need help making circuit!!!

Discussion in 'The Projects Forum' started by vane, Feb 28, 2007.

  1. vane

    Thread Starter Active Member

    Feb 28, 2007
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    i need help making a circuit that lights up two red LEDs when the voltage supplied goes below 2 volts for brake lights on my rc car thanks
    if you know anywhere where i can find an y circuit diagrams like this please help me!!!
     
  2. Papabravo

    Expert

    Feb 24, 2006
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    A simple comparator circuit should do the trick using and LM393 dual or LM339 quad. I believe they also come in single SOT23 packages, but I don't know the part number.
     
  3. vane

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    Feb 28, 2007
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    sorry im only good with simple led circuits what does that mean? im good at soldering but thats about it :) as it was my first post :)

    it musn't come on if the voltage goes above any voltage it must switch on when its 2.5v or less maybe some sort of relay that switches when it is at 2.5v or less so that i dont need an led that runs off less than a volt thanks
     
  4. Ron H

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    Apr 14, 2005
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    Why do you want the brake lights to come on when a voltage drops below 2V? Where is this voltage coming from?
     
  5. vane

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    Feb 28, 2007
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    with the speed controller when it is in brakes setting it uses less than 2v
     
  6. Ron H

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    What is your battery voltage?
     
  7. vane

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    Feb 28, 2007
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    well the main battery is 7.2 volts but it will be running a motor and a few more leds but as leds dont draw much capacity it wont make much difference please help me

    edit: dont know if you care but they are massive 3.7mah and are nickel metal hydrides
     
  8. Ron H

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    Apr 14, 2005
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    Here's about the simplest circuit I could come up with. You could make it simpler by removing hysteresis, but I'm not sure that's a good idea.
     
  9. vane

    Thread Starter Active Member

    Feb 28, 2007
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    thanks id never be able to make that circuit by myself :) im sure my dad has a few of those components lying to hand, thanks

    edit:could you write down all the components id need? thanks
     
  10. Ron H

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    I'm not sure what you need that isn't on the schematic already.:confused:
     
  11. vane

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    Feb 28, 2007
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    could you write a list of all the components i will need please
     
  12. vane

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    Feb 28, 2007
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    will that circuit work if there is like 0.01v? cause i was thinking like if you have a relay that the contacts will switch when there is the 0.01v the lights come on no matter how little power goes in cause some times when the speed controller is in brake it'll only give out 0.01v if you go into brake quickly it goes to 2v, im a bit worried about this
     
  13. Papabravo

    Expert

    Feb 24, 2006
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    incroyable! Mon Dieu!

    Here's one way to do it.
    Print a coppy of the schematic
    Take a sheet of paper or a spreadsheet
    In the leftmost column write the reference designators. e.g. R1, R2, Q1, Q2, etc.
    Next to each reference designator write the value or the part type. e.g. 10K, 2N3906
    Use a yellow highlighter to check off each item on the schematic

    Could this process be any simpler?

    When it comes to ordering the parts you have to get copies of the datasheets so you know what package the parts come in. This is actually quite a bit of work and nobody can do it for you. You absolutely must do this for yourself.
     
  14. Ron H

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    Vane, I think I got the logic backwards, i.e., the lights switch ON when the voltage is above 2V, and OFF when the voltage is below 2V. I know that's not what you want. I'm at work right now. I'll try to fix it when I get home.

    EDIT: The schematic has been fixed.
     
  15. vane

    Thread Starter Active Member

    Feb 28, 2007
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    thanks, what voltage led do i need to use for it because as i have ranted on about in previous posts it needs to work almost like a relay as i know you probably cant get 0.01v led :)
     
  16. Ron H

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    I designed it for two red LEDs, running at 10 milliamps. I have fixed the schematic (see my earlier post). The LEDs come on when the input voltage goes below 2 volts, and they go off when the voltage goes above 2.2 volts.
     
  17. vane

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    Feb 28, 2007
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  18. vane

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    Feb 28, 2007
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    and also what do the triangles at the bottom of the diagram? does that mean all the ones with a triangle are joined up? im not ery good at reading circuit diagrams, thanks
     
  19. Ron H

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    Apr 14, 2005
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    In my last post, I wrote
    Doesn't that answer your question?
    BTW, the input voltage comes from your speed controller.
    And yes, the little triangles are called "ground" symbols, but that just means that it is the common node. As you said, they are all tied together.
     
  20. vane

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    Feb 28, 2007
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    i dont mean to sound nagging but will the circuit work even if the speed control puts out 0.01v? its hard to explain what i mean
     
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