Help with a LM3914

Discussion in 'The Projects Forum' started by nmc240, Mar 26, 2010.

  1. nmc240

    Thread Starter New Member

    Mar 12, 2010
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    What I need to do is get a circuit that will measure from .3 to 4.5V and flash when it reaches 4.5V. I built the circuit and it all works except Im not sure how to get the scale I need, any help would be appreciated.

    Also what would I have to do so that V+ could be powered by either a 12V or 24V system.

    Thanks in advance for any help
     
    Last edited: Mar 26, 2010
  2. Audioguru

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    Dec 20, 2007
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    Pins 7 and 8 make an adjustable voltage reference that can make 4.5V and can be divided to get 0.3V.
    Apply the 4.5V to the top of the resistor ladder at pin 6 and apply them 0.3V to the bottom of the resistor ladder at pin 4.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    Try this:

    [​IMG]

    Note that the LED supply voltage needs to be raised; come to think of it, 7v might not be enough. 8V should work OK. I started with a 5v supply and it wasn't enough - I just swapped over to 12v.

    You won't be able to get down to 0.3v without using a negative supply; that's because of how the internal 10k divider is wired. Low end is around 0.4v. It starts flashing right about 4.4v. Decreasing the 1.4k Ohm resistor VERY slightly should bring that up a bit. You could replace the 1.4k Ohm resistor with a 1k Ohm resistor and a 500 Ohm 10-turn trimpot in series to adjust it to flash right at 4.5v.

    Your mileage may vary a bit, as these IC's can have their own individual personalities.

    If you really have to run this thing from a lower voltage, that might be done using the original circuit, and just using a voltage divider for the sig input.
     
  4. nmc240

    Thread Starter New Member

    Mar 12, 2010
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    THanks for the help guys it looks to be workin fine.
     
  5. nmc240

    Thread Starter New Member

    Mar 12, 2010
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    Two more questions about the circuit.

    Where would I have to place a pot to control the brightness?

    What would I have to do to get the circuit to latch at full scale until there was a manual reset?
     
    Last edited: Apr 2, 2010
  6. SgtWookie

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    Jul 17, 2007
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    I just realized that I didn't show all of the changes I did to the original schematic.

    Note that pin 8 is no longer connected to ground; instead it is connected to the junction of the 1.4k and 3.3k resistor:

    [​IMG]

    Brightness is controlled by ILED, which is 12.5/R1. Now, R1 is 1.4k (this is confusing, because R1 is generally connected directly to REF OUT, but they've used the reference designator R1 elsewhere in the schematic)
    Reference Out V = 1.25 *(1+R2/R1)
    ILED = 12.5/R1
    Right now, R1 is 1.4k, and R2 = 3.34k
    So, the reference out V = 1.25*(1+3340/1400) = 4.23V
    and the LED current is 8.9mA

    Unfortunately, if you change the resistance of the 1.4k resistor, you will also change the reference voltage.

    If you want to be able to adjust the LED intensity, you will either need to use a PWM circuit to control the supply to the LEDs, or use a separate voltage regulator to establish the Rhi input.
    Probably have to do something like add on another comparator to add hysteresis to the Rhi input.
     
  7. nmc240

    Thread Starter New Member

    Mar 12, 2010
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    Thanks, SgtWookie I did notice that pin 8 needed to be in the junction of R1 and R2 so that wasn't a problem.

    If I set Rhi with a voltage regulator do I just need to have a pot from 7 to ground and 8 straight to ground for the brightness control?
     
  8. Audioguru

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    Pin 7 and pin 8 make a regulated voltage reference similar to an LM317 adjustable voltage regulator. The current from pin 7 to ground controls the LED brightness.

    Rhi is set by the voltage reference at pin 7 so another voltage regulator is not needed.
     
  9. SgtWookie

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    Jul 17, 2007
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    If you set Rhi using another voltage reference (like using an LM317),
    then you could disconnect pin 6 from pin 7, replace the 3.3k resistor with a piece of wire, change the 1.4k resistor to 1k, and add a 10k pot in series with the replacement 1k resistor.
     
  10. SgtWookie

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    If you want to be able to change the LED brightness, you DO need a separate voltage reference. Otherwise, changing R1 increases the current through R2, which will change Rhi.

    Either that, or you need to use PWM to control the LED brightness.
     
  11. Audioguru

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    Simply make R1 and R2 set the voltage but having high values so the current is low and the LEDs are dim. Then add a rheostat from pin 7 to ground to increase the brightness.
     
  12. SgtWookie

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    Good idea; I hadn't considered that before.
     
  13. nmc240

    Thread Starter New Member

    Mar 12, 2010
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    That is a good idea Audioguru. by doing this i will still change the voltage at Rhi, but only by an insignificant amount if Im thinking about this correctly.

    Again thank you both for your help.
     
  14. nmc240

    Thread Starter New Member

    Mar 12, 2010
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    How large do I need to make these resistors. When I tried it with a 3.3k and 8.6K for R2 and R1 respectively and a 10k pot from pin 7 to ground it was not working and I was unable to control the brightness.

    This should have given me 0-4.5V scale with the low LED current at 3.78mA which would be dim enough for the application I need. Do I have to go lower with the LED current yet and bring it up?

    Thanks
     
  15. Audioguru

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    8.6k ohms is too high for R1 unless you account for the typical voltage error of 0.65V caused by the Adjust pin Current.

    The datasheet shows that the LED current is 3mA when the reference current is 0.2mA and the LED current is 30mA when the reference current is 3.7mA.
     
  16. nmc240

    Thread Starter New Member

    Mar 12, 2010
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    I have been working on this for quite a while now and I cant seem to get a brightness control that does not affect the voltage at Rhi.

    Audioguru I really like the idea you mentioned, but I'm just having problems implementing it. Any help would be greatly appreciated.
     
  17. SgtWookie

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    Jul 17, 2007
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    Go back to the original values that worked.

    Use a PWM circuit to supply the current to the LEDs.
     
  18. nmc240

    Thread Starter New Member

    Mar 12, 2010
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    SgtWookie, I wouldnt mind using PWM but I am not familiar with it and would have no idea how to do it.

    right now I have the Rhi set with a zener and a 1k resistor in series with a 10k pot on pin 7 for the brightness control and it works, I was just hoping that i could eliminate the zener and go at it as Audioguru suggested but I could not get it to work so I guess Im stuck with this.
     
  19. Audioguru

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    Dec 20, 2007
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    Please post the circuit you tried but didn't work then we can fix it.
    Pin 7 and pin 8 are a voltage regulator. You don't need a zener diode.
     
  20. nmc240

    Thread Starter New Member

    Mar 12, 2010
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    Here is the circuit I tried but it did not work.
     
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