Using a push to break switch to turn something on

Discussion in 'The Projects Forum' started by Tickers, Jul 5, 2011.

  1. Tickers

    Thread Starter New Member

    Dec 24, 2009
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    I have a component with about 10mm of travel, and I need to activate a lamp as soon as the component travels at all.

    The only suitable switch I could find was a push to break plunger switch, how can I use the PTB switch to get a voltage across the lamp?
     
  2. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    What kind of lamp? Lamp voltage/current?

    Ken
     
  3. Tickers

    Thread Starter New Member

    Dec 24, 2009
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    It's an array of LEDs, at around 5V and 1A.
     
  4. CDRIVE

    Senior Member

    Jul 1, 2008
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    Way too vague. Does this have to latch? If so, you need a way to unlatch. This can be done using the same switch if triggering a bistable flip flop.
     
  5. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    Echo CDrive. Can you give us the big picture...what you're trying to do with what. It's often easier to help solve your problem than help solve your solution. ;)

    Ken
     
  6. strantor

    AAC Fanatic!

    Oct 3, 2010
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    I would try using your normally closed switch the same way a NPN transistor is used in open collector output configurations, with the output going to a NPN transistor to switch your LEDs
    [​IMG]
     
    Last edited: Jul 5, 2011
  7. SgtWookie

    Expert

    Jul 17, 2007
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    I think I'd use a logic level N-ch MOSFET instead of a transistor. With 1A current required, you'd need 100mA base current to saturate a bjt (bipolar junction transistor), and that represents a substantial "tax". A Darlington would have a Vce of 0.7v or more, so that's out as well.

    Have a look at the attached schematic.

    The MOSFET could be an IRLD024, which comes in a 4-pin DIP package - nice for breadboarding; rated for 60v, and up to 2.4A collector current.
     
  8. strantor

    AAC Fanatic!

    Oct 3, 2010
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    I guess the time has come for me to get with the program. This is the 3rd or 4th time I have played the transistor card, only to be immediately trumped by the MOSFET card.
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    Well, power MOSFETs have come so far in the last 20 years or so that they're mighty hard to ignore. Their biggest downfall is their sensitivity to static electricity, but once they're in a circuit, they're pretty darn rugged.
     
    strantor likes this.
  10. castley

    Member

    Jul 17, 2011
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    If you want to keep it simple. Use the push to break switch to short out the lamp with a resistor in series with the lamp.However, the lamp would go out again as soon as the switch goes closed again. Is this what u want?
     
  11. SgtWookie

    Expert

    Jul 17, 2007
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    The LEDs use 1A current. If they're using a constant current source, then the switch would be sinking a constant 1A current; if the LEDs were using simple resistors, then the sink current could be considerably more than that.

    The MOSFET switch idea I submitted is pretty simple, and would only have a drain of 0.5mA when the switch is closed. That's a pretty significant savings in power.
     
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