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How do I reverse the pattern of a LED ligt?

  1. Sep 12, 2008 #1
    I have an ackward question.

    I've been trying to device a circuit to reverse the pattern of a LED.

    Let me give you a quick rundown of my situation and my goal.

    As it stands now.

    Imagine a an alarm system. When activated a LED is lit 24/7. When a switch is triggered the led will blink off,on,off,on,off,on until the switch is deactivated in which case it will resume to the permanently on status.

    I want to use the signal of this led +12v to control a transistor to do the opposite with a second LED.

    When this LED is on, the second LED is off. When the switch is triggered, the voltage cut from the first LED will cause the second LED to turn on.

    Basically i'm trying to see if there is a way to make it so LED #2 acts in the exact opposite of LED #1.

    I can power LED #2 from a seperate source of power. LED #1 is powered from a 12v circuit, led #2 is also 12V (can be same circuit or difference 12V no matter to me)

    Does anyone understand my goal? It's kinda complicated to explain.

    I'm trying to avoid having to stare at a bright led when it's armed constantly but still have it flicker (even if backwards to the orignal led) when triggered. Doing this I can hide the first led out of sight but still retain the same alarm alert using the second LED.
  2. jcsd
  3. Sep 12, 2008 #2


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    The simplest way to establish this complementary function is to have an LED (and resistor) in parallel with the other branch, except that the LED is turned around from the other. In this way, the first LED is on when the second is off, and vice-versa. This only works if there's enough driving current (which, given that most alarm systems are powered off of higher-current sourcing microcontrollers, is usually okay--or at least worth a try).

    I'll try to represent that here with a diode as -|>- and resistor as -\/\/\-

    o---|>----\/\/\---------o GND
    | |

    Additionally, if it's just the LED intensity that bothers you, and you have some soldering expertise (and the alarm unit uses decent-sized components), you can just solder a larger resistor value to limit current going to the LED.
  4. Sep 13, 2008 #3
    actually its the opposite. I want to put a really bright led to get my attention. but don't want the constant on status to be a distraction.

    I am at work so I don't quite see what your trying to explain. someone at work read my post and he said I made it sound way too complicated for what I want to achieve. he told me I should word it this way.

    I want to use voltages as a switch. when 12v is present on a line the switch is open. when there is no voltage on the line then the switch is closed.
    I can draw a schematic to better explain tomorrow when I can access my tablet. just picture a square circuit. 4 sides.

    each side is a component.
    1. power
    4. switch.

    I want to replace switch with something controlled by voltage. (transistor? )
    if voltage is present it is open. if voltage is not present it is closed.

    I thought about what you wrote but I am not sure how to implement that. I think I need to get that schematic up on here.

  5. Sep 13, 2008 #4
    Here's basically what I mean. In circuit #1, the two wires coming out of it is currently hooked up to a LED that is driven by 12 volts.

    I want to make the LED behave in the exact opposite. When the controller turns on the LED by turning on the +12V line, I want an LED to actually turn off. When the controller turns off the +12v line to turn the LED off, I want my led to turn on. Complete reversal of the function the controller intended me to "see".

    Thanks! I really hope my chicken scratch is legible.

    My electronics skills are pretty much 1990's so bear with me if things don't make sense. I think you'll catch my drift.


    Attached Files:

  6. Sep 13, 2008 #5
    Since you want to use a bright LED for the inverted function, it will help to add a transistor buffer to do the inversion. Do a web search on "transistor inverter" to see the basic circuit. Here's a crude ascii diagram:

    > R1
    / |
    S--^v^-- | T1 |
    R2 \ #
    | # LED
    | #
    | |
    vv vv

    S is the input signal, R1 and R2 are appropriate resistors to limit collector and base current. T1 is an NPN transistor, e.g 2N2222. # is your bright LED.

    When the signal is high, the transistor is on, taking all the LED current, so the LED is off. When signal is low, the transistor is off, and the LED gets the current/voltage, so is on.
  7. Sep 13, 2008 #6
    Since you want to use a bright LED for the inverted function, it will help to add a transistor buffer to do the inversion. Do a web search on "transistor inverter" to see the basic circuit.
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