Multiple LM3914 Bar graph

Discussion in 'The Projects Forum' started by FADM Stern GNSF, Mar 17, 2011.

  1. FADM Stern GNSF

    Thread Starter New Member

    Mar 17, 2011
    I am working on a couple of projects, both using basically the same circuit "premis". I want to make a large bar graph (using 6 to 8 LM3814's) and am having problems understanding a few points. Here are the questions I have, and hopefully someone can explain it to me so I fully understand before I cast my foil patterns.

    1 - I will be using 6 LM3914's to drive 60 LEDS to make a round guage (tachometer and speedometer). I have gone over the data sheets but have a hard time understanding the resistor values used at REFout (pin 7).

    In all the sample project schematics I have seen, they ground pin 8 on the first LM3014 and join pin 6 and 7 to the next LM3914's pin 8 and 4. They then place a resistor (ei 1K) to gnd. Now, this interconnection continues with the next LM3914's, but at each successive one the resistance basically doubles. My question is why? and is there some formula somewhere for determining the resistance values.

    2 - While a different issue, it does involve the same pin 7, specifically for LED current setting. What I need to be able to do is control (variable) the LED current to act as a dimmer (just like a car dashboard brightness knob). I can not for the life of me figure out how to amke ONE control (be it pot or OPAMP) adjust the current of all 6 LM3914's.

    I used to do a lot of electronic design work many years ago, but have not really done anything in the last 20 years, so while I know the basics I am really rusty (yea, used to build with tubes lmao). Most of my electronics were around the standard TTL and CMOS chips, plus some microcontrolers (68705 and 6800 series processors).

    Any help would be greatly appreciated. Thank you all