Super simple 14v LM339 LED meter

Discussion in 'The Projects Forum' started by Al Vince, Mar 3, 2015.

  1. Al Vince

    Thread Starter New Member

    Feb 24, 2015
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    I have a super simple project... I need a simple circuit that turns on an LED when the voltage input is at 14v or above.

    I found some circuits using the LM339, but many of them have multiple LEDs... I just need one LED that turns on when the input reaches 14v or above.

    Thanks!

    Reference circuit
    [​IMG]
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,988
    745
    just use a single op amp, or use one gate of a Lm339.
     
  3. wayneh

    Expert

    Sep 9, 2010
    12,126
    3,048
    In case it's not obvious, the input voltage seen by the comparator cannot exceed it's power supply voltage. So you don't want to feed 14V directly to a comparator supplied 9V. Divide the 14V in half to 7V and you'd be fine.
     
  4. Al Vince

    Thread Starter New Member

    Feb 24, 2015
    27
    1
    Thanks... just was just an example circuit using the LM339... but my project requires up to 14v input.

    It is a device I am designing to be used with an adjustable power supply. The device only works at 14v plus, so I wanted an LED to let the user know they have reached at least 14v, if they do not have a volt readout on their power supply.
     
  5. Al Vince

    Thread Starter New Member

    Feb 24, 2015
    27
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    I found this LM321... is this something that would work?

    LM321-LED.jpg
     
  6. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,988
    745
    yes you can use the Lm321 as a comparator it works from 3v to 32v easy, note it comes in a Sot 23 package.
     
  7. wayneh

    Expert

    Sep 9, 2010
    12,126
    3,048
    The LM321 should be a good choice for your application. The LM393 is widely used because it is widely available, for instance at the local Radio Shack. Those days are nearly gone.
     
  8. Al Vince

    Thread Starter New Member

    Feb 24, 2015
    27
    1
    Thanks for the quick response... could you give me a little help? How would I set this up so that the LED only turns on at 14v?

    I am using this in a simple LM317 adjuster circuit... I want the LED to turn on when the input to the LM317 reaches 14v or above.

    Circuit I want to add the LM321 to...
    [​IMG]
     
  9. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,988
    745
    Do you need a circuit for the comparator, or are you modifying the Lm317?
     
  10. Al Vince

    Thread Starter New Member

    Feb 24, 2015
    27
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    Not making any modifications to the LM317 circuit above except changing the pot to 2k for a max 12 volt output. I imagine I can add the LM321 circuit somewhere before the Vin of the LM317. Sorry I am just a hobbyist so much of this is out of my skillset.
     
  11. wayneh

    Expert

    Sep 9, 2010
    12,126
    3,048
    Where will the LM321 be getting its power, from that same input?
     
  12. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,988
    745
    Here is a comparator for the Lm321, it uses the same supply input as the Lm317, alter the pot Vr1 so the led is on at 14V input.

    comparator.png
     
    Last edited: Mar 3, 2015
    wayneh likes this.
  13. Al Vince

    Thread Starter New Member

    Feb 24, 2015
    27
    1
    Yes... I am making a simple voltage regulator circuit pretty much identical to the design above... it is by itself, no other circuits. I want a simple indicator (LED) when the Vin to the LM317 reaches 14v or above...
     
  14. Al Vince

    Thread Starter New Member

    Feb 24, 2015
    27
    1
    Awesome! The fact you took some time to render that is very generous! I will give it a shot!
     
    Dodgydave likes this.
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