Basic info on LEDs wanted

Discussion in 'Tips, Tutorials & Tools' started by Leif Oh, May 12, 2005.

  1. Leif Oh

    Leif Oh Member

    I notice that quite a number of people are interested in putting light-emitting diodes (LEDs) into their aircraft models for interior lighting, plus navigation & landning lights. A prime example, of course, is Mindaugas fantastic build of a large SR-71 with a bewildering myriad of LEDs already mounted.

    I, for one, would dearly require some very basic help in getting started with LEDs, and I know there are several knowledgeable people on the site who already offered some advice. Could somebody please write a few lines on the BASIC stuff on LEDs, how to connect them up, how to calculate resistors, and other things a person not really knowledgeable about electronics but willing to learn would need to know.

    I'll start you off with a practical example which I'm up against. I'd like to make an aircraft powered by two 1,5-3V electric motors, and was thinking of using a 2,4V onboard rechargeable accu for power source.

    In addition to that, I'll need to get the following LEDs aboard:

    2 white for landing and tail lights
    2 red & green for wing tip lights
    4 yellow for internal lighting

    I would want to go for mini LEDs, not the standard big ones.

    Now, would my onboard powersource handle that plus two engines? Do I need resistors; and if so, what sizes, and how do I connect them up? Should I make a bridge or something? Could I make a power central with just a few resistors in one place, or do I need separate resistors for each LED?

    I'll probably make two switches, one for engines, and one for lighting. But could I in principle have only one switch?

    Grateful for any help

    Best, Leif
  2. jrts

    jrts Active Member


    I will second that request :D
    Us wet boy's can also use them for a hell of a lot of things as well.

    Another bright idea (sorry I could not let that one go) :lol: :lol:


  3. 46rob

    46rob Member

    Not all LED's are created equally. Diferent sizes, and colors draw different amounts of current. If you go to a nearby electronics supply house and buy some, they usually come with a bit of information about max forward and reverse loads, etc. As a general rule, most (don't take this to be all) mini's will work at 1.5 V with no problem. Current draw is very low. However, the more you use, the more weight and current you'll draw. Have you considered one or two LED's and using fiber optic strands and colored filters to bring your light from the source to it's destination? Less weight and mass, as well as less current draw.
  4. dardard

    dardard Member

    The LED are semiconductor devices, they have 2 main parameters : the junction voltage and the forward current.
    Basically speaking it means that you will need to reach the junction voltgae in order to make your LED shine.
    And if you want your LED at her nominal light you must give her the nominal current (less is less bright and more is more bright until you reach the maximum current and after it is black!)

    Of course, all LEDs are polarised.

    As I see more and more people interested, I will try to make a comprehensive sheet on using LEDs (how to connect them, #packages and so on).

    It will not be difficult (I work in electronics), just need some time to write it

    Best regards
  5. Mindaugas

    Mindaugas Member

    Hello Leif,

    As I made my SR-71, I have rounded the voltage of diodes to 2V and current to 10mA. All diodes are different and all diodes have a small bias, but I don't take any attention about this.
    So this is about red, green, yellow diodes. Blue and white diodes need much more energy. For example, white diode needs a voltage of 3.6V and a current of 20mA.

    There is no difference between red standart diode and red 3mm (small) diode. If the colour is the same, all parameters are the same.

    Also, so I do not suggest using more than one flashing diode in the plane for flashing you should use NE555 timer chip or tranzistors, but this is another topic.

    So dear Leif, you should calculate everything and make diagrams on the paper first.
    Remember that when using connection in series, the current is the same for all components I=I1=I2... (these numers are indexes) and the voltage sums up U=U1+U2+U3..
    While in the parallel connection all currents sums up I=I1+I2+I3.. and the voltage is the same U=U1=U2=U3...

    Most of the systems are mixed.

    Next thing to know is Ohm law: R=U/I

    Example: if you have a serial connection with two diodes and a battery of 9V and 300mA, you need to put resistor in order not to overheat and burn diodes. So what is the resistor for this simple system? You must calculate. All currents in this system are the same (remember what I told about current in serial connections? :) ), while the voltage sums up. So lets sum up the needed voltage for both diodes: 2+2=4V. So we have 9V,300mA battery. So there is a surplus of 5V and 290mA. We don't need these.. :) Now we have the needed information, so lets count the resistor. We need to divide the "unneeded" voltage from "needed" current: R=U/I; R=5/10mA; it is the same as R=5/0.01A, so the answer is 500ohms....

    I know that I have explained very unclearly, but there was no other way.
    And now about your plane. Everything is possible, until you don't consume all your current given by the battery. In my SR-71, external power source have a current of about 300mA, while I consume only ~150mA...
    The voltage can be always changed, but in your case, you will have only 2.4V, while your engines will require 3V, so you will need to use generator or more powerful source.

    So thats all. Ask everything you would like to know.
    Good luck Leif!

  6. Jim Nunn

    Jim Nunn Member

    You should have a current limiting resistor in series with the LED other wise you risk of having the LED burn out even on a 1.5 Vdc cell. The following web site has a very good tutorial on using LED’s

    Keep in mind that LED's have a narrow field of view IE as you move off center to the LED the light out put diminishes greatly. Fiber optics are an excellent choice for putting pinpoint light sources in a model, however fiber optics have a narrower field of view then a LED. A quick trick to get a fiber optic to produce light in all directions is to very gently heat the end of the optic into a globe shape and ruff the surface with steel wool. Fibers are available in Glass and Plastic use the plastic they are much easer to work with. Most Hobby Shops that cater to the model Railroader have Fiber Optics.

    A good source for LED’s along with small switches and plugs is All Electronics
    Direct page for Miniature LED’s
    unlike other Mail order Electronics stores they will take small orders $10 or so.

    Jim Nunn
  7. Leif Oh

    Leif Oh Member

    Thank you all, and Jim in particular. That page on LEDs really contained everything I need to revise my plans properly before going to the electronics shop. Main things learnt:

    1) Always connect a LED with a series resistor (1K typically) in order to have a safe limit for current. Otherwise it will burn out.
    2) Never connect LEDs in parallell to a common resistor. A difference in voltage drop may cause one of them to suck all the current and burn out.
    3) In fact, you'll be well off connecting LEDs in series (since you get more light for the same current drain; very good point!) - but you need higher voltage.
    4) Calculate with a battery voltage of 2V per LED in series, and 2V voltage drop over the common series resistor (for rough calculations).
    5) Red, green, and yellow LEDs have a voltage drop of 2V each; blue and white ones a drop of 4V. Important fact!

    Conclusion for my little project: I'll have to increase the voltage of the on-board accu, most probably to 4.8 V (which might still be a little bit marginal for the white LEDs; will have to think about that) or 6V. This, in turn means a change of motor voltage; it's got to be 6V now. That's OK, these very cheap motors come in different voltages.

    LEDs will have to be connected in separate strands of 2 each (apart from the white ones). Like this:

    1 white; landing light (with a smaller resistor)
    1 white; tail light (with a bigger resistor)
    2 red + green; nav lights (common series resistor)
    2 yellow; interior cabin (common series resistor)
    2 yellow; interior cockpit + signal light (common series resistor)

    Very nice. All I need now is to calculate the size of the resistors when I get the actual LEDs and the data for them. And I can regulate the light for each chain of LEDs by changing the size of the resistor.

    All of this learnt from one good tip, and one good web page. Go visit!

    And many thanks again - I knew this would be good!


    PS. Incidentally, do you think it would be OK to connect two of my present 1,5-3V motors in series to a 6V accu?
  8. gchucky

    gchucky Member

    Two motors in series. On paper yes but in real practice it is better if they were in parallel. Remember, brush motor will make and break the circuit, so one motor may affect the other. You will need to find the typical current the motors will handle and place a decent wattage resistor in series. Don't forget the shared resistor for the parallel LEDs will be a different value then the single LED/resistor combination. Twice the current is going through it.
  9. Leif Oh

    Leif Oh Member

    OK. Thanks. No series then. Didn't think so either. Best - L.
  10. Mindaugas

    Mindaugas Member

    Hello Leif,

    I would like to ask you a question. Are your motors adjustable (some kind of knob, with which you change the speed of motor) ? And if not, you can use adjustable resistor from 1.5V to 3V to speed up or slower your motor.

    That would be all. Good luck!

  11. widget

    widget Member

    Though mainly a styrene and resin site has a contingent of papermodelers there. Here are a few links to technical articles regarding L.E.D.s and electronics hosted there.

    If you're interested,there are some good photography primers there also.

    Hope these help.
  12. Jim Nunn

    Jim Nunn Member

    I guess I’ll have to jump in on this one I make my living selling AC and DC motor speed controls though they are usually larger then Hobby motors generally 500 to 10,000 Hp.

    Your best bet for small DC motors is what is called a pulse width modulated (PWM) Control. This type of control is highly efficient and has little heat loss. The PWM control will also will give you a wider speed range then a simple series rheostat or Variable voltage supply. Gchucky is correct you should run the motors in parallel. However one of the advantages of using the PWM supply is that by simply adding small potentiometer (Pot) in series with the speed control pot you can set the maximum speed/voltage to your requirements.

    Now where do you get one of these controls well back to All Electronics, and no I don’t work for them but they are a great place for Electronics geeks like myself and they are located only a couple of miles from my office. This link is for a small control that sells for under $4

    Also do a Google search for “PWM speed controls†you will find a company named Velleman that produces inexpensive kits and if I remember correctly they have a larger speed control and you should find other sources closer to you.

    Jim Nunn

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