# Questions tagged [led]

An LED is a light-emitting diode. Lighting an LED is considered the "Hello world" of a circuit design, and it can be as simple as putting a series resistor or can get more complicated, involving PWM and multiplexing.

434 questions
61k views

### How can I efficiently drive an LED?

I understand that I can not connect an LED directly to a battery because it will draw too much current. Thus, there must be something else in the circuit to limit the current. What options are there? ...
82k views

### Why exactly can't a single resistor be used for many parallel LEDs?

Why can't you use a single resistor for a number of LEDs in parallel instead of one each?
19k views

### Correct formula for LED current-limiting resistor?

I'm trying to work out what value resistor to use in a LED circuit. The equation I'd use to do this is: $$R = \frac{V_{cc} - V_f}{I_f}$$ Seems logical, and makes complete sense. The answers to the ...
83k views

### Why does a resistor need to be on the anode of an LED?

Please be kind, I am an electronics nub. This is in reference to getting an LED to emit photons. From what I read (Getting Started in Electronics - Forrest Mims III and Make: Electronics) electrons ...
17k views

### Why would one drive LEDs with a common emitter?

I've seen tutorials aimed at beginners suggest the way to drive an LED from something without enough current drive is this: (option A) but why not this: (option B) Option B seems to have some ...
58k views

### How do I calculate the resistor value for a simple LED circuit?

I have a simple circuit: The max rating (current) of the LED is 30mA. How can I work out what the resistance of the resistor needs to be? By using ohm's law I found it to be $3V/0.03A = 100 \Omega$...
13k views

### 6 LEDs in parallel with a single resistor to simplify soldering

I'm trying to wire up 6 RGB LEDs in parallel, all controlled from a single source (well, three sources, one for each colour). The LEDs came supplied with resistors to limit the current of 270 Ohm for ...
289 views

### Can I use blue-green LEDs as MCU state indicators on 3.3 V power?

Modern electronics industry has developed very efficient LEDs with nice and fancy colors (“true green” , “cyan”, and “blue”) based in InGaN technology, or similar. Some of these LEDs are specified as ...
44k views

### How do I model an LED with SPICE?

What diode modifiers are used in practice to model LEDs with SPICE (Berkeley v.3f5)? These are available to me: ...
33k views

### Is a current-limiting resistor required for LEDs if the forward voltage and supply voltage are equal?

For blue LEDs with a forward voltage of 3.3 V and supply voltage of 3.3 V, is a series resistor still needed to limit current? Ohm's Law in this case says 0 Ω, but is this correct in practice? ...
21k views

### Does pulsing an LED at higher current yield greater apparent brightness?

This question implies two assumptions: The frequency of a switched LED driver is high enough that average power, not instantaneous power, should be used to determine maximum drive current. The ...
18k views

### Basic circuit to keep LED either on or off depending on night/day

How can I have a LED either on or off (not in between) depending on night or day with basic electronics? I have created the circuit below, but it only shines and in darkness it dims. The photo-...
41k views

### Calculating resistor value and power rating for LED driving

I have seen many resistances with difference Wattage i.e 1/4W, 1/2 W, 1 W, 2 W ,3W etc. If I have 3 W Load which is an (large 3W) LED in my case, which Resistance should I use? Also The LED I am ...
10k views

### How to drive a 20mA LED From a 4mA max GPIO Pin

I have an IC that has an GPIO with which I would like to drive a LED. Since the device will be running off battery, keeping the power use low (Zero maybe) while the LED is off as a priority. The ...
14k views

### What is the latency of an LED?

LEDs are known to have a very low, unnoticeable power-cycling latency, but how fast are they when measured? (nanoseconds?) In other words, how long does it take for an LED which is entirely off to ...
36k views

### How many individual lights can an arduino control

I want to use a regular Arduino board to control LEDs - what is the most lights I can control individually from the board? Is there a tutorial on how to hook it up?
12k views

### Which SIPO chip is better, 74HC4094 or 74HC595 or something else?

I want to drive lots of LEDs in a spinning persistence-of-vision (POV) device. I need one output pin per LED, because matrix techniques don't work right in spinning POV devices. A serial-input, ...
101k views

### Do I really need resistors when controlling LEDs with Arduino?

I'm just trying out Arduino Uno for the first time with 2 blinking LEDs on a breadboard. All the tutorials on the Internet seem to use a resistor. I do know the function of resistors, but does it ...
17k views

### I'd like to learn how to make my own USB gadgets

I've been writing Linux character device drivers for other people's stuff for some time. I'd like to find a new hobby and the thought of making my own USB gizmos seems really neat. I would be ...
71k views

### Does LED brightness change with voltage?

When I was young and learning about electricity, a fabulous tool for understanding voltage/current/resistance was an incandescent lightbulb (in my case it was a small 3V bulb). When you doubled the ...
55k views

### accounting for LED resistance

I'm doing a simple lab (I'm a hobby EE) to reinforce my ohm's law math and learn a little about how to do proper measurements with a multimeter. I have simple circuit with a 2.2k ohm resistor ...
7k views

### Why are the three component LEDs in an RGB LED so unbalanced?

I was recently spec-ing some RGB LEDs for a project, when I noticed that the millicandela ratings on the three colors are rarely close to the same number. (i.e. 710mcd Red, 1250mcd Green, 240mcd Blue)....
36k views

### How long would a 150 Farad Capacitor light up an LED

I have a bright white LED from a flashlight. Aproximatley How long will it light up with a 150 farad 2.5 volt capacitor. Do I need a resistor? And if so how many Ω? The capacitor is a maxwell 150 ...
7k views

### Power LED from headphone jack

I'd like to hook up an LED to a cell phone's headphone jack. When there's audio, turn on LED, when there isn't, turn it off. Is the best way to go about this just using a npn transistor? Any links to ...
3k views

### How does the current limiting resistor for an LED affect current and voltage drops?

I'm having a bit of trouble understanding currrent limiting resistors in simple LED circuits. I know that I can determine the optimal resistor like so: $\displaystyle R=\frac{V_{s}-V_{f}}{I_{f}}$ ...
5k views

### How do two-leaded color changing LEDs work?

I have a few 3mm color-changing LEDs which I harvested from a toy. They fade in between all different colors, and then after a little while, they blink crazily between all the colors. After a few ...
219k views

### What is “forward” and “reverse” voltage when working with diodes?

What is the difference between "forward" and "reverse" voltages when working with diodes and LEDs? I realize this question is answered elsewhere on the interwebs such as wikipedia, but I am looking ...
2k views

### Power LED bed lamp from mains

I'm attempting to build an LED bed lamp that will be powered by 240V mains. The LEDs are these ones which if I use with a 12V supply and have calculated correctly will draw 0.24W each. I plan to use 5 ...
18k views

### Why do LEDs not obey Ohm's law?

In a previous question, it was brought to me that LEDs do not obey the Ohm's law. (See Calculate expected voltage around a resistor) Simply put: how is that? What makes them behave so differently? ...
2k views

### Driving LED with PIC 5V

I have always been taught to driver an LED using an MCP using the following method (Driving the pin high) as shown in the diagram below: However I have recently seen LED's being driven using the ...
6k views

### Use of LED driven over-current in a strobe lamp

I'd like a strobe lamp using RGB LEDs as it's light source. I want to pulse the LEDs with very short duration pulses (ideally microseconds or less) at around 100 Hz. The total on time for the LEDs ...
569 views

### Lighting either of two LEDs, or none, based on digital input

I ran into a problem which I tought was extremely simple, and after giving it some thought for more than 2 hours I had to recognise it actually is quite complicated. The problem is the following: I ...
2k views

### Understand 2 transistors (NPN & PNP) flasher circuit working

I implemented this circuit on a breadboard and it works. However on Proteus ISIS it doesn't (I have to change the 330k to 1500k for it to work on Proteus). I have two questions related to it. 1- Why ...
23k views

### Hooking up multiple RGB LEDs while using a minimal number of PWM pins on an Arduino?

I currently have an anode RGB connected to the 11, 10, & 9 PWM pins on my arduino. However I would like to add 3 more LED's to my project, but I don't want to necessarily take up every single PWM ...
47k views

### How do LED light bulbs work?

I have been using and making LED flashlights for years now and I am VERY familiar with all of the circuitry and technology inside of them, but I just got my first LED house light bulb that runs off of ...
12k views

### simple, energy efficient circuit to make single IR LED blink

I'm using a 850nm IR LED (spec sheet), and in order to extend my battery life, I'd like to make the LED blink (UF 1.9 V, IF 100 mA). I found a couple of links 1, 2, 3 but they either a) require two ...
3k views

### Why are there only RGB and RGBW-LEDs, but none with more chips inside one housing?

In another thread the question emerged, where the LEDs with 5 or more dies of different colours are. (Kudos to Brian Drummond) The background is as follows: RGB LEDs are very popular for creating ...
7k views

### Single LED single transistor circuit not working

I was making a circuit with 1 LED and 1 transistor. pretty primarily i was trying to use the transistor as a normal switch. Here is the diagram below. So, this is an NPN transistor and i connected ...
9k views

### Arduino, NPN and common cathode RGBs

I'm trying to design a mood light system using 4 RGB LEDs which will be powered by an external 9V source and an Arduino for processing. Link to Fritzing file Part list: 3 x 2n2222 transistors (NPN) ...
9k views

### Best way to control 130 - 140 LEDS with Arduino Uno 32/64 bit led drivers?

I am new to electronic prototyping and had a few questions. I need to control 132 LEDs individually. I want to make 3 rows ( 2 rows of 60 and 1 row of 12) I think i can matrix them but i would still ...
3k views

### Changing the resistor to be in front or behind an LED doesn't affect brightness?

Note: I am very new to the more advanced side of electricity. I do not have a firm grasp on the exact mechanics. Lets say I have an example circuit: ...
8k views

### LED as 230V AC indicator

Answers to other questions here (e.g. 1) show capacitors used to drop AC voltages to levels suitable for a LED. I've seen much simpler circuits such as the one below simulate this circuit – ...
34k views

### Is it OK to put LEDs in parallel? [duplicate]

I wonder if it's a good idea to put LEDs in parallel, as below: I've heard that it might not be because the voltage threshold won't be exactly the same for each LED, so they'll shine all with very ...
42k views

### How do filament LED bulbs work, looking very similar to incandescent bulbs?

Filament LED bulbs use very narrow strips containing the LED, which look similar to filaments of an old incandescent bulb: They look like a normal bulb with large "old-style" coal filaments, and are ...
25k views

### Turning on multiple leds with arduino using transistor

I want to be able to turn on 50 red LEDs with one pin of an Arduino. I don't want individual control, I just want to be able to turn them all on or all off. I found this example. It uses a 2N3904 ...
4k views

### What transistor do I need to drive many LEDs and how should I hook it up?

I know that a transistor amplifies a signal, but I've never used one. I have this design here and I want to add 15 LEDs connected to each pin, instead of 1. How I could incorporate a transistor to ...
4k views

### LED strips: 46% resistive losses?

I recently bought these LED strips. They are supposed to run on 12V/72W and have 300 x the SMD 5050 LEDs on it. Schematic: simulate this circuit – Schematic created using CircuitLab The ...
3k views

### LED accent light that slowly dims when removed from power source

We've all played with different transformers and power supplies that have an LED indicator that dims before going out entirely when the power supply is unplugged. I'm working on designing an accent ...
Traditionally I have always used a $1\:\mathrm{k\Omega}$ resistor as a current limiting resistor for driving an LED direct from a TTL (or 5V CMOS) output. I can't remember how I came by this value -...