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Low voltage indicator

Discussion in 'Electronic Basics' started by Fredrik Jonsson, Oct 14, 2004.

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  1. Hi,

    I am building a small circuit that will have a supply voltage of 9V from
    a battery. I would like to detect when the battery is wearing out, using
    a led that lights up when the voltage passes under a certain level.
    Does anybody have an idea for a simple circuit that will achieve this?

    /Fredrik
     
  2. CFoley1064

    CFoley1064 Guest

    Subject: Low voltage indicator
    Hi, Frederik. Look at the National Semiconductor LM10, a low power op amp with
    built-in reference. If you look at the circuit titled "Battery Threshold
    Indicator" in the upper right hand corner of p. 14 of the datasheet, you'll
    have exactly what you need, as long as your threshold is 6V. You'll have to
    tweak resistor values a little for higher voltage, but the circuit will work
    as-is with a 9V battery. It uses less tha 0.25mA when V is above Vth, and a
    little over 5mA when the LED is on (use a high-intensity LED).

    http://www.national.com/pf/LM/LM10.html#datasheet

    Good luck
    Chris
     
  3. As another approach to the dedicated IC route suggested by Chris, you
    could try this one: http://www.4qdtec.com/batty.html
     
  4. Jack

    Jack Guest

    Hi, Frederik. Look at the National Semiconductor LM10, a low power op amp with
    Hi Chris,

    Is measuring voltage across a battery, a good parameter to judge
    wearing of batter?

    I having about 5years old ni-cd aa battery, charge it up they will
    gain enough voltage but can't supply any more current once you put a
    load.

    I don't know which type to wearing did op referred to, is that standby
    wearing or load wearing?

    Regards
     
  5. In the case of alkaline 9V (which is probably what this OP was asking about), it
    probably makes sense. Alkaline's have a drooping V as they discharge and a 9V
    battery stacks six of these, so the slope is fairly pronounced. So, I'd guess a
    voltage threshold would be reasonable, in this case.
    Nicad discharge curves are flatter, I believe. Their voltage drops very rapidly
    right at first to about 1.2V per cell and then stays close to that until near
    dead. A voltage threshold would probably be less useful here. There are a
    number of Coulomb-counting chips now that try and keep a kind of "fuel gauge"
    for Nicad and NiMH, but in the case of a 9V __alkaline__ this probably isn't
    needed.

    So I imagine, anyway.

    Jon
     
  6. Jack// ani

    Jack// ani Guest

    Yep, i observed, its very apparent.
     
  7. You probably need to specify the battery type. Some battery types are easier
    than others to monitor.

    You also don't specify how much current draw for the circuit can be tolerated.
    A lot of this will depend on just how long you expect to be using the battery,
    whether or not you expect this detection circuit to operate all the time or just
    when your own small circuit is running, how much current draw your circuit takes
    and how often it's running -- questions like that.

    One problem is that when your battery gets very, very low, it won't have enough
    voltage to keep the detection circuit operating as designed. So while it may
    work okay when the battery gets lower than some threshold voltage, there will be
    yet another point below which it will cease to operate altogether.

    Another issue is that running an LED continuously when the battery is low is
    probably a poorer choice, as it takes away the maximum possible energy from the
    battery at a time when the battery is at its weakest. A possible solution to
    this is to blink the LED periodically. This reduces the average consumption at
    this more difficult time. Pulsing the LED may reduce the average by one or two
    orders -- which can be helpful.

    And, of course, there is the issue of simple vs complex and how to trade off all
    these issues. You might look at Maxim's voltage monitor ICs -- some draw only a
    few microamps (thinking perhaps of the MAX6459.)

    Jon
     
  8. Rubicon

    Rubicon Guest

    Hello,

    I've been following this post and I have a question regarding low
    voltage indicator circuits.

    How could you "buffer" the indicator circuit to take any sudden but
    temporary drops in voltage from the battery powered circuit it's
    monitoring into account? A circuit that occasionally activates a motor
    or solenoid for example.

    I'm thinking of using a low voltage detecting circuit to trigger a 555
    to run a 3VDC motor for approx 15 seconds. It's also remotely
    triggered and once the motor starts turning I think that it'll just
    keep on being re-triggered by a low voltage circuit until the battery
    dies.

    The battery was to be a 9V alkaline but I read that they actually
    supply only 50mA before a dramatic drop in voltage occurs. True or not
    I'm not sure.

    Any help appreciated,

    Andrew.
     
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