SPDT Relay with Resistors

Discussion in 'General Electronics Chat' started by IGill, Jan 5, 2013.

  1. IGill

    Thread Starter New Member

    Jan 5, 2013
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    0
    I am quite new to electronics and would like a little help on something that I assume must be very common.

    I want to be able to fulfill the basic function of a SPDT relay, ie. current high powers one circuit, whilst current low powers an alternative circuit.

    In a relay a spring activates an alternative switch if there is no current. I can't figure out how to use no current to activate a transistor as a switch.

    This must be a common requirement of circuits and as I continue to study I will probably come across the solution. In the mean time I would really appreciate someone giving me a nudge in the right direction.

    Ian
     
  2. SPQR

    Member

    Nov 4, 2011
    379
    48
    Hello and welcome.

    HERE and THERE and EVERYWHERE are sites about transistors.

    Although there are many different types, in terms of your question you can think of just two - the PNP and the NPN.

    With an NPN transistor, if you apply a positive signal to the base, the transistor will turn ON, and with a negative signal it will turn OFF.

    With a PNP transistor, if you apply a positive signal to the base, the transistor will turn OFF, and with a negative signal it will turn ON.

    You couldn't use a single transistor like a SPDT relay, but two transistors might do the trick.

    Let's see how others might approach this question.
     
  3. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
    197
    Google Transistor push-pull.
     
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  4. IGill

    Thread Starter New Member

    Jan 5, 2013
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    0
    Thanks for that, it goes part way to solving my problem. I am getting a little confused by varying transistors, particularly since some suppliers use the opposite casing to others (the D the other way round).

    So if I connect the bases of an NPN and a PNP together then either one will be activated depending on the polarity?

    If I have an OFF-ON + pulse can I change this to a + - pulse by connecting it to ground via a suitable resistor?
     
  5. SPQR

    Member

    Nov 4, 2011
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    48
    Can you give us more details on exactly what you want to do?
     
  6. IGill

    Thread Starter New Member

    Jan 5, 2013
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    0
    I was thinking about using the output from an NE555 to generate a pulse. Depending on weather the output is high or low one or other set of LEDs light up.

    I'm just trying to get my head around some basics at the moment. What I was really after was a simple circuit that does the basic equivalent of

    If high then
    circuit A
    else
    circuit B
    end if
     
  7. SPQR

    Member

    Nov 4, 2011
    379
    48
    Ok, then I've done the same thing with a 555.

    Assuming 5-9V power supply:

    + terminal
    330 ohm resistor
    LED + lead
    LED
    LED - lead
    connection from 555
    330 ohm resistor
    LED + lead
    LED
    LED - lead
    - terminal

    If the pulse is "HIGH", the upper LED is shut off and the lower one conducts.
    If the pulse is "LOW", the upper LED conducts and the lower one is shut off.

    I made a little board with a 555, two pots for the two control resistors, 12 capacitors to allow the capacitance to vary, and the output is connected to the LED circuit above and a voltage comparator to allow connections to another circuit.

    At this minute, I'm watching the LEDs alternately flash.

    Top...Bottom...Top...Bottom...Top...Bottom...Top...Bottom...Top...Bottom...Top...Bottom...:)
     
  8. IGill

    Thread Starter New Member

    Jan 5, 2013
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    0
    Thanks for that. It helps to get input from somebody who knows what they are doing. I'm finding electronics to be like maths, everything you learn builds on what you've already got to grips with. But sometimes it takes a while for the penny to drop.

    By the way, from your name, are you by any chance a Roman soldier?
     
  9. SPQR

    Member

    Nov 4, 2011
    379
    48
    I'm not sure I know what I'm doing, but eventually I get things figured out.

    In terms of SPQR, I'm as WASPy as they come.
    But I'm an Italophile, so I'm sure I was a Roman Senator in a prior life...:D
     
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