Auto Unlatching Switch

Discussion in 'General Electronics Chat' started by winders, Jul 9, 2012.

  1. winders

    Thread Starter New Member

    Jul 9, 2012
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    Folks,

    I am looking for a latching switch that will unlatch itself if power is removed from the line side of the switch. In other words, if power is lost to the circuit, the switch will be in the off position when power to the circuit is restored.

    The application is 12v and less than 10 amps. The smaller the better. Push button is fine. This is for a camera system in a race car.

    Thanks,

    Scott
     
  2. #12

    Expert

    Nov 30, 2010
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    Will a relay be sufficient?
     
  3. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    Do you also want the ability to switch it off too?

    Ken
     
  4. winders

    Thread Starter New Member

    Jul 9, 2012
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    A relay won't work because I need the switch off by default even if it was on before the power to the line side was interrupted..

    Yes, I need to be able to turn the switch on and off manually while power is supplied to the line side.

    Scott
     
  5. #12

    Expert

    Nov 30, 2010
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    Are you assuming that a relay circuit can not be configured to be off by default or is there some requirement you haven't told us about?
     
  6. strantor

    AAC Fanatic!

    Oct 3, 2010
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    I don't see any reason a relay wouldn't wouldn't meet the requirements you've given in post #1 & #4. Actually, your description of requirements is the description of the operation of a relay.
     
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  7. winders

    Thread Starter New Member

    Jul 9, 2012
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    I don't think so.....or are we talking about a relay of which I am unaware?

    Here is more detail:

    A camera is configured to automatically start recording when it receives power. The camera automatically shuts off 1 minute after power is removed. The switch that controls the power to the camera is on switched power controlled by the ignition toggle switch on the dash. Just before I go out on the track , I turn on the camera power switch and the camera automatically starts recording. When I come off the track, I turn off the ignition switch which removes power from the camera power switch. The camera turns off after 1 minute. I have not turned off the camera power switch though. If I were to turn on the ignition switch, the camera would get power and start recording.

    What I need is a switch that will turn the camera on and supply power until I switch it off or power is removed. If power is removed from the line side of the camera power switch, the switch turns off so it is in the off position if power is supplied to the line side again.

    Is there a relay that does this?

    Scott
     
  8. #12

    Expert

    Nov 30, 2010
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    Does the vehicle use 12 volts for its ignition?
    Is the "unknown camera power" the same 12 volts that everything else uses?
    (Meanwhile, I'm checking TOS.)
     
  9. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    I think this is what people are thinking.
    One normally-open pushbutton switch
    One normally-closed pushbutton switch
    One 12V SPST or SPDT relay

    When the ignition is turned on, no power is supplied to the camera.
    Once the ignition is on, push the ON button and the camera is powered.
    While the ignition is on, push the OFF button and the camera power is cut.
    or
    When the ignition is turned off, the camera power is cut.


    Ken
     
    Last edited: Jul 9, 2012
    BSomer, strantor and #12 like this.
  10. winders

    Thread Starter New Member

    Jul 9, 2012
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    Yes, the same 12 volts...

    Scott
     
  11. strantor

    AAC Fanatic!

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    Precisely. I had drawn it up, but then got called to go save the day. I get back and see you've already posted it and saved me the time of scanning and editing.
     
  12. #12

    Expert

    Nov 30, 2010
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    Thank Dog. I can quit playing, "20 questions".
     
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  13. winders

    Thread Starter New Member

    Jul 9, 2012
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    I don't see how this solves the problem.

    What I want to accomplish is to have the camera power switch work like a momentary switch except that once on, it stays on until power on the line side is removed or it is pushed again.

    Think about the rear window defroster switch in many modern cars. It's a momentary switch that toggles power to the defroster elements. Typically, it defaults to off when the ignition is turned off. Yes, there is more logic behind that switch. But that is basically what I am looking for here.

    Using the relay setup laid out in a previous post seems like a crazy to do the same thing as no relay and just a toggle switch.

    Scott
     
  14. strantor

    AAC Fanatic!

    Oct 3, 2010
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    The circuit given in post #9 does exactly what you described here.
    When you push the ON button, current flows through the ON button and through the relay coil, which actuates the relay.
    when the relay actuates, the contact (upper right, diagonal line) switches from NC to NO.
    Once the contact is in the NO position, current flows through the contact and down through the relay coil, keeping it energized. So you can remove your finger and it will stay energized. (the relay feeds itself to keep itself energized) This all happens in less than 1/10 of a blink of an eye. This is called a relay latching circuit.
    When the relay contact is in the NO position, +12V is present at the point marked +12v to camera.
    In order to stop the camera, you can press OFF which will break the connection, deenergize the relay, and remove power to the camera, or you can turn the ignition off and remove power to the whole circuit.

    There is no single switch that does what you want, that I am aware of. You will need the relay to accomplish this task. However, if you want to eliminate the OFF button, you can use a momentary toggle switch like this one (see data sheet). For example, if you were to use this switch that I linked to, replace the OFF button with terminals 1 & 2 of the momentary switch, and replace the ON button with terminals 4 & 5 of the momentary switch.
     
  15. winders

    Thread Starter New Member

    Jul 9, 2012
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    Okay guys, I see it now. I didn't think about using a relay power output to power its own coil.

    Should I use the normally closed momentary "Off" switch to interrupt output power or ground? I would think interrupting ground would be better....

    Thanks again!

    Scott
     
  16. BSomer

    Member

    Dec 28, 2011
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    Okay... One switch, no relay...


    edit> see post #24 for correct schematic.
     
    Last edited: Jul 10, 2012
  17. strantor

    AAC Fanatic!

    Oct 3, 2010
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    It doesn't matter. If you wired it up exactly as drawn, it would work right the first time and avoid confusion.
    Oh no! erectronics!
     
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  18. winders

    Thread Starter New Member

    Jul 9, 2012
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    Cool!

    Anyone in the mood to build this in a package that could be used in a race car? How much would it cost?

    Scott
     
  19. winders

    Thread Starter New Member

    Jul 9, 2012
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    Do I have this correct?:

    You need a normally open (NO) momentary switch for "On", a normally closed (NC) momentary switch for "Off", and a Bosch 0-332-204-150 relay.

    On the relay:

    Pin 30: Main Power In
    Pin 87: Main Power Out
    Pin 86: Switched Power (relay control)
    Pin 85: Relay Ground

    You wire up the relay so:

    Pin 30 is connected to +12 that is switched by the ignition switch.

    Pin 86 is connected to +12 that is switched by the ignition switch and has the NO momentary switch in-line.

    Pin 87 (main power out) goes to the camera and is jumped to pin 86 (relay control).

    Pin 85 is connected to ground and has the NC momentary switch in-line.

    When you press the "On" momentary switch, the relay is energized and provides power to the camera and to its own coil so it stays "latched". Pressing the "Off" momentary switch removes ground from the relay coil and it disengages. If you turn the ignition switch off, the coil disengages since there is no power.

    Correct?

    Scott
     
  20. BSomer

    Member

    Dec 28, 2011
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    I know! What was I thinking??? :eek:

    Yup. That will work just right.
     
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