEDs in parallel:
(0.015*3)*(0.015*3)*240=0.486 Watts
That is way over the 1/4W allowed and is too close to the max for a 1/2W resistor. So in this case you would want something like a 1W resistor.
In general it's better to hook the LEDs up in series as it keeps the current low and the dropping resistor throws away less power. But you can't put too many of them in series before your voltage drop across the LEDs exceeds the input voltage(in which case the LEDs will not light). in the case of 12 volts input practically you wouldn't want to connect more than 6-8 of them in series.
And, as you probably know current only flows in one direction so if it is hooked up backwards no current will flow and the LED will not light.
And that is my Cliff Clavin moment for the day....
I know this may not be the "conventional" model railroading way to lite structures with LED's but I found their method interesting.
http://www.thefiberopticstore.com/fiberprojecthome.htm
Tim
I used Polly Scale SCL Hopper Car Beige but I am not totally happy with the colour. It is still a little harsh for incandescent lighting. I tried straight yellow but the colour was way too strong. For the next batch I will add some yellow to the beige. If you don't like the colour after painting the LED, you can take it off with regular paint thinner on a rag even after it is dry. Before going the paint route I would suggest doing a search for warm white LEDs. I bought my supply 2 years ago when warm white LEDs were still expensive so I opted to go with regular white and paint. I believe that the selection has increased and the prices have come down for warm white LEDs since then.
My solution for the resistor issue is to put a 470 1/2 watt resistor in series with each LED. My head spins when I try to understand the formulas for single resistors with multiple LEDs. I admit that the resistors don't look right if you are going to model a removable roof. I am contemplating putting the bus wires and resistors under the structure and running fine wires up through the walls to get them out of view. By the way, I have been buying my electronic parts from Digi-key. They have a great on line catalogue that shows prices in either US or Canadian dollars and their service is excellent. Stuff arrives here in Canada in 3-4 days max. I have not compared prices recently with some of the newer LED suppliers but when I bought my supply of LEDs 2 years ago their prices were good.
Another approach is the Miniatronics building interior light. It is a series of surface mount yeloglo LEDs mounted on a PC board with a power supply than can accept AC, Dc or DCC power. The board segments can be separated; each segment has a solder pad for electrical connection. The sections are attached with double stick tapehttp://www.walthers.com/exec/productinfo/475-100IBL01
http://www.miniatronics.com/Merchant2/merchant.mvc?Screen=PROD&Store_Code=M&Product_Code=100-IBL-01&Category_Code=&Product_Count=7
I used 2 sets to light a roundhouse I made for the Bothbay RR Village model layout
George In Midcoast Maine, 'bout halfway up the Rockland branch
Got off my bottom and rewired the engine shop so you can see how the LEDs work, sort of. I haven't got the best camera. The walls have yet to be sealed against light bleeding through.
George
Can you show us a picture of the roundhouse lit (hopefully a better quality picture then the one I posted)?
FYI Digi-Key sells a 3mm clear white LED part #160-1782-ND for $33.28/100 (CDN $) and the resistors run around $5.50 for 200. They also have a surface mount warm white LED part #CLM3C-MKW-CWAXB233CT-ND for $47.08/100. The only problem with the surface mount style would be figuring out how to attach leads. Others have mentioned in related threads that you can buy LED Christmas light strings where the LEDs are easy to remove for very good prices, at least in the US. The only LED strings I have seen in Canada have very bulky difficult to remove sockets around the LEDs and could be hard to hide in some instances. NOMA sells a 100 light string for around $33.00 CDN regular price.
Just a note to add to this thread. It is christmas time and Walmart has a string of small square LED Christmas tree lights(soft white) available on their shelves right now. I purchased a string of them which equates to 150 LED units for around $7.00. All that is needed is to solder one LED lead to a brass rod and the other lead to a resistor and another brass rod in the ceiling of the building. Once the brass rods are wired to a power source and the LED polarity is all the same, the building will be lighted.....chuck