A Little Help with a LM3914 situation

Discussion in 'General Electronics Chat' started by Mudcat, Feb 21, 2007.

  1. Mudcat

    Thread Starter New Member

    Feb 21, 2007
    Newbie here with a question on developing a flashing circuit based on the LM3914 chip.

    The NS datasheet includes a flashing circuit using a single LM3914 chip. I'm building a circuit using three LM3914 chips to drive 3 x 10 LED bar graphs (30 LEDs in bar mode - I have a jones for light bars). Anyway I want them (all 30 LEDs) to flash when the signal voltage exceeds 10 VDC. The single chip method taps into pin 18 then through a cap and resistor to the pin 8 junction with resistor after pins 6 & 7. This did not work with all three chips. Any hints/help/snide comments?

  2. beenthere

    Retired Moderator

    Apr 20, 2004
    If you just want all the LED's to go at 10 volts, a single compatator is all you need.
  3. Mudcat

    Thread Starter New Member

    Feb 21, 2007
    please be more specific? Where would I put the comparator, and what comparator? I've attached both the National Semiconductor drawing and a drawing of my circuit for any help.

  4. kubeek

    AAC Fanatic!

    Sep 20, 2005
    How are the three circuits connected together?
  5. nomurphy

    AAC Fanatic!

    Aug 8, 2005
    No guarantees, but see the attached.

    The idea, with a single unit, is that when the top LED comes on, its cathode goes low and pulls-down on the HI reference which causes all the leds to turn on. The cap charges and discharges accordingly and the top LED cathode alternates between Hi/Lo.

    The problem could be the daisy-chain of the HI ref into the LO ref of the next unit, which means the added circuit will be pulling down on both references and it won't flash.