LED Indicator question

Discussion in 'The Projects Forum' started by unclebob, Sep 19, 2012.

  1. unclebob

    Thread Starter New Member

    Aug 3, 2012
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    I have the following connections:

    0V, 12V switched, and 12V.

    I want to install a pair of LED's that indicate in the switched 12v is on or off.

    I was thinking along the lines of the following:

    0V - neg of LED1,
    switched - Pos of LED1, NEG of LED2
    12V is Pos of LED2

    Would this work in principle? would LED's be OK if it received 12V on both legs?

    Where would i place resistors? Would i need a set in the switched and 12v? or could i get away with just in the switched?
     
  2. unclebob

    Thread Starter New Member

    Aug 3, 2012
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    Some background to this,

    My house alarm unit has an output that allows me to connect a light to indicate if the alarm is set or not (if its set, light is on).

    I want to use that to show two lights, set and unset.

    Not that it should make a difference but then i want to do that all over again in a second location.

    so 4 leds, - two green and two red. Two red when 12v applied, two green when 0v applied.
     
  3. absf

    Senior Member

    Dec 29, 2010
    1,493
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    Not sure if I understand your description correctly. If you want both Red or Green LED lighted at the same time, you can connect them in series.

    Attached is the circuit diagram in simulation. You can omit the ampmeter and SPDT switch. Just connect you switched point to "XXX"

    Allen
     
  4. unclebob

    Thread Starter New Member

    Aug 3, 2012
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    hi absf,

    Yes i want both red's on at the same time, when the alarm is armed, and both greens on when the alarm isnt armed.

    I'm hoping to do it with just resistors and the led's if possible, not much space behind the keypad to do anything else...
     
  5. ScottWang

    Moderator

    Aug 23, 2012
    4,855
    767
    The resistors were designed for LEDs protection, you better add them.

    Maybe you can use the smaller resistors, as 1/4W or 1/8W or SMD chip.

    The circuits as absf showed, it could be increase the value of resistors for R1 and R3 from 4.7K ~10K(if Vf_LED=3V), it depends on the Vf of LED.

    Show your keypad's space, let's see what we can do.
     
  6. unclebob

    Thread Starter New Member

    Aug 3, 2012
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    Sorry my last post may have been a bit confusing,

    I'm hoping that i can get away with using ONLY resistors and LED's, rather then using transistors etc.

    There are two keypads, in two locations,

    The keypads have 12v and 0v already, and i will be sending 12v switched through one of the spare wires in the 6 core alarm wire.

    I haven't opened up the keypad to find out just how much space there is as yet, i was thinking of cutting a Perspex plate behind the keypad which will glow red/green (but this is yet to be investigated!)
     
  7. Markd77

    Senior Member

    Sep 7, 2009
    2,803
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    I see a slight problem with the circuit, the base current for Q2 flows through D1 and D2 so the red LEDs will never be off, just dimmer. I don't know the best way to avoid that.
    Also the switched 12V may just be floating when off rather than at 0V so a 10K resistor to ground from the base of Q1 could be a good idea.
     
    absf likes this.
  8. Markd77

    Senior Member

    Sep 7, 2009
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    It depends on exactly what the 12V switched is:
    If when it isn't 12V it is connected to 0V then it should work.

    If when it isn't 12V it is unconnected to anything (floating) then it won't work. Current will from from 12V, through both sets of LEDs and to ground, so all four LEDs would be lit with the switch off, but not as brightly as the two that would be lit with the switch on.
     
    Last edited: Sep 20, 2012
  9. unclebob

    Thread Starter New Member

    Aug 3, 2012
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    Was thinking along the lines of this?

    The resistors being something like 10k as suggested by markd77

    Would the green one be OK with receiving 12v on both sides?

    I assume that if the switched isnt quite 0v or 12v that the other LED will be lit a little, and likewise the one that is suppose to be lit, would be a bit dimmer?
     
    Last edited: Sep 20, 2012
  10. unclebob

    Thread Starter New Member

    Aug 3, 2012
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    Sorry markd77 - updated the drawing before i realised you replied!

    I will get the multimeter out tonight and check to see exactly what i get when its on and off. Will also need to think about the 12v on both sides. would a diode over come this?
     
  11. unclebob

    Thread Starter New Member

    Aug 3, 2012
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    So maybe something like this?
     
  12. Markd77

    Senior Member

    Sep 7, 2009
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    12V on both sides is the same for the LED as 0V on both sides, it's no problem.
    The 10k suggestion was for something else, just calculate the resistor normally, 1k is probably about right.
    Here's a possible circuit, just one PNP transistor and it assumes the 12V can supply enough current for the LED (highly likely):
    [​IMG]

    If your switched 12V is the same as shown here, then with the switch off both LEDs will have the same current flowing through them
     
    unclebob likes this.
  13. unclebob

    Thread Starter New Member

    Aug 3, 2012
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    Thanks Markd77, I assume the circuit above is if things aren't quite 12v and 0v?

    I'm hoping to do this without too many components (lack of space behind keypads), and as my 12v and 0v will be coming from there, i cant put them in the main panel side.

    Assuming that the panel does output 12v and 0v cleanly, can i do it without the transistor?
     
  14. Markd77

    Senior Member

    Sep 7, 2009
    2,803
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    Yes, it's if off isn't 0V and capable of sinking current.


    Yes, your circuit looks fine, give it a go.
     
  15. unclebob

    Thread Starter New Member

    Aug 3, 2012
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    Thanks Mark77d.

    Btw - do you mind if i send you a pm re transistors? just got a quick question which is on another post but not sure if im understanding right. Its related to the phantomlink you did some programming for!)
     
  16. unclebob

    Thread Starter New Member

    Aug 3, 2012
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    The output I'm using is described in the manual as:
    "Open-collector transistor output, 500mA, 12VDC, negative applied."

    500mA ok to run 4 led's? or no where near?
     
  17. Markd77

    Senior Member

    Sep 7, 2009
    2,803
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    OK.
    I'm not sure what "negative applied" means.
    Open collector means that it can supply 500mA in the 0V state but next to no current in the 12V state, I think you need a transistor circuit.
    This is open collector:

    http://www.icenta.co.uk/pdfs/open_collector_output.pdf
     
  18. unclebob

    Thread Starter New Member

    Aug 3, 2012
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    Ill go with your circuit layout - any recommendations for transistor to use?
     
  19. Markd77

    Senior Member

    Sep 7, 2009
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  20. unclebob

    Thread Starter New Member

    Aug 3, 2012
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    Thanks!

    for your assistance! much appreciated!
     
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