Making a display panel for switch monitors

Discussion in 'The Projects Forum' started by seabear, Feb 20, 2009.

  1. seabear

    Thread Starter Active Member

    Feb 20, 2009
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    Note: my electronics experience is moderate, I can read schematics and can easily develop circuits... but I am a bit weak in designing them.

    I would like to build a panel of LED indicators in my RV that monitors the position of the external compartment doors and other closable or lockable devices. I could use magnetic switches or such and use the 12v power that is constantly available to me. I would like the position of each switch to be indicated as open or closed by either green/red LEDs or by dual color LEDs. If you could help with a simple schematic with suggested values for the resistors and such, I would be very greatful. thanks.
     
  2. mik3

    Senior Member

    Feb 4, 2008
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    Post more information about the specifications of the magnetic switches and the leds.

    Do you need only to turn on/off the leds by only the open/close of the switches?
     
  3. seabear

    Thread Starter Active Member

    Feb 20, 2009
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    Yes, I just want the opening of a door or lock to light the red LED on the panel, and the shutting or locking of a door to light the green LED. Note, only one color will be on at any one time.
     
  4. mik3

    Senior Member

    Feb 4, 2008
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    Assuming that your leds can handle 20mA use 680 ohm resistors to limit the current through them and just use the magnetic switches like normal switches.

    Note that the 680 ohm resistor won't give exactly 20mA current through the leds but it will vary close to it because red and green leds have different voltage drops across them.
     
  5. seabear

    Thread Starter Active Member

    Feb 20, 2009
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    I guess what I don't understand is exactly how the LED that shows an open circuit gets its voltage. In other words, if the switch is normally closed then I can see how the current flows through the switch and the appropriate LED, but when the switch is open, how does the current flow through the OTHER LED.? thanks
     
  6. mik3

    Senior Member

    Feb 4, 2008
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    An easy solution is to use a SPDT relay, connect one led on the NC contact, the other on the NO contact and connect the common contact to 12V via a 680 ohm resistor.

    When the switch will be open the led on the NC contact will light otherwise the led on the NO will light.
     
  7. seabear

    Thread Starter Active Member

    Feb 20, 2009
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    Thanks for both suggestions. But I don't understand the one from Alberto. Looking at the diagram, the way that the two LEDs are wired, doesn't it imply that the +12v would have to change direction in order for one or the other to light up? In other words the output at pin 2 is always going to be +12v whether the switch is open or closed.
     
  8. mik3

    Senior Member

    Feb 4, 2008
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    Aberto made a mistake.

    One of the leds has to be connected from the output of the inverter to ground via a resistor and the other from the output of the inverter to Vcc via a resistor.
     
  9. seabear

    Thread Starter Active Member

    Feb 20, 2009
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    Great, that makes more sense to me. Thanks for clarifying and all your help.
     
  10. mik3

    Senior Member

    Feb 4, 2008
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    You are welcome ;)

    If more problems occur come back here :p
     
  11. mik3

    Senior Member

    Feb 4, 2008
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    Albetro in this way both leds will blow. Each leds has to have its own resistor.
     
  12. mik3

    Senior Member

    Feb 4, 2008
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    Now it is better!

    Don't worry Alberto, everyone has bad days.
     
  13. seabear

    Thread Starter Active Member

    Feb 20, 2009
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    Mik3 and Alberto, thanks for your prior help.. a new question for you on this circuit. In an attempt to make my panel more efficient I was thinking about using dual-colored LEDs instead of single red and green LEDs. The problem is that there is only 3 leads, one common and two voltage on the LED. The circuit as drawn requires two different common/ground points, one of them switched and the other one static. Is there a way to work these in or am I just going to be limited to the two discrete LEDs? thanks.
     
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