6v and 12v Low Voltage Alarm

Discussion in 'The Projects Forum' started by jj_alukkas, Jun 30, 2009.

  1. jj_alukkas

    Thread Starter Well-Known Member

    Jan 8, 2009
    751
    5
    Hi everyone,
    I need a simple circuit which runs a buzzer when voltage falls below a specific value. I need 2 ckts to monitor batteries of 6v and 12v.. Could somebody help me for a simple solution using zenners and transistors?
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,647
    2,346
    Hello,

    Are the circuits to be on separate units?
    You can use a comparator with a reference on one pin and the battery voltage on the other pin.
    like the circuits halfway down this page of the eBook:
    http://www.allaboutcircuits.com/vol_3/chpt_8/12.html

    Greetings,
    Bertus
     
  3. bertus

    Administrator

    Apr 5, 2008
    15,647
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  4. jj_alukkas

    Thread Starter Well-Known Member

    Jan 8, 2009
    751
    5
    Yes, the circuits are seperate.. a 6v tester for a 6v battery.. And thanks for that above link.. i hope it should do the job.. But I have a doubt.. Can I monitor other voltages by replacing that zenner?? also what should I modify for a more precise value, say 6.2v??
     
  5. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    You can replace the zener with a 4.7 Volts zener.
    This way you can adjust the voltage from 4.7 Volts and up.

    Greetings,
    Bertus
     
  6. jj_alukkas

    Thread Starter Well-Known Member

    Jan 8, 2009
    751
    5
    Well, I breadboarded this circuit

    [​IMG]

    But the problem is, pin 6 is always high for any voltage no matter what value the 100k preset is set to. I used a 5v6 zenner diode. What could be the problem?? Also how can I confirm the value of a zenner? Would connecting it in parallel and in reverse biased with an RPS and voltmeter show the cutoff point??
     
  7. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    What is the IC that you are using?
    If it is an LM339, you have the wrong pins connected.
    If it is an LM741, you are using the wrong IC; you should be using a comparator instead of an operational amplifier.

    Another possible problem is low Zener current. Zeners need to have a certain range of current flowing through them in order to operate in their linear region; usually somewhere between 5mA and 30mA - but that varies, depending upon the particular Zener.

    You can actually use an LED instead of a Zener. Not quite as accurate, but all you're looking for is something reasonably stable anyway.
     
    Last edited: Jul 3, 2009
  8. jj_alukkas

    Thread Starter Well-Known Member

    Jan 8, 2009
    751
    5
    Ohh Sorry that I didnt specify the IC .. I searched and found this online
    [​IMG]

    So when i posted it, searched again but found another one similar but didnt look deep ..

    The site claimed this circuit to work with 741.. So can you help me with a simple circuit for a low voltage buzzer, one for 6v and another for 12v..???
     
  9. jj_alukkas

    Thread Starter Well-Known Member

    Jan 8, 2009
    751
    5
    The above circuit really does work !! The problem was that the zenner is wrongly drawn.. I reversed it and used a 3v3 zenner as a 6v circuit and it works fine after some adjustment on the preset.
     
  10. bertus

    Administrator

    Apr 5, 2008
    15,647
    2,346
    Hello,

    If you want more accuracy you can use a TL431 in place of the zener.
    The TL431 is a 2.5 Volts voltage reference. (see datasheet).

    Greetings,
    Bertus
     
  11. jj_alukkas

    Thread Starter Well-Known Member

    Jan 8, 2009
    751
    5
    Thanks for that info Bertus. I will use it for precision applications.
     
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