can I use a PNP transistor to invert switch operation?

Discussion in 'General Electronics Chat' started by jarl, Apr 6, 2012.

  1. jarl

    Thread Starter Member

    May 16, 2009
    16
    0
    Hi Guys. I'm sure that this has been done a million times before, but try putting "making an open switch closed" or similar into google and see how far that gets you :rolleyes:

    I have two problems. One is a custom SPST push switch- replacement is not an option unfortunately. I want to invert it's operation- I want it to pass current when in the "open" position, but not when in the "closed" position. Is there a simple chip I can use to achieve this?

    The second is very similar, but it's a tilt switch. The switch opens when horizontal, I want it closed!

    The solution I have thought of uses a PNP transistor - Chop the wire after the switch. Connect the switch end of the cut wire to the transistor base, connect the free wire to the transistor collector, and connect the transistor emitter to the pre-switch wire.

    When base voltage is low (switch open) then current will flow though the transistor, and when base voltage is high (switch closed) the transistor will prevent current flow. Is this right?

    What about the gain element of a transistor? Could that cause damage to other electrics downstream? As I understand it, the gain element only applies when the transistor is not fully on or fully off, which are the only two states I'll be using it in, so I don't need to worry.

    Finally, is there a better way to achieve what I'm trying to do??!

    Thanks!
     
    Last edited: Apr 6, 2012
  2. panic mode

    Senior Member

    Oct 10, 2011
    1,320
    304
    can you post your circuit?
     
  3. #12

    Expert

    Nov 30, 2010
    16,298
    6,810
    Without any details, the best I could recommend is a relay. Transistors don't universally work with both polarities, AC voltage, any and all levels of current, any and all levels of voltage, etc.

    Must have details.
     
  4. crutschow

    Expert

    Mar 14, 2008
    13,016
    3,235
    Yes, we need details.

    What is the current voltage and current level you need to switch?

    Is the voltage AC or DC?
     
  5. jarl

    Thread Starter Member

    May 16, 2009
    16
    0
  6. panic mode

    Senior Member

    Oct 10, 2011
    1,320
    304
    add 1k resistor from base to DC ground
     
    jarl likes this.
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