# Desperate help needed...How do I power 8 LEDs?

Thread Starter

#### albathistleman

Joined Jan 24, 2021
7
I am building a model car for my daughter's birthday and lighting it with LEDs..
There are 4 white LEDs, 2 Yellow LEDs, and 2 red LEDs. These all have been tested with 3 v dc and light properly. as it stands, Ill need 24 v dc to power them..
Now comes the problem... I have 1 7.4 v purple (Ultraviolet) , and 2 pink. These also are 7.4 v. They do, however, light up with 9v.
My question is this...how can I, and what voltage do I need to power ALL at the same time. (I'm really a NOOB at electronics) so be simple please! Lol!

#### BobTPH

Joined Jun 5, 2013
9,170
No LED has a forward voltage of 7.4V as far as I know. You might have a device that has two in series and has a 7.4V forward voltage.

Please link to the specs of all of your LEDS.

Do not apply a voltage directly to an LED. That is a great way to destroy them.

Bob

#### SamR

Joined Mar 19, 2019
5,060
To begin with... LEDs are not Voltage driven devices but current. Yes, they have a forward breakover voltage that must be met. Which means they need a resistor in series with them to limit their current. Don't know the product specifications for what you have but run of the mill LEDs are typically limited to less than 20mA. So V/R=I and calculate it out with ~10mA as a good target.

EDIT: Take a look here Notes on LEDs (gizmology.net)

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#### djsfantasi

Joined Apr 11, 2010
9,183
First, you won’t need 24V to power the first eight LEDs. You can put them all in parallel and it would look like all you need is 3V.

But you’ll need a little bit more. As SamR pointed out, you’ll need resistors to limit current to the LEDs. So maybe 5V at a minimum. You’ll need to know the safe maximum current they can take to calculate the resistor. To confuse things further, you don’t want the maximum current, but maybe 50% of the maximum current.

Unless the LEDs have a built in resistor. We need to know more about the LEDs you have.

But all of the LEDs in parallel will use the most current from a battery. So put pairs of LEDs with a resistor and use a higher voltage. Don’t use a 9V battery. It dies quickly.

As far as the other LEDs, we need to know more about them.

There’s a lot of unknowns. Posting links to the LEDs you want to use will help.

Thread Starter

#### albathistleman

Joined Jan 24, 2021
7
No LED has a forward voltage of 7.4V as far as I know. You might have a device that has two in series and has a 7.4V forward voltage.

Please link to the specs of all of your LEDS.

Do not apply a voltage directly to an LED. That is a great way to destroy them.

Bob
Thanks so much for the quick reply...Im at my wits end here...
Here is the link to the 7.4 volt modules I purchased..
https://www.racedayquads.com/products/tiny-leds-super-bright-4-6s-choose-your-color
I CAN, however, replace them with the 3mm LEDs used elsewhere in the model, but I still have no idea how to power them without having a dozen 3v CR 2032 batteries in the base...
The model has 4 headlights (3mm LED @ 3v each) 2 marker lights..again 3 v each, 2 tail lights (3mm/3v) and 6 pink 3mm 3v LEDs...

Thread Starter

#### albathistleman

Joined Jan 24, 2021
7
Thanks so much for the quick reply...Im at my wits end here...
Here is the link to the 7.4 volt modules I purchased..
https://www.racedayquads.com/products/tiny-leds-super-bright-4-6s-choose-your-color
I CAN, however, replace them with the 3mm LEDs used elsewhere in the model, but I still have no idea how to power them without having a dozen 3v CR 2032 batteries in the base...
The model has 4 headlights (3mm LED @ 3v each) 2 marker lights..again 3 v each, 2 tail lights (3mm/3v) and 6 pink 3mm 3v LEDs...
The total number of 3 v LEDs will be 14...

#### djsfantasi

Joined Apr 11, 2010
9,183
How about a link to the other leds?

The led modules contain a built-in resistor. So it must be powered by 7.4V. Five AAA batteries could be used to power them. You could try hoking then up to two CR2032 batteries to see if they work on the lower voltage.

You don’t need a dozen CR2032 batteries for the remainder of the leds. If they do have a built-in resistor, the simplest approach is to connect all the positive leads together to the positive side of a CR2032. Then connect all the negative leads together to the negative side of the battery. You probably want a switch in the positive lead.

IF the led modules work on 6V, wire the same wiring technique will work.

You may want to try this on your workbench and sacrifice some batteries, to see if they last long enough for you.

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Thread Starter

#### albathistleman

Joined Jan 24, 2021
7
To begin with... LEDs are not Voltage driven devices but current. Yes, they have a forward breakover voltage that must be met. Which means they need a resistor in series with them to limit their current. Don't know the product specifications for what you have but run of the mill LEDs are typically limited to less than 20mA. So V/R=I and calculate it out with ~10mA as a good target.

EDIT: Take a look here Notes on LEDs (gizmology.net)
Thanks for the help..much appreciated..
So there are (will be) a total of 14 3mm LEDs, with a voltage of 3v each.
Instead of having 14 CR 2032 batteries jammed into the base of the display case, I'm lost to find out how to power them with an outside source (AC adapter or something)
I have decided to eliminate the 7.4 v modules as I've figured they weren't compatible with this particular circuit.

Thread Starter

#### albathistleman

Joined Jan 24, 2021
7
First, you won’t need 24V to power the first eight LEDs. You can put them all in parallel and it would look like all you need is 3V.

But you’ll need a little bit more. As SamR pointed out, you’ll need resistors to limit current to the LEDs. So maybe 5V at a minimum. You’ll need to know the safe maximum current they can take to calculate the resistor. To confuse things further, you don’t want the maximum current, but maybe 50% of the maximum current.

Unless the LEDs have a built in resistor. We need to know more about the LEDs you have.

But all of the LEDs in parallel will use the most current from a battery. So put pairs of LEDs with a resistor and use a higher voltage. Don’t use a 9V battery. It dies quickly.

As far as the other LEDs, we need to know more about them.

There’s a lot of unknowns. Posting links to the LEDs you want to use will help.
Thanks for the help..much appreciated..
So there are (will be) a total of 14 3mm LEDs, with a voltage of 3v each.
Instead of having 14 CR 2032 batteries jammed into the base of the display case, I'm lost to find out how to power them with an outside source (AC adapter or something)
I have decided to eliminate the 7.4 v modules as I've figured they weren't compatible with this particular circuit.

Thread Starter

#### albathistleman

Joined Jan 24, 2021
7
How about a link to the other leds?

The led modules contain a built-in resistor. So it must be powered by 7.4V. Five AAA batteries could be used to power them. You could try hoking then up to two CR2032 batteries to see if they work on the lower voltage.

You don’t need a dozen CR2032 batteries for the remainder of the leds. If they do have a built-in resistor, the simplest approach is to connect all the positive leads together to the positive side of a CR2032. Then connect all the negative leads together to the negative side of the battery. You probably want a switch in the positive lead.

IF the led modules work on 6V, wire the same wiring technique will work.

You may want to try this on your workbench and sacrifice some batteries, to see if they last long enough for you.
The other LEDs being used are 3mm. At this moment, I can't remember where I bought them, but they are bulk, and the package says 2v, but they light nicely on a CR2032.
So would series be better, Or parallel? I also have 100ohm 1/4 watt resistors on hand...

Thread Starter

#### albathistleman

Joined Jan 24, 2021
7
How about a link to the other leds?

The led modules contain a built-in resistor. So it must be powered by 7.4V. Five AAA batteries could be used to power them. You could try hoking then up to two CR2032 batteries to see if they work on the lower voltage.

You don’t need a dozen CR2032 batteries for the remainder of the leds. If they do have a built-in resistor, the simplest approach is to connect all the positive leads together to the positive side of a CR2032. Then connect all the negative leads together to the negative side of the battery. You probably want a switch in the positive lead.

IF the led modules work on 6V, wire the same wiring technique will work.

You may want to try this on your workbench and sacrifice some batteries, to see if they last long enough for you.
The other LEDs being used are 3mm. At this moment, I can't remember where I bought them, but they are bulk, and the package says 2v, but they light nicely on a CR2032.
So would series be better, Or parallel? I also have 100ohm 1/4 watt resistors on hand...

#### djsfantasi

Joined Apr 11, 2010
9,183
Thanks for the help..much appreciated..
So there are (will be) a total of 14 3mm LEDs, with a voltage of 3v each.
Instead of having 14 CR 2032 batteries jammed into the base of the display case, I'm lost to find out how to power them with an outside source (AC adapter or something)
I have decided to eliminate the 7.4 v modules as I've figured they weren't compatible with this particular circuit.
See my last post. Just wire them all in parallel and use a 3V AC Adapter. It has to have enough power for all your leds. With 14 leds (or however many you end up with) and using a typical led current of 15mA, you will need a supply more than 14 x 0.015 or 0.6A 0.21A. A 1A 300mA supply would work.

Woodland Scenics makes an led lighting system for model railroaders. Read about it in my link. It might help you.

EDIT: Corrected incorrect figures from calculator fubar.

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#### djsfantasi

Joined Apr 11, 2010
9,183
The other LEDs being used are 3mm. At this moment, I can't remember where I bought them, but they are bulk, and the package says 2v, but they light nicely on a CR2032.
So would series be better, Or parallel? I also have 100ohm 1/4 watt resistors on hand...
They work with a CR2032 because the battery has an internal resistance.

With an external power supply, you won’t be so lucky.

If you can find a 3V 1A power supply, the 100 ohm resistor may be enough. Wire one in series with an led and apply power. The led should not be excessively bright. Then, wire all of the led and resistor combos in parallel to the power supply.

#### KeepItSimpleStupid

Joined Mar 4, 2014
5,088
If you get out the excavator, the specs of the "7.4 LED" module is here: https://tinysleds.com/collections/all-leds/products/3-6s-led

It's designed to operate at 12-24V. Each LED is 1/2 a Watt. 1.5W total. It's a 300 ohm resistor.

P=V*V/R; 1.5=((3*V)^2)/300

https://www.wolframalpha.com/input/?i=1.5=((3*V)^2)/300;+solve+for+V

The LED voltage = 5*0.7 = 3.5V; This is the voltage the LED will drop at max power.

I=V/R; 18=V1+V2; 18=300*I + 3.5*3

I picked 18, because it's the center of the operating point (24-12)/2+12 = 18

https://www.wolframalpha.com/input/?i=18=300*A+++3.5*3;+solve+for+A

I=1/40 or 25 mA

This is very expected. So, Vr of those LED's is about 3.5V and the designed brightness is 25 mA

math is fun when you don;t have to get bogged down in the Algebra.

So, what that means is, you can use the LEDs individually as long as the voltage is greater than 3.5V and they will draw 25 mA each.

A 9V battery is probably a good choice. R=(9-3.5)/0.025 <= 220
220 is a standard value, I think. 1/2W ought to work without checking the numbers.

So, each LED gets a 220 ohm resistor in series and can operate off of 9V.

9V can also work directly. You just would have to adjust the brightness of your other LEDs.

LED's
they are a current device.
they have a voltage drop that varies with color. red is usually 2.1
there is also a range of voltage drops
they have a typical operating current. used to be 10-20 mA
if you want to normalize the intensities, binning by Vf is the way to go.
The eye has different perceptions of intensity based on color.

You can buy LED lamps that operate on 120 VAC
You can buy 5V LEDs and 12V LEDS and even 24DC/24VAC LED LAMPS.

#### KeepItSimpleStupid

Joined Mar 4, 2014
5,088
Anyway, back to your problem. 14 LEDs that drop around 3V. group by color.
Take a typical laptop supply 18V. 12V supplies are common. 5V at 1A is common,.

2 groups of 5 and 1 of 4 leds.

It depends on the colors. You have to check the resistor wattage necessary,

You really want to group the colors together and use a resistor.

You might have to experiment and add a pad for another resistor so you can vary the intensity a bit.

You have multiple colors, and many leds and the eye has different sensitivities.

So make the design goal 12V and use one of these https://www.superbrightleds.com/mor...lor+LED+Dimmer+Switches&utm_medium=LDP-2A-WHT for variable intensity.

#### KeepItSimpleStupid

Joined Mar 4, 2014
5,088
Comment about coaxial DC Barrel connectors. 5.5/2.1 and 5.5/2.5 are both common parts. There are adapters between each series. Don't assume that the barrel is negative. the 5.5/2.1 and 5.5/2.5 both "FIT" the 5.5/2.1, but only one is correct,

You could get a potentiometer and then figure out how the brightnesses equalize and then get fixed resistors.
As long as you know 10mA, 15mA and 20mA are perceived the same intensity, scale everything by current accordingly.

I hope I didn;t make things too complicated for you?

#### BobTPH

Joined Jun 5, 2013
9,170
I am going to assume you you have “standard” generic LEDs that are rated at 20mA.

The White ones will be about 3V and the colored ones 2V at that current. You can put them all in series with a single resistor and run them off your 24V supply.

The total voltage will be:

4 x 3 + 4 x 2 = 20V.

Let’s initially design for 10mA. You need a resistor that will drop 4V at 10 mA. This gives you the equation :

V = I R

4 = 0.01 R

R = 400.

Bob

#### KeepItSimpleStupid

Joined Mar 4, 2014
5,088
@BobTPH True, but likely the LED will appear at different brightness based on color and Vf.

#### BobaMosfet

Joined Jul 1, 2009
2,119
I am building a model car for my daughter's birthday and lighting it with LEDs..
There are 4 white LEDs, 2 Yellow LEDs, and 2 red LEDs. These all have been tested with 3 v dc and light properly. as it stands, Ill need 24 v dc to power them..
Now comes the problem... I have 1 7.4 v purple (Ultraviolet) , and 2 pink. These also are 7.4 v. They do, however, light up with 9v.
My question is this...how can I, and what voltage do I need to power ALL at the same time. (I'm really a NOOB at electronics) so be simple please! Lol!
Title: Understanding Basic Electronics, 1st Ed.
Publisher: The American Radio Relay League
ISBN: 0-87259-398-3

You need resistors and can power the LEDs in parallel with one another, but in series with specific resistors for each type of LED to provide the necessary current and voltage for each. Let's simply assume that 10mA is sufficient for each LED (should be). And your voltage source is 9V.

The White, Yellow, and Red LEDs:

R = E/I ; Ohm's Law to calculate resistors needed:
R = (9-3)/0.010
R = 600 Ohm ; You need eight 600-Ohm 1/8th Watt resistors

A Purple LED should only use perhaps 3.4V. Here is a datasheet of one as an example:

https://cdn-shop.adafruit.com/datasheets/1793datasheet.pdf

R = E/I
R = (9-3.4)/0.010
R = 560 Ohm ; You need one 560-Ohm 1/8th Watt resistor

Likely the same with the pink LEDs (you can do the math).

Put one associated resistor with each LED in series. Then connect each of those 1 LED + 1 Resistor circuits in parallel to your 9V power-supply. Or better yet, a 4.5V power-supply (like 3 AA batteries in series).

Watts Law can be used to determine actual Wattage used.

#### djsfantasi

Joined Apr 11, 2010
9,183
Just an aside to @boba mosfett

I agree with your comment about purple LEDs. If they are standalone. The TS was originally using a module designed for RC control and 7.4V battery packs. In that specific case, the resistor is included in the module. The TS did state he was going to use standalone LEDs instead, as it was easier for him to understand.