Circuit to detect how many batteries I have

Discussion in 'The Projects Forum' started by summersab, Nov 8, 2012.

  1. summersab

    Thread Starter Active Member

    Apr 8, 2010
    132
    0
    Alright, so, my background is more mechanical engineering and computer science in nature, and it's been quite a long time since I've done anything EE related. So, know that I know the basics, but beyond that, I'm pretty rough, here.

    I guessthat the easiest way to do this is to write out the logic in pseudocode (that's the CS major in me). Suppose I have a bunch of AA batteries, and based on how many there are, I want to direct the output to a different circuit. Assuming a AA battery is 1.5V:

    if (Vin <= 1.5V)
    Vin >> circuit1
    if (1.5V < Vin <= 3V)
    Vin >> circuit2
    if (3V < Vin <= 4.5V)
    Vin >> circuit3

    And so on. So, if I put in one battery, the voltage is directed to a circuit that, say, lights a bulb. If I put in two batteries, a second bulb (and only the second bulb) lights up. If I put in three batteries, a third bulb (and only the third bulb) lights.

    Assume I don't know the resistance of the circuits; all I know is Vin, and I want to switch between circuits based on these thresholds. Is this possible?
     
  2. tshuck

    Well-Known Member

    Oct 18, 2012
    3,531
    675
    Using some relays/transistors could get this going, though you will run into a problem in that a battery's voltage will not always be 1.5V, effectively hosing anything you do...

    You could, run a supervisory circuit, which would use fixed voltage references and enable/disable devices according to voltage ranges that would coincide with the ranges you'd expect from these batteries...
     
  3. JohnInTX

    Moderator

    Jun 26, 2012
    2,347
    1,029
    This will do it

    LM3914 LED Bar/Graph generator. Scale the input with a voltage divider.. Use the DOT mode.
     
    summersab likes this.
  4. summersab

    Thread Starter Active Member

    Apr 8, 2010
    132
    0
    Beautiful!
     
  5. wayneh

    Expert

    Sep 9, 2010
    12,145
    3,055
    It is beautiful, but won't save you from the inherent problem of trying to count batteries based on their combined voltage. 4 cells run down to 1.2V each will still look a lot like 3 fresh cells at 1.5V each. If all you really care about IS the combined voltage, then you're good to go.

    You could also just dedicate a cheap multimeter to the chore. They're cheaper than the parts you'll need to build your LM3914 circuit. No pretty lights though.
     
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