Push button opens the circuit

Discussion in 'General Electronics Chat' started by tekinzen, Nov 27, 2009.

  1. tekinzen

    Thread Starter Member

    Nov 22, 2009
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    Could I have ordered the wrong type of push button? When I push the button the circuit opens and stays closed when the button is not pressed - the opposite behavior I was expecting from a on/off button... at least that's what I thought it to be :)
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    It sounds like you ordered a N.C. (normally closed) pushbutton switch, where what you wanted was a N.O. (normally open) switch.

    If you post a schematic of your circuit (.PNG format preferred) we may be able to figure out a way to resolve your dilemma. Either that, or the easy way would be to simply order the correct type switch.

    If your pushbutton switch has three terminals, it may have both NC and NO connections with a COM (common) terminal.
     
  3. tekinzen

    Thread Starter Member

    Nov 22, 2009
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    Now I'll know... My circuit is not worthy of a schematic at all. I just have it connected to a battery. It also has four terminals, I assume two for "input" two for "output" if I can say that.
     
  4. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    Most push button switches are available as N.O. (normally open) and N.C. (normally closed) versions. Some also have BOTH avaiable...see if your switch has three terminals.

    Eric
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    Wow, there's an echo in here. :rolleyes: ;)

    Nope, it has four terminals.
     
  6. eblc1388

    Senior Member

    Nov 28, 2008
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    The number of terminals does not necessary indicates the overall switch function. A four-terminal push button could have two pairs of normally open, normally close or one of each.

    I have a NO/NC push button that has four terminals.

    The "common" is inside the push button and is not accessible to user. Two of the terminals becomes open while the other two close when user pushes the button.
     
  7. Audioguru

    New Member

    Dec 20, 2007
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    Doors open and close, not switches.

    Switches turn on electricity or turn it off.

    Do you open and close a light bulb?

    I think you have a button that turns on the electricity when you press it then it turns off the electricity when you release it.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    Gee, but the "open" and "close" terminology has been around since long before any of us were thought of.

    Probably had a lot to do with those big old "knife" type switches.
     
  9. tekinzen

    Thread Starter Member

    Nov 22, 2009
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    This is the button in question: http://www.sparkfun.com/commerce/product_info.php?products_id=97

    I'm not familiar with the term "Single Pull Single Throw" but the description said "momentary on switches" - I thought that meant "ON switch" not "OFF switch". I was looking for a button that "closes" a circuit when pressed.

    Mine does the opposite :(
     
  10. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    The drawing shows a SPST-NO with dual contacts
     
  11. tekinzen

    Thread Starter Member

    Nov 22, 2009
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    "SPST-NO" as in Normally Open when button is pressed or depressed?
    It would make sense if it were "open when pressed" since this is how it behaves in my case.

    Also, how could you tell? I'm trying to buy a "SPST-NC" for inputs, send logic 1 to a PIC, etc.
     
  12. eblc1388

    Senior Member

    Nov 28, 2008
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    "Normally" means the state the switch or contact when it is not acted upon by user or via external force caused either mechanically or electrically. So a normally open contact will be like an open circuit and current cannot flow pass the contact.

    The link you have provided indicates a switch which closes when user presses it but yours actual switch appeared to perform oppositely. It is possible that you have ordered the wrong switch or sparkfun simply has supplied you with a wrong type of switch without knowing about it.

    You can email sparkfun to clear up the matter.
     
  13. SgtWookie

    Expert

    Jul 17, 2007
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    It may also be that you have wired loosely across two terminals that are connected internally, and when you're pressing the switch button, the loose wires break connection.

    Check the switch terminals with a multimeter set to Ohms, without the button being pressed and nothing else connected to it.
     
  14. tekinzen

    Thread Starter Member

    Nov 22, 2009
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    I got my button replacements from the vendor and happily I went straight ahead and put them in my simple, battery + button circuit only to realize they work in the same way as my old buttons.

    I either don't understand how to wire them or I was sent the wrong type of push buttons again...

    (attached is a drawing of my circuit made in paint)
     
    Last edited: Dec 5, 2009
  15. SgtWookie

    Expert

    Jul 17, 2007
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    Ack! :eek:

    You're shorting out the battery when you push the button.

    Connect it to your load like this instead (squiggle represents your load):

    [​IMG]
     
  16. wii552

    New Member

    Dec 2, 2009
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    yeah, tac switches are like that a lot. when you look at the four leads, you should be able to see 2 pairs-some of the leads are closer to each other than others. Count the two pairs as single contacts. From the what-barely-counts-as-one datasheet, it looks like you got the right kind of switch.

    Look at the attached pic for how I THINK you should hook it up. L is LOAD
    (sorry for cruddy image, mad in gimp and was rushed)
     
    Last edited: Dec 5, 2009
  17. tekinzen

    Thread Starter Member

    Nov 22, 2009
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    That's great, it works now. Thanks a bunch!!!
     
  18. zgozvrm

    Member

    Oct 24, 2009
    115
    2
    The phrase in the Sparkfun's description of the button is incorrect. It should have read, "Single POLE, Single Throw" (not "Single PULL ...").

    The number of poles that a switch has refers to the number of separate, isolated circuits that it can switch simultaneously.

    The number of throws a switch has, refers to the number of positions the switch has that are capable of completing a circuit. Typically, each pole has the same number of throws.

    Your switch is single pole, since it is only capable of switching one circuit. It is single throw, since (although it has 2 positions) it can only "make," or complete, a circuit in one of those positions.

    Note that pins 1 & 2 are electrically the same, as are pins 3 & 4. The extra pins are there both for convenience, and for mechanical stability, when soldering the switch onto a circuit board.

    [​IMG]


    See Wikipedia for more info: http://en.wikipedia.org/wiki/Switch
     
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