Tri-colour voltage indicator circuit wanted

Discussion in 'The Projects Forum' started by clockworks, Aug 24, 2009.

  1. clockworks

    Thread Starter New Member

    Aug 24, 2009
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    I'm looking for a simple DIY circuit to show the battery voltage in a radio control transmitter. It uses 4 AA cells.
    I'd prefer to use a single tri-colour LED, and would like the LED to show green above 4.8v, amber between 4.8 and 4.4v, then red below 4.3v.

    I've seen circuits for LED "voltmeters", but they normally need a 9v supply. This needs to run off the battery that it's monitoring (i.e, down to 4v).

    Anyone got a suitable circuit?

    Thanks
     
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    It looks like a job for two volatage comparators.
    The red led must be on below 4.8
    and the green led must be on above 4.4.

    On the page of posistive feedback the comparators are shown.
    http://www.allaboutcircuits.com/vol_3/chpt_8/12.html

    A little question: the tri-color led is it common-cathode or common anode?
    What are the Vf's of the leds @ wanted current?

    Greetings,
    Bertus
     
    Last edited: Aug 24, 2009
  3. clockworks

    Thread Starter New Member

    Aug 24, 2009
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    0
    I will buy the LED to suit the circuit, but let's assume that the LED is common cathode. Red is 30mA at 2v, green is 25mA at 2.2v.

    I have no experience of designing electronic circuits. I can build stuff by following instructions. My knowledge of semiconductors is limited to high school physics, 35 years ago.

    Thanks

    Steve
     
  4. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Let me know the led data when you have it and I will look for a circuit.

    Greetings,
    Bertus
     
  5. jj_alukkas

    Well-Known Member

    Jan 8, 2009
    751
    5
    Assumption with CC and CA devices will result is lots of problems. First have the device, then try for the design.
     
  6. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Here is a circuit that I think does as you described. Green[G] above 4.8V,amber[A] between 4.4 & 4.8V, empty hole between 4.3 & 4.4, red below 4.3V. Possibly could feed AND H to both R & A to make something like orange? The V divider is just symbolic, and would be made up of a resistor string or 4 sep. pots between gnd. &+V.
     
  7. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    This is about what the V divider would look like.Note that A & B comparators should use the same tap on divider. And you wanted it simple, sorry.
     
  8. clockworks

    Thread Starter New Member

    Aug 24, 2009
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    OK, here's the specs of the LED I'll be using:

    colour IF max VF max VF typ VR max
    Red 30mA 2.0 2.4 5.0
    Green 30mA 3.4 4.0 5.0

    It's common cathode.

    Thanks
    Steve
     
  9. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    What do you get when red & green are mixed?, expect not verry pretty but may give a third "color". Here's my attempt to fit your specs.
     
  10. Søren

    Senior Member

    Sep 2, 2006
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    28
    Hi,

    This is the basic circuit you need, but to measure it's own supply down to 4.4V, you might need another op-amp and the input and reference voltage needs to be scaled down, but that is just dividing down - you could use eg. a red LED for the reference voltage, depending on how precise you need it?
    [​IMG]
     
  11. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Sorry for the delay.
    @ soeren, the LM358 still can be used.
    I completed your schematic.
    Thanks for the boost.

    [​IMG]

    See the datasheets for more info.

    Greetings,
    Bertus
     
  12. Søren

    Senior Member

    Sep 2, 2006
    472
    28
    Hi,

    Seems I forgot to post the final circuit, but here it is:
    Battery Voltage Detector

    Bertus <- Consider that it is going on a battery, so it needs to be as lean as possible current-vise.
     
  13. clockworks

    Thread Starter New Member

    Aug 24, 2009
    4
    0
    Thanks for the schematics. I'll order some parts and give them a try.

    Just one question - Soeren: what's the value of C2? Schematic shows 1..10nF.
     
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