Simple Battery Monitor

Discussion in 'General Electronics Chat' started by Joster, Jun 26, 2014.

  1. Joster

    Thread Starter Member

    Jun 12, 2013
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    Hi All!

    I want to know when my battery is at 14V. I can use a comparator using a reference voltage to get a change in state on the output when 14V is reached. I need to convert that change in state to a 1 second pulse to drive a mosfet. What circuit do I use? Maybe there is One that will give me a 1 sec pulse when ever there is a change in state from low to high or high to low??

    Thanks!

    Joster
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Use an opamp comparator to detect the voltage, and feed the op amp into a 555 astable to get the pulses.
     
  3. Joster

    Thread Starter Member

    Jun 12, 2013
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    ok thanks for responding!

    would the astable 555 not just keep giving pulses? I only want a pulse if the output of the comparator changes state. So I need a pulse which is activated by either a rising edge or a falling edge...
     
  4. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    The op amp would give a pulse out on detection, if you want to extend it then use a 555 monostable .
     
  5. wayneh

    Expert

    Sep 9, 2010
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    A monostable multivibrator is known as a "one shot".

    The 555 normally triggers on a negative going edge. I'm trying to recall how to get a state-change pulse. Using another 555 is the brute force solution. I think there's a more elegant approach.
     
  6. Joster

    Thread Starter Member

    Jun 12, 2013
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    ok...but the battery is going to hit 14V and keep climbing up until I turn the charger off so the output of the op amp will be high once the battery gets to 14V and then remain high...see what I mean?
     
  7. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    So whats the mosfet for then?
     
  8. Joster

    Thread Starter Member

    Jun 12, 2013
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    I am connecting it to the input of my alarm system. when the input of the alarm system sees a short I get an email that by battery is charged. So really I just need to short the two terminals of the alarm input together... I was thinking of using a relay for that...hense the mosfet or bjt...
     
  9. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    ok so use the comparator to turn off the charger using a relay, as well as triggering the alarm then.
     
  10. Brownout

    Well-Known Member

    Jan 10, 2012
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    If you use the monostable, it will give you a single pulse out.
     
  11. Joster

    Thread Starter Member

    Jun 12, 2013
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    now there is an idea! i think thats the winner!

    I dont think the monostable is edge triggered is it?
     
  12. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    yes its negative edge triggered, take pin 2 to 0v to trigger
     
  13. Brownout

    Well-Known Member

    Jan 10, 2012
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    Last edited: Jun 26, 2014
  14. Joster

    Thread Starter Member

    Jun 12, 2013
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    I need a solid bat ref but a 10V zener across the battery doesn't work like i thought it would...I need a solid vref for the op amp and want to do it from the bat. the bat will be anywhere from 11v to 14v but i need 10v vef. tried two 5v zeners and current limiting resistor...tried without the resistor still no luck...am i going to have to power it off another supply??? hopefully not

    thanks
     
  15. inwo

    Well-Known Member

    Nov 7, 2013
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    What didn't work?
     
  16. Brownout

    Well-Known Member

    Jan 10, 2012
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    Never use a zener without a current limiting resistor.
     
  17. Joster

    Thread Starter Member

    Jun 12, 2013
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    i thought that i could get a solid 10v reference voltage across the battery with the two 5v zeners and a resistor?
     
  18. Brownout

    Well-Known Member

    Jan 10, 2012
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    What did you get? What was the value of the resistor? Why make us guess?
     
  19. Joster

    Thread Starter Member

    Jun 12, 2013
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    the resistor is 6.8k

    voltages measured are in pic
     
  20. Brownout

    Well-Known Member

    Jan 10, 2012
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    Connect the components in this order: Resistor, Zener, Zener. Like this

    V+
    R
    Z
    Z
    gnd.

    Now take the reference voltage between the resistor and the top zener. You might try a lower value resistor too. Try 300-1k
     
    Last edited: Jun 27, 2014
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