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Simple Battery Monitor Circuit Required

Discussion in 'Electronic Basics' started by Nigel Heather, Jan 19, 2005.

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  1. I'm trying to think up a very simple circuit which will monitor battery
    level. Requirements are

    Vbatt > Threshold --- LED is off
    Vbatt < Threshold --- LED is on
    Vbatt = Threshold --- Not fussed LED could be on or off
    Accuracy of t=Threshold is not too important

    Here's the catch. I'm looking for a very low component count - I'm think a
    transitor, zener, LED and one or two resistors.

    I can think of circuits using comparators, ICs, multi-stage transistors but
    I feel there is a clever circuit that I'm missing.

    Clever ideas would be most welcome.


  2. Sorry didn't think it mattered that much and I didn't want to constrain the
    creative juices - around the 6 - 6.4V mark. Nominal around 7.4V.
    Undepleted O/C voltage around 8.2V. Can you guess what it is?


  3. The circuit depends very much on the specific threshold voltage you
    have in mind but haven't specified.
  4. Not interested in guessing. But to put my original point into
    perspective, how would the general concept you have in mind vary if
    the battery was a 1.2 volt cell or a 300 volt UPS battery?

    Now, the next question: How concerned are you with power consumption,
    both when the LED is on and when it is off?
  5. full name

    full name Guest

  6. smpowell

    smpowell Guest

    I suggest looking at a 1381 Voltage Trigger.
  7. Clarence_A

    Clarence_A Guest

  8. Not so clever, but here is a circuit that may work:
    (view in courier font, or other monospaced font)

    Vin ---------o----------------.
    | |
    Q1 |------. Q2 |------|___|-----.
    /| | /| 100k |
    | | | |
    | | | |
    | '-------o |
    LED V | |
    - .-. 6.2V -
    | 10k | | Zener ^
    .------o | | |
    | | '-' |
    | .-. | |
    | 1k | | | |
    | | | | |
    | '-' | |
    | | | |
    | |-+ 2N5460 | |
    '---<| | |
    |-+ P-JFET | |
    | | |
    GND ---------o----------------o------------------'

    (created by AACircuit v1.28 beta 10/06/04

    If you omit the JFET, the LED will get brighter as the voltage increases
    until it turns off. With the JFET, the brightness stays fairly constant
    for a few volts until it turns off. The JFET forms a simple constant
    current sink.

    That's an N-JFET in the picture. You can also use a P-JFET; just put the
    resistor below it, and connect the gate to below the resistor. You may
    have to experiment with the value of the resistor, depending on the JFET.

    If you wanted to get bold, you could use the beta of Q1 to limit the
    current through the LED, and omit the JFET and collector resistor of Q1
    altogether. Just increase the 10k resistor to 220k. Again, it'll
    brighten as the voltage comes up, until it turns off.

    For a battery monitor, however, you may want to consider a flashing
    circuit, since this one will drain off 2mA at right before the LED turns
    off, and thus decrease battery life. A little flashing 555 circuit,
    along with a uPower comparator and voltage reference, might be a better
    option; it would be more noticable, and probably consume far less power.

    Robert Monsen

    "Your Highness, I have no need of this hypothesis."
    - Pierre Laplace (1749-1827), to Napoleon,
    on why his works on celestial mechanics make no mention of God.
  9. jsmith

    jsmith Guest

    Geese, what a lot of irrelevant questions!!!
  10. John Fields

    John Fields Guest

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