Why am I seeing noise on my LM3914 circuit only when all LEDs are OFF

[mactonight]

Hi everyone,
I'm working on a basic LED bar graph using an LM3914 and a potentiometer as the input voltage source. The idea is to light up more LEDs as I increase the pot, and turn them off as I decrease it.

Here’s the weird part:
When I turn the potentiometer all the way down (so that all LEDs are OFF), I see a lot of noise on my oscilloscope at the LM3914 input pin (pin 5).
But when I turn the pot up and light up some or all LEDs, the noise completely disappears.

I double-checked with a multimeter, and the input voltage when the pot is at minimum is exactly 0.000V. So the oscilloscope seems to be picking up something that’s not really there.

Things I've already tried:

Added a 100nF ceramic capacitor from the pot wiper to GND

Changed the pot from 1k to 50k → gave more control

Added bypass capacitors near LM3914 Vcc and GND

Shortened oscilloscope probe ground wire

Used a pull-down resistor on the input

Still, I get noise only when input = 0V and all LEDs are OFF.

My conclusion so far:
It’s probably due to the oscilloscope itself (cheap Chinese model) introducing or picking up noise when there's no activity on the line.

Any thoughts? Is this a common issue? How would you deal with this, especially when trying to prove to a teacher or supervisor that the circuit is fine?

[jim-p]

If you take a about 1 foot long piece of wire and connect it between oscope input and ground, do you seee noise?

Is everything built on a solderless breadboard?

[mactonight]

i did what you said and there is no difference.And yes everything has built on a solderless
breadboard

[TomGeorge]

Hi,

Now is the time to ask for some images of your project.

Can you please post a copy of your circuit, a picture of a hand drawn circuit in jpg, png?
Hand drawn and photographed is perfectly acceptable.
Please include ALL hardware, power supplies, component names and pin labels.

Thanks... Tom....

[jim-p]

@mactonight

i did what you said and there is no difference.

Then obviously it is noise pickup by your scope.
Does the scope auto scale the input?

[gilshultz]

Without an annotated schematic, I can only take a SWAG and say the issue is likely with your SMPS (Switch Mode Power Supply) not having enough load. Check the input power with an oscilloscope (output of power supply) I suspect it will be present. Try adding a load to the output; that may resolve the issue.