# 40 LEDs on 9V - 12V battery

Hi, I want to run 40 LEDs in total in my project, using 9V or 12V battery, without using any boards. LEDs can light up to around 40 - 50% of it's brightness.

LEDs specifications:
Orange/Red - 2V - 2.2V (20mA)
Blue - 3V - 3.2V (20mA)

I can use 28 orange + 6 red that makes 34 LEDs 2V and the remaining 6 must be blue 3V. I was thinking about parallel connections 8x4=32 + 2 remaining LEDs and then add 6 blue ones. If there is some problem with that, I can also separate half of it and make 2 sections to power it by 2x 9V batteries. All 2V LEDs on first battery and 3V LEDs on second battery or just make it half (17x2V + 3x3V) (17x2V + 3x3V). Please help, thank you for any help.

No, you need a current limiting resistor in each series string. Those forward voltage drops are only approximate and will change with age and temperature so you must have control of the current and a resistor is the simplest way.

I tried 4 LEDs with 56 ohm resistor and power it by 9V. Nothing lights up.

Like this, but im not sure if I can add 3V LEDs with a calculated resistor beside it or it will mess up.

Nevris:
I tried 4 LEDs with 56 ohm resistor and power it by 9V. Nothing lights up.

Not really surprising!

I see two problems. These LEDs - which you have not specified - may well require more than 2 V each for one, and I do not know what you mean by a "9V battery". I trust you do not mean a "PP3" or "smoke alarm" battery with all those parallel groups of LEDs.

I specified, all of them as in diagram are from ranges 2 - 2.2V and other that are not there are 3 - 3.2V these are 3mm ultrabright frosted LEDs. Yes I mean 9V battery heavy duty or alkaine haha If thats a problem just to light all of the 40 leds together or separately to bright in half or less than half of it's brightness I can go with 12V power supply.

I tried 4 LEDs with 56 ohm resistor and power it by 9V. Nothing lights up.

That resistor is way too low. The lower the value the worse the regulation effect if the resistor. Calculate the resistor so that it drops the same voltage as an LED. Assuming you want 20mA for the LEDs and 2V for the voltage drop then the resistor should be
2 / 0.02 ohms or 100R.

The fact that nothing lights up is due to either a power supply issue of an error in the wiring / connections.

Start off with just one series led chain and get the resistor right then add more, one at a time in parallel.

If you get nothing from one chain either increase the voltage or reduce the number of LEDs you have in the chain.

Grumpy_Mike:
Assuming you want 20mA for the LEDs and 2V for the voltage drop then the resistor should be 2 / 0.02 ohms or 100R.

Im not sure about these calculations as everyone gives different theory. Using Ohms Law the resistor for the LED is calculated across resistor not LED so R=V/I, not adding LED voltage drop, it's 7/0.02=350 ohms for 1 LED running at max brightness.

I managed to power 15 LEDs with 470 ohm resistor at each LED in parallel. (R calculation per LED: I=V/R, 7/470=0.014 or 0.015A.

Is that fine?

Is that fine?

Yes

Im not sure about these calculations as everyone gives different theory.

No they don’t.
I think you are miss understanding that these are not the same problem.
Ohm’s law is always the same. It applies to all linear devices which means you apply it to resistors not to LEDs.

I was suggesting a way of deciding what size of resistor you needed as a minimum to achieve some sort of constant current regulation. My suggestion that you made the volts drop across the resistor the same as the volts drop across one LED applies to only a string of series LEDs. With a single LED you still calculate the resistor by the voltage drop you want to loose after the forward drop of the LED is considered.

In effect these are two different questions and so have two different answers.

Ah okey. I tried to put 2 LEDs in one string but no matter what resistor I put there, nothing happens. I think I will stick with what I have, upgrade power supply to 12V and recalculate resistor values.

tried to put 2 LEDs in one string but no matter what resistor I put there, nothing happens.

Then you have something wrong. If it is all connected correctly and you have not blown your LEDs then you should get something with a 470R resistor. Is your battery flat? What voltage do you measure out of your battery when you try and power these two LEDs?

I just measured voltage now and it have around 3V that's why.

I have one more question. If I wanted to add WeMos D1 board to it and power it by using old power supply Switch Mode Power Supply (SMPS) 12V (DC) - 2A or AC Adapter 19V (DC) - 4.75A? The WeMos jack input power range is 7-24 or 9-24V.

It would appear either of those would do for - specifically - the WeMOS D1 board which has the same configuration as the Arduino Duemilanove or UNO.

Not to be confused with the WeMOS D1 Mini which is more practical but operates on no more than 5 V.

Paul__B:
It would appear either of those would do for - specifically - the WeMOS D1 board which has the same configuration as the Arduino Duemilanove or UNO.

Ok, thanks

Paul__B:
Not to be confused with the WeMOS D1 Mini which is more practical but operates on no more than 5 V.

Yeah, I did check that just to be sure

Ok so I added WeMos D1 R1 to my project, connected 28 orange LEDs to one pin with a TP2222 NPN transistor. For that transistor I have connected 6 strings of LEDs, from string 1 to 5, 5 LEDs each, the last string 3 LEDs. Im going to use another NPN transistor C2001 on a second pin to run 6, 3V LEDs. 2 strings for 3 LEDs.
Im using WeMos because I wanted to program these 28 orange LEDs and the rest 12 just going high.
My setup is powered with 12V and Vin-GND connecting to my breadboard. Is that setup fine?

Is that setup fine?

You make no mention of a current limiting resistor in each series strip.

Grumpy_Mike:
You make no mention of a current limiting resistor in each series strip.

For 5 orange strings, 10k on each, 6th orange string for 3 LEDs 15k and for blue ones ill see later.

For 5 orange strings, 10k on each, 6th orange string for 3 LEDs 15k and for blue ones ill see later.

Those sound way too high, what current do you want the LEDs to work at, and what voltage are you working with?

Grumpy_Mike:
Those sound way too high, what current do you want the LEDs to work at, and what voltage are you working with?

I tested it and it works as I want, very dim lighting at 0.2mA, working on 12V and as I checked with multimeter +-11.90V.

it works as I want, very dim lighting at 0.2mA,

Would have been good to know that at the start.

If that is what you want then fine, it is OK.