Voltage indicator circuit

Discussion in 'The Projects Forum' started by SHIBASHRIT MOHANTY, May 16, 2015.

  1. SHIBASHRIT MOHANTY

    Thread Starter New Member

    May 16, 2015
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    I am not very familiar with circuits. Ok, so I needed to design a circuit that will have an LED which will light up whenever the supply voltage to the circuit exceeds 40V. And I have no clue where to start from. Help will be really appreciated. Thanks in advance. :)
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
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  3. ebeowulf17

    Active Member

    Aug 12, 2014
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  4. MrChips

    Moderator

    Oct 2, 2009
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    No. Use a voltage divider to bring down the input voltage to a suitable value to suit the supply and reference voltages.
     
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  5. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    use resistors to bring it down to a lower value, like 4v,,,,,
     
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  6. ebeowulf17

    Active Member

    Aug 12, 2014
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    :rolleyes:
    Embarrassingly simple now that you've shown me. Should've thought of that. Next time I'll know. Thanks!
     
  7. Reloadron

    Active Member

    Jan 15, 2015
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    I agree as to using a comparator circuit. However, while I like most of the AAC reading material I have to disagree with the above link. The linked pages refer to the ancient 1458 Dual Operational Amplifier which consist of two LM741 operational amplifiers on a single chip. The 1458 and 741 are both very old operational amplifiers and really do not make very good comparators. They will work but I simply do not see them as a good choice.

    I would consider a chip aimed at being a comparator like the LM339 quad comparator, LM393 dual or the LM311 single comparator. I also like this reading on the subject of comparators. Since you mention 40 Volts you will need to likely do a little precision dividing down that voltage and scale it. Maybe 5:1 so the 40 volts becomes 8 volts to the comparator circuit. You would need a single comparator, a LED, a 10K pot, a 15 Volt power supply and a few resistors. Not much to it. I can draw you a basic circuit if you would like.

    Again and just my thinking I would go with a LM311. The 1458 or 741 can be used and they mention this in the link I provided.

    Ron
     
  8. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    What kind of precision do you need, and how crisp an indication? A common low-cost linear regulator circuit has a zener in the emitter of an NPN transistor. This puts the base up at a higher voltage, and reduces the variability of Vbe relative to the zener voltage, so that the base is basically the input to a comparator. The turn-on knee is softer than what you would get using an LM393, but things just can't get more simple. The overall body count for the circuit is less than with a comparator IC when you add the IC and voltage reference decoupling, plus you don't need a separate comparator supply - the whole thing will run directly off of the 40 V source.

    ak
     
    Last edited: May 17, 2015
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