Do I need a relay to simulate SPDT switch

Discussion in 'The Projects Forum' started by Granz, Apr 27, 2009.

  1. Granz

    Thread Starter Member

    Apr 27, 2009
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    I have a single throw switch available but I'd like to have a portion of the circuit active when the switch is open and a different portion of the circuit active when the switch is (momentarily) closed. The two portions of the circuit should never be active at the same time. Do I need a solid state relay for this? I feel like I should be able to get the job done using only transistors but I can't figure it out.
     
  2. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    How much separation (in mS) do you want between the two switches?

    Have you considered either an NPN and PNP pair (or equivalent mosfets) or simply taking your signal, inverting it and using the inverted signal for one switch and the non-inverted for the other.

    John
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    Why don't you post your circuit so that we have something to look at?
     
  4. Granz

    Thread Starter Member

    Apr 27, 2009
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    I think an NPN / PNP pair is probably what will do it.

    The circuit can be as simple as a battery with two resistors in parallel.
    If I hold down a momentary SPST tactile switch, charge only flows through resistor A.
    When I release the switch, charge only flows through resistor B.
     
  5. Granz

    Thread Starter Member

    Apr 27, 2009
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    This is what I had in mind.
    The idea is that charge will flow to R7 when the switch is open. When the switch is held closed it will stop flow to R7 and allow flow to R6 instead.
    Not sure if this will work though. It seems conflicting that when the switch is closed, both the emitter and base of the PNP will be high.
     
  6. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    When the switch is closed, won't the base of T4 still be tied to ground? Thus, it will still be on.

    John
     
  7. Granz

    Thread Starter Member

    Apr 27, 2009
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    Yes, so I guess this wouldn't work as a way to switch between R7 and R6 but it's just a starting point. I have a feeling some configuration of transistors would do the trick rather than using a solid state relay.
     
  8. CDRIVE

    Senior Member

    Jul 1, 2008
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    Please clarify something for me. You said that the two portions of the circuit will never be active at the same time but you want the switch action to be momentary. does this mean you want to use a push button or spring loaded toggle sw? Do you want the last switched on circuit to remain on when the switch is opened?

    Rephrased: Are you trying create a sort of Push to switch to A - Push to switch to B circuit? If so, that's what a Flip-Flop does.
     
  9. Granz

    Thread Starter Member

    Apr 27, 2009
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    The switch action is momentary...it's a momentary normally open SPST.
    But the last switched on portion of the circuit should not remain on like a latch. It should go back to the other portion of the circuit.

    Not push to A push to B.
    Rather ...push an hold for B....otherwise A.


    I've started trying to simulate JFETs in Microcap but I've just confused myself.
     
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