LED lighting

Discussion in 'Tips, Tutorials & Tools' started by rmks2000, Nov 28, 2006.

  1. rmks2000

    rmks2000 Member

    I was looking to add LED lighting to some models and stumbled upon the schematic for the TinTin rocket. Can someone help explain the it so that I could apply this to other projects?
  2. SteveM

    SteveM Member

    Maybe this will help?
  3. Bowdenja

    Bowdenja Active Member

    And here is a neat site that will help you figure the circuit.


    I use their suggestions for the input.

    I also found out that you SHOULD make your LED circuits in series, instead of parallel........fun I know, but it's has something to do with the LED heating up and changing voltage across it, makes the parallel circuit flaky.

    Of course I'm using them to convert/make flashlights, but you can use it to figure out the lower voltage LEDs.

  4. rmks2000

    rmks2000 Member

    Thanks guys, great info! This will also help to convert my magnifier headlamp that eats batteries like crazy.
  5. jleslie48

    jleslie48 Member

    just bought a string of 10 mini leds powered by 2 'c' batteries from michaels craft store for $3. I cut off 9 of the bulbs and ran the one remaining bulb into my model:

    http://jleslie48.com/ww01/ww03/ww00819b.jpg (140k enlargement)

    now all I have to do is buy 9 more battery housings for the other bulbs (radio shack??? nahhh I'll make the battery housing out of card, paper clips, pen springs and washers...)
  6. rmks2000

    rmks2000 Member

    Cool! Did you add a resistor? I like the idea of making your own battery cases. That 's real 'out of the box' thinking. (Sorry about the pun).
  7. Bowdenja

    Bowdenja Active Member

    From what I've read you HAVE to put a resistor inline with LEDs, something to do with stablizing the voltage going into it.
  8. SteveM

    SteveM Member

    Actually it is to set the current flowing through it. An LED, like all diodes, has a relatively fixed voltage across it regardless of the current flowing through it. The resistor in series sets the current through the LED according to ohm's law V=IR where V=Supply Voltage minus Diode voltage. You will want to calculate the resistance to produce the current suggested for the diode (typically 10 to 20 mA).
  9. jleslie48

    jleslie48 Member

    I hooked up a battery to the the light and it worked. ;)))

    thats it. oh, stopped by at radio shack on the way home. $1 for a battery case, cheap enough...
  10. Stev0

    Stev0 Active Member

    You could make a box from card that only allows light to the windows and spray it black both sides.

    Looks like it's gonna rock!
  11. rmks2000

    rmks2000 Member

    Without a resistor, I believe that the LED will have a shortened life.
  12. Bowdenja

    Bowdenja Active Member

    Thanks SteveM.......... man I knew there was a good reason why I forgot all my electrical stuff.........:grin:

    I do know too much and "poof!" no LED..............

    I usually go with 3.5v 20 Ma......... the new ones I just got will got up to 3.8 and 25 mA, but I'm going to run them at 3.6/3.7 at 25.

    It's really hard getting that dang diode to unscrew to put another one in:grin:
  13. Maurice

    Maurice Member


    Actually the value of the resitor needs to be determined by both the voltage across the combined led and resistor and the current that can be allowed through the led.
    Alternatively view that as the resitance of the led being (roughly) constant so you need to ensure that the voltage and current are balanced,
    Both the voltage of the supply that has been chosen and the current that can be allowed through any given led are known so
    R = V / I
    can be applied to find the value of the total resistance needed.
    (since the resistance of the led is small it can be ignored for practical purposes)


    P.S. Completely off topic but while I think of it, if you haven't been there recently use the last few minutes of the existence of the Smartgroups message archive to check out the early months after the move from e-groups. You may find it quite remarkable.
  14. SteveM

    SteveM Member

    Actually, that is what I said.
  15. Maurice

    Maurice Member

    Could've sworn you said

    Now off you go to check those archives and remember I am hoping you will discover remakability.
  16. SteveM

    SteveM Member

    I recommend you check the archives as well.

    I said "You will want to calculate the resistance to produce the current suggested for the diode (typically 10 to 20 mA)."

    Since the voltage across the resistor is determined by the difference between the supply voltage and the forward voltage of the diode, the value of the resistor determines the current through the diode by the equation V=IR as I said before. Exactly what current you want to be flowing through the diode is given by the manufacturer's recommendation. But you can pump more current through it and get a brighter light at a reduced lifetime by using a smaller resistor.
  17. Maurice

    Maurice Member

    You think I haven't ?
    Your response leaves me little alternative but to assume you don't need to.
  18. SteveM

    SteveM Member

    I am completely mystified by what you are accusing me of.

    Here is my original response which you demanded I re-read:
    I am saying two things:
    1) the purpose of the resistor is to set the current flowing through the circuit.
    2) you calculate the desired resistance from the desired current and the voltage across the resistor (which is fixed by the difference between the supply voltage and the diode voltage).

    You said:
    Which I previously said was the same as what I said, but now that I re-read it, is actually incorrect. It is just the voltage across the resistor that is used to calculate the proper resistance. Once the diode begins to conduct, it maintains essentially a fixed voltage across it regardless of the current through it and so the current is determined entirely by the resistor.

    I really do not understand what you think is incorrect about what I am saying.
  19. Zathros

    Zathros Guest

    Why not set the whole thing up in parallel and use a small pot to set the voltage. A simple test of the voltage would allow you to set the pot to the correct voltage and the pot would suck up anything extra. Just use a pot with sufficient wattage rating to suck up the heat . _Power_
    CurrentxVoltage or
    Power over Current Times voltage. This would give you a flexible system in which you could add or remove more LED's. Now shake hands and stop arguing.
  20. SteveM

    SteveM Member

    No. LED's should not be connected in parallel with only a single resistor supplying all of them. In parallel, each LED should have its own resistor. If you have enough supply voltage, you can connect LEDs in series with a single resistor. In series, all the LED's need to have the same recommended operating current.

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