help with voltage circuit

Discussion in 'The Projects Forum' started by david1234, Jul 31, 2014.

  1. david1234

    Thread Starter Member

    Nov 27, 2013
    81
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    Hello
    i want to built a circuit that will give me "1" when the voltage is higher then 10.5V and "0" when lower
    this circuit need to be connected to a battery and a digital I\O - so when it does under 10.5V I will know I need to cahnge the battery soon.
    so I only need it to be as small as possible and will use very low current
    I was thinking of using LM741 (will work on 12-10V ?)and connect to him a D flipflop maybe ?
    this is a sketch of what I have in mind
    but how do I make it compare from 10.5V -
    all use the same battery
    Thanks ,
     
  2. alfacliff

    Well-Known Member

    Dec 13, 2013
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    why not a zener diode to determine the voltage? a 10.5 vot zener with a resistor in the ground line. if theres voltage drop across the resistor, thevoltage is above 10.5 volts.
     
  3. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    you need a comparator circuit using an op amp, adjust the Vr1 so the led is on at 10.5V supply
     
  4. Johann

    Senior Member

    Nov 27, 2006
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  5. bertus

    Administrator

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  6. inwo

    Well-Known Member

    Nov 7, 2013
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    How about a voltage divider set to the lower point of a schmitt trigger?
     
  7. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    If I read you correctly you just want a light to come ON when it is time to change the battery? That is very do-able. (If instead you are either turning things off or turning on a charger things get more complicated as the voltage you are reading change in both cases.)

    Basically you need three things: a (single) comparator (with specs to work on a single supply in that 10-11 volt range), a voltage reference of well below the 10V, and some resistors to make a voltage divider.

    The reference is to give you a fixed voltage for comparison no matter what the battery is doing, that is why it is a lower voltage then 10V. The resistive divider is to get a voltage equal to the reference when the battery drops to the 10 volts.

    The comparator compares these two voltages, and then drives the LED. I could sketch this up at another time (no schematic stuff on this machine).
     
  8. pnJunction

    New Member

    Jul 31, 2014
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  9. alfacliff

    Well-Known Member

    Dec 13, 2013
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    comparators are not needed, if you want one, use it. there is a similar circuit used on ower supplies called an overvoltage crowbar consists mainly of a zenerdiode, scr and a few resistors. when the voltage rises to the zeners conduction point, it triggers the scr, shorting the output. similarly, a zener in series with a resistor will conduct if the voltage stays above the zener voltage, giving you a drop across the resistor you can use for your alarm.
     
  10. AlphaDesign888

    New Member

    Jul 27, 2014
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    An LM393 dual comparator is the textbook solution for this problem.

    Why go against the grain?
     
  11. Johann

    Senior Member

    Nov 27, 2006
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    True, however, LM358 configured as a comparitor works well; I've used it in many temperature controllers with great success.
    The LM 393 is the correct choice, theoretically. It is an open-collector output device and when used, should always have the pull-up resistor on its output. This resistor is not needed when you use the LM358 (as comparitor).
     
  12. david1234

    Thread Starter Member

    Nov 27, 2013
    81
    1
    I understand I need to use LM393 with this circuit ,but
    where do I make the calculation about the voltage I want it to go "high"?
    what resistors I need to change in order to compare above and lower then 10.5v?

    and the battery is always connected to it so all the power for the circuit and the compering will do from the same power source -not a problem right?
     
  13. AlphaDesign888

    New Member

    Jul 27, 2014
    189
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    Not really. The LM358 output cannot source to the supply rail or sink to absolute ground.
    You can get away with it in some applications though I guess.
     
  14. AlphaDesign888

    New Member

    Jul 27, 2014
    189
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    OHMs law is your friend. It is all OHMs law that you need to learn.
     
  15. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    Your obviously not listening, you need to make a comparator , not a schmitt trigger, replace the 741 op amp for your op amp , and adjust the Vr2 so the led comes on at 10.5v, the led will go out at higher voltage.
     
  16. AlphaDesign888

    New Member

    Jul 27, 2014
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    Should we give him an schematic on an silver plater?
     
  17. david1234

    Thread Starter Member

    Nov 27, 2013
    81
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    I'm listening
    so you are saying this is a much better\nicer\clean solution ,right?

    I will build this , but why not use the 741?
    what do I need to use?

    Thanks,
     
  18. AlphaDesign888

    New Member

    Jul 27, 2014
    189
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    The LM393 is the professional textbook solution.

    What will the load on the output be?
     
  19. david1234

    Thread Starter Member

    Nov 27, 2013
    81
    1
    I need to connect it to a digital Input of a device
    and this is his spec
    Connection type TTL
    Voltage
    To Trigger (high) Logic Level 0: 0 ~ 0.4 V
    Normal (low) Logic Level 1: 3.3 ~ 30 V
    Current 10mA~100mA
     
  20. AlphaDesign888

    New Member

    Jul 27, 2014
    189
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    High and low are reversed it seems.

    Normal TTL (Transistor Transistor Logic) -- chips are as follows:

    High > 2VDC
    Low < 0.8VDC

    Current drain would be in the range of (I've actually forgotten :D)
    I did study this back in 1997. Long time ago!
     
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