Logic controlled bipolar switch

Discussion in 'General Electronics Chat' started by mikep56, Aug 25, 2011.

  1. mikep56

    Thread Starter New Member

    Feb 27, 2009
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    Hi All,
    I am looking for a simple schematic of an electronic switch. This switch will be driven from a TTL level signal. The output of the switch will be +5 and -5, depending on the input level.
    I have used this before, but I do not remember the exact details. I think that it had an NPN and a PNP transistor with the emitters connected, and the +5 fed to the NPN collector through a resistor, and the -5 fed to the PNP collecctor through a resistor. The tied emitters is the switched voltage point.
    Thanks for any help on this.
    Regards,
    mike
     
  2. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    Do you mean a level shifter?

    1. What's the output voltage for 0V input? -5V?
    2. Whhat's the output voltage for 5V input? +5V?

    Or is there any reason to call the circuit a "switch"?
     
  3. mikep56

    Thread Starter New Member

    Feb 27, 2009
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    Hi praondevou,
    For 0V in the output is -5V, for +5V in the output is +5V. So it level shifts a logic 0 to -5V and a logic 1 to +5V.
    I guess I mean a level shifter.

    Regards,
    Mike
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    I'd posted something like this before; it's pretty simple, just five resistors and a couple of transistors.

    As shown, it'll be able to source current from +5v pretty well, but depending on your application, you might require more current sinking down to -5v. If your load is light, this should work fine.
     
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  5. mikep56

    Thread Starter New Member

    Feb 27, 2009
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    Hi SgtWookie,
    Thanks for the circuit diagram. It will be used to drive a '4066 analog switch, so the current draw is minimal. As I said in previous posts, it will take a logic 0 and output -5V, and a logic 1 and output +5V; to drive the switch inputs of a '4066.
    Thanks again for all the feedback.
    Regards,
    mike
     
  6. John P

    AAC Fanatic!

    Oct 14, 2008
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  7. SgtWookie

    Expert

    Jul 17, 2007
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    Ahh, I see. Yes, it'll work fine for that.

    If you are switching audio, you may wish to slow the transition down quite a bit to avoid "popping" noises. If the 4066 inputs change slowly, they'll go through a resistive state rather than a fast switch from one state to the other; giving the voltage levels on either side of the switched points more time to equalize.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    Hi John,
    Our OP is probably using the 4066 for switching audio in a preamp or something similar. It's pretty usual for such audio amps to have bipolar supplies. If they simply used ground and +5v for the 4066, the lower half of the waveform would be lost.

    They're probably controlling the 4066 using a microcontroller or other logic level device; so they needed an interface between the different logic levels.
     
  9. mikep56

    Thread Starter New Member

    Feb 27, 2009
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    0
    Hi Sgt Wookie and John P,
    Actually the '4066 will run with +- supplies, and will have faster switching times and a higher off resistance. It gets closer to simulating an electronic relay.
    I am using it to switch low level analog voltages.
    Regards,
    mike
     
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