LM3914 Linked together and really bright!

Discussion in 'The Projects Forum' started by Unforgiven, Feb 16, 2013.

  1. Unforgiven

    Thread Starter New Member

    Feb 15, 2013
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    I have been reading non stop about LM3914's and have some questions. I have read through the datasheet a whole lot and still fail to understand things. My plan is to have 20 LED's in 4 rows so 80 LED's. The LED's will be powered by 12 volts and the range for the bar graph will be 0 to 5V. I am looking for the 4 rows to go up together as the 5 volt signal increases.

    So the things I am looking to do is to make this thing as bright as possible! Can I get some push in the right direction? Basically which particular LED's would be the most beneficial to use? Also, would I just be needing to cascade two LM3914's together and then split to 0-5volt signal to each set of two? Ideas?
     
  2. Unforgiven

    Thread Starter New Member

    Feb 15, 2013
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  3. thatoneguy

    AAC Fanatic!

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    What is your desired outcome?

    A large spectrum analyzer or animation?
     
  4. Unforgiven

    Thread Starter New Member

    Feb 15, 2013
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  5. Unforgiven

    Thread Starter New Member

    Feb 15, 2013
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    also, im unsure of my power voltage. it's going to be around 12 volts so how do i keep the varying of the 12 volts from hurting anything, do i use a special diode to keep everything at 12 volts and if so where would it go?
     
    Last edited: Feb 16, 2013
  6. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Why would the 12 Volts vary?
    Are you going to use it in a car?

    Bertus
     
  7. tubeguy

    Well-Known Member

    Nov 3, 2012
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    How far away ?


    What is your 12vdc supply ?
     
  8. #12

    Expert

    Nov 30, 2010
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    I did this for a church. Trying to get the pastor to speak up so he could be heard. The pastor could see the LEDs at 50 feet, and they were standard, 20 ma LEDs in 1985.
     
  9. Unforgiven

    Thread Starter New Member

    Feb 15, 2013
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    Ya this is for farm equipment. And it's going to be attached to the battery of a tractor, I need to measure what the volts are while it's running still, so i can better calculate. So how do I keep the voltage constant going into the LM3914?

    And the distance could be anything that's why I just need to make it bright as possible? I've been reading a lot and I think I've read almost every LM3914 article. I also figured that I can probably just use 10 LEDs spread apart a bit so that would solve having to cascade them, but what's the most amps the chip can send to an LED? If I get an LED that can handle more amps can the LM3914 send it with right resistor attached to the pin?
     
    Last edited: Feb 17, 2013
  10. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    If you want really bright, you need to use external driver transistors like a 2N3906. They are very cheap. If you want a "go- no go" indicator easy to read set it up so that you have four green LEDs that light when the voltage is in the "good" range, three amber LEDs for the "marginal" voltage and three RED LEDs for when the battery voltage drops too low.

    If you are concerned about regulating the voltage to the 3914, just use an LM317 set to 8V. There is no advantage running th 3914 at higher voltages. The power voltage which runs the LEDs does not need to be regulated.

    I honestly don't understand what you are trying to do here. Is the charging system of the tractor not working and you are running the battery dead?
     
  11. tubeguy

    Well-Known Member

    Nov 3, 2012
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    The TI datasheet states 30ma max current, but your LED's have a 20ma recommended current. So set the current for 20 ma.
    This chip limits current so you could string a few LEDs in series depending on the supply voltage. They would get the same current with about a 2.1 volt drop for each. The total drop can't exceed the supply voltage minus the dropout voltage which appears to be 1.5volts. (not positive on the dropout voltage).

    I agree with bountyhunters suggestions - multicolored LED's.
     
  12. Unforgiven

    Thread Starter New Member

    Feb 15, 2013
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    ok thanks guys. i appreciate the help. the whole project is just for a 0-5volt pressure sensor, the battery and tractor is just going to be where i am getting the power from. so basically as the pressure increases in the hydraulic lines I will be able to see the LED's going up.

    now I am not sure exactly which pressure will be my minimum voltage and which pressure will be my max voltage. so i am just getting a 0-5 volt pressure sensor that will cover the whole range and then want to be able to adjust the minimum and maximum voltage to match the exact pressures i want.

    so this means, i should put a variable resistor on pin 4 (rLo) and a variable resistor on pin 6 (rHi) correct?
     
  13. Unforgiven

    Thread Starter New Member

    Feb 15, 2013
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    i have been simulating this thing all day now on Proteus DEMO and I can't get this thing to work right. i need to be able to adjust the minimum value and the maximum value because i am not sure if the bottom LED needs to be on at 2V or 1.5V or the top LED on at 4V or 4.5V. Ideas? I've tried the first one on the datasheet but I cant get the brightness out of it, so I tried the one for variable voltage range of 4.46 -5.46 (??) and it didnt work right for me either. Anyone got a good one to try?
     
  14. KJ6EAD

    Senior Member

    Apr 30, 2011
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  15. Unforgiven

    Thread Starter New Member

    Feb 15, 2013
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    awesome. i shall read.
     
  16. thatoneguy

    AAC Fanatic!

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    Do you have a link to the pressure sensor? The output may have some odd requirements for conditioning prior to using as a signal.
     
  17. Unforgiven

    Thread Starter New Member

    Feb 15, 2013
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    Ive been looking at these sensors. Not entirely sure which one I am going to go with yet, but I know I am going to need a maximum pressure read of 2500 psi.

    On a more exciting note though, I finally got my simulation to work based off the article KJ6EAD. I am stuggling now though to get 4 LED's to light up per row. So why can't I splice the 0-5 volt signal to 4 LM3914's?

    I attached a picture of my circuit that lets me adjust the low and the high.
     
  18. tubeguy

    Well-Known Member

    Nov 3, 2012
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    Because the chip limits the LED current you really can put more than one LED in series on each output. The limiting factor is that the supply must be a few more volts than the total Vf forward voltage drop of the LED's 4*2.1v = 8.4v plus the internal drop of the IC circuit.
    (And, the supply must not exceed the limits of the chip)

    If you supply the chip with 10-12 volts you should be able to string 3-4 in-series per output
     
  19. Unforgiven

    Thread Starter New Member

    Feb 15, 2013
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    I tried that in the simulator. I did 4 per pin and I tried in series and in parallel and by the time it gets to the top they are very faint. Also the LED closest to chip was the brightest in the row of 4. So basically in a grid 10 rows by 4 columns, the lower left (pin 1) inner LED was brightest and top row (pin 10) outside LED wasn't even lit. Ideas? Just run 10 LEDs per chip and 4 chips? Oh I also need bar mode, so that going to cause problems with wattage overload on chip if I've read correctly. Maybe just do the relay set up for each output and hook however many LEDs to each relay?
     
    Last edited: Feb 18, 2013
  20. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Do you have a link for the sensors?

    The ones I've found for that pressure and rated for machinery use have a supply voltage between 16 and 36V, and an output of 4-20mA current loop, or 0.1-10V Three Wire.
     
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