A potentiometer in between 9 leds and common ground

So my connection is 9 leds (3.2V, 25mA) from 9 pins. And instead of individual resistors, I am Thinking to adding one potentiometer in between common ground and cathode. Will that protect all leds from burning out?
Will that be enough for all?
Pls tell what you think guys…

You need a resistor for each LED.

groundFungus:
You need a resistor for each LED.

So how much ohms do you think is required?
AND ALSO, after doing all this
If i add a potentiometer like in my case, could you adjust the equivalent brightness???

So my connection is 9 leds (3.2V, 25mA) from 9 pins.

You won’t be lighting all the LEDs at the same time.

TheMemberFormerlyKnownAsAWOL:
You won't be lighting all the LEDs at the same time.

Not all the time. But yes in certain instances.

To use the pot to adjust LED brightness wire the pot to an analog input, measure the pot voltage and use that value to set PWM outputs to the brightness. For 9 LEDs ( one each on 9 pins) you will need an Arduino with 9 PWM pins or an external PWM chip or shield.

groundFungus:
To use the pot to adjust LED brightness wire the pot to an analog input, measure the pot voltage and use that value to set PWM outputs to the brightness. For 9 LEDs ( one each on 9 pins) you will need an Arduino with 9 PWM pins or an external PWM chip or shield.

But instead of that couldnt we adjust the flow between them and therby adjusting brightness?
I mean could that happen?

bensontom98:
Not all the time. But yes in certain instances.

The circumstances where you want to smoke your processor?

TheMemberFormerlyKnownAsAWOL:
The circumstances where you want to smoke your processor?

Ok you weren't asking
So if i plug 9 individual resistors, all light it up at the same time, will i damage the board??

No.

So what do you think should be the ohms of the resistors? Can you give a maximum and a minimum value?

What board are you using? Is it powered from 5V or 3.3V?

How to calculate LED resistor.

LED resistor calculator.

I am using uno. The board is powered through the vin pin.
Idk to calculate
.

Supply voltage of the LED minus the forward voltage of the LED, divided by the current.
Go for the next highest preferred value resistor.

9 * 25mA is 125mA more than the Uno’s processor is rated for.

Will that protect all leds from burning out?

Maybe, depending on the adjustment of the pot. :wink: (The pot should have a series resistor to limit the adjustment range.)

The BIG problem is - The resistor (or pot) controls the total current which gets divided among the LEDs. With 9 LEDs sharing 25mA, each LED will only get 2.8mA. The more LEDs you turn-on the dimmer they will get.

Depending on manufacturing variations the current won't divide exactly-equally so the LEDs may have different brightness.

The bottom line - It's "bad practice" to share a current limiting resistor.

DVDdoug:
Maybe, depending on the adjustment of the pot. :wink: (The pot should have a series resistor to limit the adjustment range.)

The BIG problem is - The resistor (or pot) controls the total current which gets divided among the LEDs. With 9 LEDs sharing 25mA, each LED will only get 2.8mA. The more LEDs you turn-on the dimmer they will get.

Depending on manufacturing variations the current won't divide exactly-equally so the LEDs may have different brightness.

The bottom line - It's "bad practice" to share a current limiting resistor.

Then the 9 led arent sharing 25mA, they are being supplied individually by the I/O pins.
So what about placing 1k ohm resistor behind every leds, will i get some glow?(not expecting bright though)

TheMemberFormerlyKnownAsAWOL:
Supply voltage of the LED minus the forward voltage of the LED, divided by the current.
Go for the next highest preferred value resistor.

9 * 25mA is 125mA more than the Uno's processor is rated for.

Re-calculate and I will agree :smiley:

So what about placing 1k ohm resistor behind every leds, will i get some glow?(not expecting bright though)

What is stopping you from wiring up one LED with a 1K resistor to find out?

Hi
The OP is using the LEDs in this project;

Can you please post a copy of your circuit, in CAD or a picture of a hand drawn circuit in jpg, png?

Tom... :slight_smile:

Another point to bear in mind is that LEDs don't actually dim like an incandescent bulb. That is why the advice has been given to use PWM, it's the only way to get decent co-relation between control operation and brightness.