How to use a drop in voltage to trigger a switch?

Discussion in 'The Projects Forum' started by PulseLED, Aug 11, 2015.

  1. PulseLED

    Thread Starter New Member

    Jan 24, 2014
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    The problem, I want something to turn on when a drop in voltage is detected.

    I know this can be done with a relay, but its not very versatile, unresponsive and slow.

    I'm looking for the opposite of a transistor I guess. Something that will detect a slight drop in current, then allow more current through another circuit. What is the simplest way to accomplish this?
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    A little more on the intention/circuit/devices are required, in some cases a simple comparitor will do it, LM311 etc.
    Max.
     
  3. ScottWang

    Moderator

    Aug 23, 2012
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    Your problem is not descried very clearly as how is the voltage ?V and current ?mA ?
     
  4. #12

    Expert

    Nov 30, 2010
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    Anti-transistors haven't been invented. We just wire up the regular kind to work the logic backwards.
     
  5. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Need more specifics:

    Rate of change of voltage?
    Magnitude of the voltage before and after the drop?
    What happens when the voltage goes back up?
     
  6. PulseLED

    Thread Starter New Member

    Jan 24, 2014
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    The specifics are not known,I was more after a few examples of how this is accomplished so that I can research it and test myself. Voltage range is about 9v DC but I'm unsure of the actual trigger voltage drop etc.

    How are they wired up?

    Is there a simple example of this?
     
  7. #12

    Expert

    Nov 30, 2010
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    Google is your friend.
     
  8. MaxHeadRoom

    Expert

    Jul 18, 2013
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    I have used the LM311 to detect a change in a 240vdc mobile application.
    The output will turn on/off as the set point is exceeded or below.
    Max.
     
  9. TheButtonThief

    Active Member

    Feb 26, 2011
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    Either use a PNP transistor or still use a NPN transistor but flip over your voltage divider.

    You could also take a look at schmitt triggers or op-amps as comparators if you want to learn something a little more complex.
     
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