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Ohms to trigger a switch

Discussion in 'General Electronics Discussion' started by Alan P, Mar 12, 2015.

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  1. Alan P

    Alan P

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    Mar 12, 2015
    Hi Guys,

    I'm trying to design a circuit to trigger a relay based on an ohms reading. I need to trigger the relay at 150 ohms. I believe I could use an LT1721 to do this, however I'm unsure of the resistor values or the circuit design. Any help would be appreciated. Thanks.
     
  2. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Hi Alan can you explain a bit more about your project? Where is this 150R coming from?
    Adam
     
  3. Alan P

    Alan P

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    Mar 12, 2015
    Hi Adam,

    The signal is coming from a sending unit which is being used to monitor fluid levels in liquid storage tanks. The sending units are similar to fuel sending units which basically act as potentiometers with a reading of 16 ohms empty and 158 ohms at full. I need to create a safety switch to prevent overfilling of the tanks. By creating a circuit which could cut out the pump at approx 150 ohms, I could use the existing sending units as the source input signal.
     
  4. ADRT

    ADRT

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    Nov 25, 2014
    Last edited: Mar 12, 2015
    davenn likes this.
  5. ADRT

    ADRT

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    Nov 25, 2014
    Here's a better one. Substitute thermistor with your sending unit.

    image.jpg
     
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  6. Alan P

    Alan P

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    Mar 12, 2015
    @ADRT, If I"m understanding this right, R3 provides for an adjustable trigger signal? Also, is there a specific Comparator used in this circuit?
     
  7. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Yes that circuit was just what I had in mind :). Nice one
    Adam
     
  8. Alan P

    Alan P

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    Mar 12, 2015
    I ran a few more tests on sender. At Empty it's around 5vdc and Full is about .8vdc which is my trigger voltage. Any suggestions on the comparator?
     
  9. ADRT

    ADRT

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    Nov 25, 2014
    This is the original source of the circuit. If you read further down you will find some recommendations on comparitors as well as some equations for figuring out which resistors to use for your application. Hope this helps and good luck. : ) oh and yes R3 is for adjustment.
     
    Alan P likes this.
  10. Alan P

    Alan P

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    Mar 12, 2015
    Thanks ADRT Much Appreciated!
     
  11. Alan P

    Alan P

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    Mar 12, 2015
    Sorry, what's the link? Did I miss something?
     
  12. davenn

    davenn Moderator

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    Sep 5, 2009

    Yeah he forgot to add it to his last post ;)
     
    ADRT likes this.
  13. ADRT

    ADRT

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    Nov 25, 2014
  14. duke37

    duke37

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    Jan 9, 2011
    A comparator normally needs a pull-up resistor on its output.
     
  15. ADRT

    ADRT

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    Nov 25, 2014
    Is that always duke37? I noticed that on some of the datasheets for different comparitors sometimes they use a pull up and sometimes they don't depending on the application. Do you think there should be one here?
     
  16. duke37

    duke37

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    Jan 9, 2011
    You will need enough base current to turn Q1 hard on so you will need a pull-up resistor if the comparator does not pull up.

    If you wish to look at several different levels of voltage, then you could investigate the LM3914 which has 10 comparators on board.
     
    ADRT likes this.
  17. Colin Mitchell

    Colin Mitchell

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    Aug 31, 2014
    HighFuel-2.gif
    Here is a simple transistor circuit
     
  18. Alan P

    Alan P

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    Mar 12, 2015
    What's protecting the sender from +12v in the transistor circuit?
     
    Last edited: Mar 13, 2015
  19. Colin Mitchell

    Colin Mitchell

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    Aug 31, 2014
    You said the voltage is 5v when empty, so there is a safety resistor in the circuit.
     
  20. Alan P

    Alan P

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    Mar 12, 2015
    My circuit reading skills leave a lot to be desired. The fuel sender + enters at the bottom right? I think I got it. I'll wire it up on a bread board and give it a shot. Thanks.
     
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