Relay to switch two inputs separately to single output

Discussion in 'General Electronics Chat' started by Ryan McGuire, Aug 24, 2016.

  1. Ryan McGuire

    Thread Starter New Member

    Aug 24, 2016
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    I'm definitely a novice, so thank you in advance for your patience -- hopefully this isn't an obvious question. What I'm looking to accomplish is essentially switching two separate inputs to a single output. I would like a simple SPST off-on push-button switch that will act as the signal to a relay that will have two separate inputs and one output. When the push button is off I want one of the two inputs on the relay to connect to the output. When the push button is on I want the second input only to connect to output. This is a 12vdc setup.

    I suppose my first question is: can you take a SPDT type C relay (which I understand to mean one throw is NO, one is NC) and operate it in reverse so that the two outputs are actually my source voltage and the single input becomes my output? If not, what type of relay will accomplish switching between two separate inputs and one output so that only one input or the other is connected?

    For some background, the two inputs are basically signals to a timer. The timer is configured so that when the signal is applied the output is powered, and in that state if the signal is removed the output is disconnected after a set time (https://www.amazon.com/gp/product/B015Z2EQCU). I have two signal wires, one of them is always on, the other stays on only while the vehicle is running. I want to be able to push / arm the push button causing the always-on signal to be connected to the timer so that the output never turns off -- or disarm the push-button so that the other signal (only while vehicle is running) connects which would cause the timer output to be turned off after x seconds of the vehicle being turned off.

    Thank you in advance!
     
  2. #12

    Expert

    Nov 30, 2010
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    Piece o' cake.
     
    cmartinez likes this.
  3. Ryan McGuire

    Thread Starter New Member

    Aug 24, 2016
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    Hi, thanks for the reply. Can you help a layman out to understand what physically I would use to accomplish this? What type of relay?

    Thank You
     
  4. #12

    Expert

    Nov 30, 2010
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    A single pole single throw switch and a 12 volt automotive relay seems obvious.
    If your "signals" are using tiny currents, like a milliamp or so, you might need gold plated contacts. They are sometimes called, "dry" contacts. Automotive relays aren't built like that. They are designed for massive current like 10 amps.
     
  5. Ryan McGuire

    Thread Starter New Member

    Aug 24, 2016
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    I do get that, per my original post, but it's the 12v automotive relay that I'm struggling with. I don't know what kind to get! My only thought is to get a SPDT type C relay and reverse the way it works by putting the two inputs on the output side and the output on the input side. This obviously isn't how this is designed to work, so I didn't see this as a solution. I don't believe a DPST relay will work as the poles should not be connected, they should be separate and only one at a time should be connected to output.

    I understand I need a switch and a relay. What type of relay I need is where I need the help.

    Thanks Again

    Edit: Perhaps what I'm missing is that automotive relays are fundamentally different and can be configured in a variety of different ways. Looking at it I believe an automotive "changeover" / type B relay is what I'm after where terminals 87 and 87a are my separate inputs, 30 is my output, and 85/86 are common. Does this sound right?
     
    Last edited: Aug 24, 2016
  6. MaxHeadRoom

    Expert

    Jul 18, 2013
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    A standard automotive relay from any auto store or wrecker (probabally get a hand full for $5).
    There are two basic types spst and spdt, you want the latter.
    If single pole will suit your application?
    Max.
     
  7. #12

    Expert

    Nov 30, 2010
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    Baloney. Those contacts don't know and don't care which way the current is flowing. They are just two pieces of metal that touch each other.
     
  8. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Any reason you need the relay at all?
    This could just be done with a spdt switch...
     
  9. Ryan McGuire

    Thread Starter New Member

    Aug 24, 2016
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    So to use a SPDT switch I would have the "SP" side as my output and the "DT" side as my two inputs. If that's ok and it's assumed that a switch can operate in either direction that is good for me. I had thought I read somewhere that this was a bad idea.
     
  10. #12

    Expert

    Nov 30, 2010
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    Switch contacts are just like relay contacts. They are just two pieces of metal that touch each other.
    If you have a lot of amps, you use thicker metal. If you need tiny little milliamps, you use gold plated metal.
     
  11. cmartinez

    AAC Fanatic!

    Jan 17, 2007
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    What device in your car is it exactly that you want to control with either the relay or the switch?
     
  12. Tonyr1084

    Active Member

    Sep 24, 2015
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    Ryan: You're making it WAY more complicated than it is, or needs to be. I'll be back in a few minutes with a picture. Normally a switch is configured with a common point that switches between one of two contacts. However, there is nothing that says it must be set up that way. You can use ANY switch (SPDT) ( as long as it meets current and voltage minimum requirements ) either way. SC (or switch common) can be thrown between S1 (one of the two contacts) or S2 (the other contact). So you can send a single signal in either direction, SC to S1 or SC to S2, using SC as the source and S1 and S2 as the destinations. But you can reverse the connections and use S1 and S2 as separate sources and the SC as the destination.

    As promised, I'll be back shortly with a drawing. Watch for my edit.

    OK, so, in the drawing you see one source (a 12 volt battery) being sent to one of two loads (L1 or L2). They can be anything that operates on 12 volts. Just assume for a moment one is a red light and the other is a green light. Certainly doesn't matter what color you imagine, just understand that an SPDT switch will direct current between a source and two destinations - OR as in the drawing below, two different sources (a 12 volt battery and a 6 volt battery), the switch chooses between one battery or the other and sends it to L1 (whatever L1 may be).

    Really, it's that easy. Don't try to complicate it any further. If this didn't clear it up for you then I'm at a loss to help any further.

    'av'a g'day mate.
     
    Last edited: Aug 24, 2016
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