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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.

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4answers
57k 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? ...
115
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5answers
78k 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?
91
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8answers
79k 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 ...
69
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10answers
13k views

Why did LEDs take so long to appear as light bulbs?

LEDs are an old technology, why did the industries take so long to put them into light bulbs? Was there any technological gap missing?
61
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3answers
6k views

Why is kerosene stopping my red LEDs from illuminating?

I initially posted this on chemistry.stackexchange but didn't get any answers, so I'm reposting it here. Long story short - we have an electronic product that is submerged in fuels (kerosene being ...
59
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7answers
204k 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 ...
51
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5answers
16k 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 ...
49
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5answers
11k views

Are LEDs better than we think?

Conventional wisdom about LEDs says their maximum reverse voltage \$V_{R(max)}\$ is quite limited, usually in the 5V-8V range. So, for experimentation purposes, I wanted to bring an LED into ...
46
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10answers
14k views

Why hasn't my LED burned out by now?

I have a 15 year old ReplayTV DVR. I got it used, I'm not sure when, but probably about a decade ago. It's turned on almost all the time, and it has a blue power LED on the front. The LED FAQ says ...
45
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6answers
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 ...
41
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6answers
96k 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 ...
39
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7answers
17k views

Why can't I light a LED with a 1.5 V battery?

The LED I am using requires a higher voltage to light than I have supplied, and as a result, it doesn't light at all. I would expect at least a dim light, but light is not generated. Why this ...
39
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2answers
11k 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 ...
38
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9answers
31k 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? ...
37
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2answers
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 ...
36
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5answers
6k views

Why should you use two resistors in parallel on an LED?

So I was looking over the Arduino R3 schematics and noticed this little design choice: What is the reason for doing something like this? I mean it's hard to know what the designers were thinking, but ...
35
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4answers
7k views

Why are LEDs on most embedded designs inverted?

I noticed, that on all my evaluation boards that I had up to this point in time. The LEDs were all connected in active low to the Microcontroller Port. I understand that from a safety view it's ...
35
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5answers
5k views

Why do 7 segment LED displays break apart when I brush my teeth?

This question might sound silly, but it is very serious (although geeky, I must admit). When I use my electric toothbrush in front of my alarm clock (one of those with a big red LED display), the ...
34
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6answers
41k 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: ...
33
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5answers
6k views

Detecting light with an LED

How do I detect light with an LED?
32
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2answers
3k views

Why are SMD LEDs shipped in a sealed bag containing desiccant?

I had a selection of 0805 LEDs in my last Mouser order and was a bit surprised to discover they arrived in heat-sealed (rather than folded and taped with sticker) bags containing a desiccant bag and ...
31
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7answers
28k views

Correcting for non-linear brightness in LEDs when using PWM

When driving an LED with PWM, the brightness (as I perceive it) does not scale linearly with duty cycle. The brightness is slow to ramp up, then increases exponentially with duty cycle. Can anyone ...
30
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7answers
26k views

What is the advantage of a “straw hat” (inverted cone) LED?

LEDs with a concave cone-shaped lens are sometimes called "straw hat" LEDs. What is the advantage compared to the more common hemispherical lens?
29
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7answers
23k views

If Infrared not visible, why the red LEDs?

If infrared light is not visible to the human eye, why do most tv remote controls and security cameras appear to have a visible red colored LED lit when the infrared light is being emitted? Is that ...
29
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11answers
16k 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 ...
29
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7answers
63k views

Easy way to figure out a LED's Vf in order to pick an appropriate resistor

I was wondering what the easiest way to figure out a LED's forward voltage is, using measurement tools. I know we can assume red LEDs to be around 1.8V - 2.2V, and that we have similar information ...
29
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4answers
6k 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)....
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3answers
2k views

The mysterious case of the random leakage current

Disclaimer: I already know the solution of this circuit debugging conundrum (and it is not trivial), and I'll post it after some time, so that other people could give their contribution to this ...
27
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2answers
5k views

How efficient are LEDs?

Generally I see efficiency related to the lumens per watt, but what is the actual typical efficiency of LEDs in terms of electrical energy in to optical energy out? What sort of conversions apply?
27
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3answers
465 views

Spectrum of a LED

As I understand it a LED emits a photon when an excited electron falls back to a lower orbit, and this is always the same energy (read: wavelength). So then why is the spectrum of a LED a bell-shaped ...
26
votes
10answers
14k views

Doesn't using resistors serial to LEDs all the time waste a lot of energy?

I was wondering for a long time why in schemes which use LEDs for lighting it's very common to put a resistor to go with the LED, and finally it seems like the answer in this question explains why. (...
26
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15answers
25k views

Controlling 500 LEDs with PWM

I'm considering taking on a project that would involve individually addressing 500 LEDs, optimally with PWM support for each. I'm planning on using an Arduino since I already have one, but I'm open ...
24
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10answers
89k views

Blink an LED with just a capacitor?

Is it possible to blink an LED using just a capacitor? (and maybe a resistor). For example, if I want to LED to blink once every 2 seconds. Is that possible? I know it can be done with a 555 as well ...
24
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9answers
33k views

LED and lamp in series - why does the bulb not light?

My 6 year old son has just started experimenting with a Snap Circuits style kit and already we have a very basic question. If we arrange an LED and lamp in parallel powered by batteries then both LED ...
24
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6answers
98k views

Why doesn't my 5050 led strip draw as much power as I expect?

I've been playing with a 5m long RGB LED strip, with 300x surface mount 5050 RGB LEDs, but can't understand why the strip isn't as bright or drawing as much power as I was expecting. I've had a look ...
23
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3answers
8k views

How are LEDs considered efficient?

I've always found circuits containing LEDs hard to understand, please bear with me. I know most people find it easy, but I'm confused by them so some of my assumptions might not be correct, please ...
23
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3answers
3k views

Is the LED drop voltage difference between colors linked to the different wavelength energy?

To me appears that LEDs that emitt light with less energy (e.g. IR and red) have less voltage forward drop than the ones with more energy associated to their wavelength (such as blue or UV). That ...
23
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4answers
3k views

Does (and when how) the lifetime of a LED depend on the PWM frequency

Let's assume two options for driving a standard LED at its rated current. PWM set to 50% duty cycle at 10 kHz PWM set to 50% duty cycle at 50 kHz Technically both LEDs would produce the ...
22
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4answers
4k views

Why does an LED have to be a diode?

I know LED stands for Light Emitting Diode; but why does this component need to be a diode to emit light? My question assumes that the "leds" we see everywhere (for lighting, screens, etc) are ...
21
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2answers
2k views

Why do these LEDs have different brightnesses

I did a search for 0603 sized high brightness Kingbright LEDs on element14, results are here . Why is there such a range of luminous intensity, even within the same colour, despite forward voltage, ...
21
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3answers
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 ...
21
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8answers
12k views

fastest way of doing on/off-modulation of a LED?

I need to modulate a LED quickly (multi-megahertz range) on/off. It's a high-power LED. I have had some trouble locating any well-known methods to do this. Simply switching the voltage with a FET ...
20
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4answers
3k views

What makes a LED a laser diode?

High-level survey of this question is fine: After reading https://en.wikipedia.org/wiki/Laser_diode I still can't tell if the electronics that enable a diode to lase are different from those that ...
20
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3answers
2k views

What's the purpose of traces that are later punched out?

I've found this odd feature on an FPC in a Camera than handles button and switch input. You can see there are traces that look like they were once connected and later punched out to be cut. The one ...
19
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6answers
59k 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 ...
19
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1answer
4k views

What makes a good RGB LED

I am looking for supplier for RGB LEDs for an upcoming project RGB POV LED Globe. Normally I would go to my local electronics supply story, but this project requires lots (~1000) of LEDs and my local ...
19
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1answer
2k views

How do LEDs slowly degrade in normal use?

As I understand it, an LED normally has a service life of perhaps 25 years with output that decays exponentially as a function of time and current. What causes the degradation? I'm guessing that ...
18
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7answers
35k 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?
18
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5answers
9k views

current sourcing, current sinking

I'm a student studying electronics and I'm having trouble understanding the concept behind current sourcing and current sinking. We've covered it in a lab using a 7404 and an LED and all that. Just ...
18
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3answers
30k views

Are UV LEDs really dangerous?

I just bought some UV LEDs I bought them for testing banknotes and identity papers. I read the documentation and saw some warnings about UV light in which the vendor advises not to look directly at ...