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Normally closed contact switch and Garage door

Discussion in 'Electronic Basics' started by Moloney, Jan 13, 2004.

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  1. Moloney

    Moloney Guest

    I am trying to detect whether or not my garage door is open. I purchased a
    normally closed contact switch to place on the bottom of the railing and
    bottom of the door. Being new to electronics I only understand the basics,
    but would like to try and use the N/C contact to send some type of signal to
    me that my garage door is open.

    Could I hook this N/C contact to a breadboard and have an LED light up when
    the door is open? What would I need to detect if the circuit is opened or
    closed?
     
  2. Byron A Jeff

    Byron A Jeff Guest

    -I am trying to detect whether or not my garage door is open. I purchased a
    -normally closed contact switch to place on the bottom of the railing and
    -bottom of the door. Being new to electronics I only understand the basics,
    -but would like to try and use the N/C contact to send some type of signal to
    -me that my garage door is open.

    Sounds simple enough.

    -
    -Could I hook this N/C contact to a breadboard and have an LED light up when
    -the door is open?

    Sure.

    - What would I need to detect if the circuit is opened or closed?

    You already have the detector, the NC contact. The only problem is how to hook
    it up so that the LED lights up when the door is opened. The simplest way is
    to utilize the fact that an LED must have a current limiting resistor so that
    it won't burn up. So let's start with a simple LED circuit:

    V----R----A----LED-----GND

    V is a positive voltage source, presumably a DC wall wart. R is the current
    limiting resistor (more on that in a bit), A is just a mark point which I'll
    get to in a second, just note there's no physical device at point A yet, LED
    is the LED (duh?!) and GND is the ground end of the wall wart. If you wire
    this on a breadboard, current will flow through the resistor and the LED and
    the LED will light. To pick a resistor value divide the voltage by 10
    milliamps (.010A) an use a resistor that's close to that value. So
    for example if you have a 9 volt wall wart, divide 9 by 0.01 giving 900. So
    pick a resistor around 900 ohms.

    Now on to the dectector. It's normally closed which means that it's an unbroken
    circuit when the door is closed. It wouldn't help to wire the detector between
    point A and the LED because then the LED would be on when the door is closed.
    But what you can do is use the detector as a bypass around the LED. So if you
    wire it between point A and ground, when the door is closed then the current
    will flow from point A through the detector and into ground (or vice versa ;-)
    bypassing the LED. So the LED will go out because it doesn't have current
    running through it. However when the door opens and the detector opens, then
    current can no longer flow directly between point A and GND, so it must then
    flow through the LED, lighting it.

    Hope this helps,

    BAJ
     
  3. Byron A Jeff

    Byron A Jeff Guest

    -In article <>,
    --I am trying to detect whether or not my garage door is open. I purchased a
    --normally closed contact switch to place on the bottom of the railing and
    --bottom of the door. Being new to electronics I only understand the basics,
    --but would like to try and use the N/C contact to send some type of signal to
    --me that my garage door is open.

    -- What would I need to detect if the circuit is opened or closed?
    -
    -You already have the detector, the NC contact. The only problem is how to hook
    -it up so that the LED lights up when the door is opened. The simplest way is
    -to utilize the fact that an LED must have a current limiting resistor so that
    -it won't burn up.

    -So let's start with a simple LED circuit:
    -
    -V----R----A----LED-----GND
    -

    One last point I forgot. Point A has to between the resistor and the LED. The
    reason for this is so that the resistor can limit the amount of current drawn
    from the wall wart. If point A were between V and R, then the closed circuit
    would be a dead short on the wall wart, which leads to all kinds of problems.

    That's why the fact that the LED requires a current limiting resistor is
    important.

    BAJ
     
  4. John Fields

    John Fields Guest

    ---
    Being new to electronics, the distinction between a normally closed and
    a normally open switch may not be clear to you. A normally open switch
    is one in which the contacts would be open when not being actuated by
    the garage door, and a normally closed switch would be one where the
    contacts would only be open when the garage door forced them open.

    Viewing the following with a fixed pitch font like Courier,

    If you wanted to use a normally open switch you could do this:

    +V
    |
    [R]
    |
    +--[<LED]--O |
    | |---DOOR OPEN
    +----------O |
    |
    |
    GND


    If, on the other hand, you wanted to use a normally closed switch you
    could do this:


    +V
    |
    [R]
    |
    |A
    [LED]
    |
    O|
    |---DOOR OPEN
    O|
    |
    |
    GND

    For a 20mA red LED and a 5V supply the value of the resistor would be
    about 150 ohms and it would dissipate about 167mW of power in the first
    example (with the door closed), which would be worst case, so you could
    use a 150 ohm 1/4 watt resistor.

    If your switch is really normally closed then the circuit would only be
    drawing current when the door is open, as opposed to all the time if you
    used a normally open switch.
     


  5. John,

    Did you mean:

    +V
    |
    [R]
    |
    |A
    [LED]
    |
    |O
    |-----DOOR OPENS CONTACTS
    |O
    |
    |
    GND
     
  6. John Fields

    John Fields Guest

    ---
    No, I meant that the contacts are closed when the door is open.

    Perhaps this would have made it clearer:

    +V
    |
    [R]
    |
    |A
    [LED]
    |
    O|
    |<---CLOSED WITH DOOR OPEN
    O|
    |
    |
    GND


    What the OP was looking for was an LED ON when the door was open, and
    your way will turn the LED OFF when the door opens
     
  7. I reckon the title "Normally closed contact switch" let me
    to believe the objective was to show the OP a way to wire NC
    as well as alternative NO. I should have read the text of
    the message instead of assuming anything...

    Best,

    SBK

     
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