Simple low battery indicator

Discussion in 'The Projects Forum' started by pityocamptes, Nov 19, 2012.

  1. pityocamptes

    Thread Starter Member

    Jul 25, 2012
    75
    0
    I've been searching for a few days now and can't seem to find anything that may work. I need a low battery indicator (that draws very little power) for a circuit I am using. The battery bank I expect to peak out around 18 volts or there abouts, and eventually decrease to about 6 volts before my peripheral devices crap out. So, does anyone have any idea how to make a low cost, simple low battery monitor that can be powered from unregulated source that will swing between a minimum of 6 volts to a max of around 18 volts?

    I would like it to light a red led when source voltage drops below around 8 or 9 volts, so I guess something adjustable would be great (or how to calculate voltage sets). Also, I would like a green led to be a part of the circuit for anything above 8 or 9 volts but not lit 100% of the time, maybe a pushbutton could be incorporated to at least show that it is still above 8 or 9 volts as the led constantly on will only pull power.

    Anyone with a schematic? Thanks for the help.
     
  2. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    An LM10 has an opamp that can be used as a comparator and has a precision adjustable voltage reference.
    Its minimum supply voltage is 1.1V and its maximum is 40V. Its current is only 270uA.
    It is in an 8-pins DIL case.
     
  3. pityocamptes

    Thread Starter Member

    Jul 25, 2012
    75
    0
    Would you by any chance have a schematic? Thanks.
     
  4. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Here is the schematic for an LM10 to make its output go low when the battery voltage drops to 6V or less:
     
  5. KrisBlueNZ

    Member

    Oct 17, 2012
    111
    14
    Do you have a figure for maximum current consumption with no LEDs lit?

    Search Digikey or Mouser for comparators with a built-in reference. There are many low-power options from companies like Maxim and Linear Technology. There may be a problem with that high maximum supply voltage; you may need a low-current regulator (again search on Digikey or Mouser). It's a tradeoff between simplicity / low cost, and low current.

    Hewlett-Packard (now Avago) make some high-efficiency red LEDs - HLMP-D150 and HLMP-K150 I think. These glow quite brightly at only 1 mA. Their green ones aren't as efficient though.
     
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