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LM3914 Problems

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tvsound

New Member
Hi,

I am trying to build a circuit that has 3 cascading LM3914 chips driving 30 leds in dot mode and am encountering the following problem...

When the signal input is above the V-Hi reference level the top led of each chip stays on.

So...even though the "bargraph" does what I want...ie, as the signal voltage drops the leds go from "top" (led 1) to "bottom" (led 30) when the signal voltage is at it's highest (in this instance 1.25V) as well as led No.1 being on (which I want) led No.11 & LED No. 21 are also on.

I can see that if I was running this in "bar" mode it wouldn't be a problem but I really need to run this in "dot" mode.

Any Suggestions???...I am currently copying the circuit out of the NS datasheet onto a breadboard and am fairly sure I am not making any mistakes.

Please feel free to enlighten me...and as I am very dim and a newbie at electronics please feel free to make your answers as basic as you wish!!!!...and if you want to include a circuit diagram that would be great!!!

Kind Regards
TVSound:;)
 

audioguru

Well-Known Member
Most Helpful Member
The datasheet for the LM3914 explains and shows how the pin 1 (drive to the 11th LED) of the second LM3914 is connected to pin 9 of the first LM3914 to shut off LED #10.
The same would be pin 1 of the third LM3914 connected to pin 9 of the second LM3914 to shut off LED #20.

They also show a 20k resistor in parallel with the 9th and in your circuit the 19th LEDs to stop them from glowing faintly when they are supposed to be turned off.
 
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