Need a simple LED DC voltage indicator circuit

Discussion in 'The Projects Forum' started by doug08, Jan 31, 2011.

  1. doug08

    Thread Starter Member

    Jan 30, 2011
    153
    2
    I need a circuit which will illuminate an LED at any specified DC voltage under 12v. There are circuits out there, (with zeners/resistors...)but they are no good. All those circuits gradually light the LED. I need a circuit that will instantly light the LED once my set voltage is reached. Must be someone who knows a simple circuit to do this.


    Thanks
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    1,728
    "Any specified voltage under 12v" - that's a pretty wide range.

    The indicator circuit would need to be powered independently of the voltage being measured.
    The comparator or opamp's common mode range would need to include ground. An LM324 opamp or LM339 comparator are both available at your local Radio Shack, if you happen to be in the States.

    Do you have any issues with what I've written thus far?

    As far as the voltage you're trying to get an indication on - what is the source impedance, or do you know?
     
  3. doug08

    Thread Starter Member

    Jan 30, 2011
    153
    2
    I made a few battery chargers, some constant current, some not. All I would like to do is add a feature to the device which allows me to see what the voltage is during the charging process(w/o having to hook up a multimeter to see the voltage). When the light comes on, it will be at a set voltage I choose.

    Thanks
     
  4. wayneh

    Expert

    Sep 9, 2010
    12,094
    3,033
    There is, and some of them are here!

    You just need to provide a bit more detail such as lowest/highest voltage set points, available power supply options, acceptable precision (±0.01v, 0.1v, 1v), total current (just an LED?), and anything else that might matter. For instance, have you tried anything yourself yet? What circuits did you look at that don't work for you?
     
  5. wayneh

    Expert

    Sep 9, 2010
    12,094
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    Do you have a regulated supply voltage that is always higher than your desired set-point? If so, you can set up a voltage-divider (resistor and pot in series) to provide your setpoint reference. Then, compare the battery voltage to that reference with a comparator (such as LM339). Put the battery on the inverting input and the reference on the non-inverting, so that when the battery voltage exceeds the reference, the comparator turns "off". In that state it can sink enough current to light an LED. (Hook the comparator output to a MOSFET if you want to power anything else.) You could use the 3 other comparators on the quad LM339 and 3 additional voltage references to give 4 setpoints with just the one LM339 IC, if you want to be elaborate.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    See the attached circuit and simulation.

    D1 is a 5v Zener diode that establishes a fixed voltage reference.
    R1 supplies nominally 10mA current through D1.
    VR1 is a potentiometer that allows you to select any voltage between 5.1v and ground/0v as the reference input.

    The R2/R3 voltage divider network scale down the input voltage to be within the same range as the reference. 12v / (R2+R3)/R3 = 4.857v, which is close enough.

    R4 and R5 limit the current through the LEDs. You may find that one or the other of the LEDs don't completely turn off due to the relatively heavy load for the opamp's output. You could use 2.2k resistors for them instead. The LEDs would not be as bright, but you will still be able to see them.

    Right at the threshold, both LEDs may be lit, or they may flash on and off. You may wish to add a bit of hysteresis to eliminate this flashing. To add hysteresis, connect a 1 MEG resistor from the output to the REF input.

    Note that the supply is 12v DC. If you wish to use a different voltage supply, you will need to re-calculate resistors R1, R4 and R5.
     
  7. doug08

    Thread Starter Member

    Jan 30, 2011
    153
    2
    So far I have tried this circuit with a 9.1v zener instead:
    http://electroschematics.com/993/9volts-battery-indicator/


    It works, but I need it to turn on the led at 9.9v. I replaced one resistor at a time with a potentiometer to adjust the value up and down to get the led to illuminate at a higher voltage...no luck. I also used a 9.1v zener with an led and resistor...another waste of time. The led gradually gets brighter. You don't even need the zener, you can just use a resistor. I'll have to go buy some IC's to put these other circuits together. Thanks guys.
     
  8. wayneh

    Expert

    Sep 9, 2010
    12,094
    3,033
    Sarge's circuit is a little different than what I described but is exactly the same concept; compare battery voltage to a reference voltage. You could eliminate the zener if you already have a voltage reference somewhere else in your circuit.
     
  9. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,535
    It is a pretty common circuit. I've seen it done at Alcatel with 1458's. I was thinking in term of comparators with this one.

    [​IMG]

    Indicator circuit
     
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