Need help with Citcuit Design.

Discussion in 'General Electronics Chat' started by rb8720, Feb 21, 2008.

  1. rb8720

    Thread Starter New Member

    Feb 21, 2008
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    First let me say that I am very new to this so please bear with me. I need help to design a Circuit that when power is applied it will open one switch and close another and when poser is removed it will do just the opposite.
    Basicall i need a circuit to act like a SPDT switch. Thanks in advance for you help.

    RB8720
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    Two relays with DPDT contacts can do that. What kind of power are you switching - what voltage and how many amps?
     
  3. rb8720

    Thread Starter New Member

    Feb 21, 2008
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    low power 3-5 volts. I also need components to be as small as possible. Tring to automate what is normally a manual process.

    One Solution that i am considerig is using a microcontroller to controll2 transistors. For example if input pin is high then output pin a is on and B is off and if input pin is low then output pin A is off and B is on. I has hoping however that there is a simpler solution.

    Thanks for the help.

    RB8720
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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    In terms of size and cost, the two transistor solution is better. DPDT PCB relays use more power and take up significantly more space, plus they will set you back about $8.00 apiece.
     
  5. nomurphy

    AAC Fanatic!

    Aug 8, 2005
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    If you use one DPDT relay and use the N.O. contacts for one circuit and the N.C. contacts for the other circuit, you should get what you want.

    When power is applied to the coil, the N.O. contacts will close and apply power to circuit A, while the N.C. contacts of the other pole open and removes power from circuit-B. When power is removed from the coil, it will be the opposite. Provided you remove power only from the coil, and still have power applied to the contacts.

    You could do the same thing with a solid state relay or analog mux (provided you maintain power to the device itself).
     
  6. rb8720

    Thread Starter New Member

    Feb 21, 2008
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    Thanks for your replys guys. I'll build a test curcuist using some DSDT relays ands see how that works. Thanks for pointing me in the right direction.

    RB8720
     
  7. beenthere

    Retired Moderator

    Apr 20, 2004
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    Two DPDT relays? Glad someone reads this stuff too. I must have had relay logic on my mind.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    Wait a minute! :rolleyes:

    All he needs is a SPDT relay! You can get those things in REALLY compact sizes.

    Radio Shack carries stuff like this: 5V SPDT relay, $4.69
    [​IMG]
    Link: http://www.radioshack.com/product/i...&sr=1&origkw=Relay&kw=relay&parentPage=search

    That relay is fairly small, but far smaller relays are available from other sources like Mouser and Digikey. I simply point out Radio Shack because they're everywhere, and although the in-store price is high, that would be justified if only a single part is needed to accomplish the job (shipping would kill any price advantage on a single item)
     
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