Hot answers tagged

59

Leaving it on would use more energy, absolutely. Sometimes, people try to convince themselves that turning a light on and off uses more energy because there is some high inrush current, or some such thing. Firstly, incandescent lights hardly have any inrush current, because they don't have any capacitors to charge, and they need not strike an arc in the ...


58

The Edison lamp base design dates predates all twentieth-century safety regulations (because it predates the twentieth century altogether). Having light bulbs screw in and out is not great in environments where vibration is a problem, and replacing a bulb which has just burned out while in use may require use of a cloth to hold it, but it is generally ...


56

The existing answers miss the core of the question. An LED needs to be a diode, specifically because the way the charge carriers recombine in the forward-biased diode junction releases the correct amount of energy to create photons in the visible range. Passing a current through a chunk of semiconductor with no diode junction in it would simply produce heat....


52

The simple answer is that they are using near IR. LED manufacturers have a good handle on how to make them so they are affordable. Their center frequencies may be invisible to the M-1 eyeball (i.e. human eye), but unless they put a filter in front of the LEDs (which cause them to produce less illumination) there will be some of it that you can see. ...


47

Here's your opportunity. The market's looking for that right now so build a better mousetrap. The USDOE and California CEC want to murder the Edison base to finally stop people from using incandescent bulbs, and enable fixture designs that don't have to worry so much about dissipating heat. They mandated GU24 in 2008, which solves some of your concerns. ...


43

Well in your eBay-Link they are providing enough material to debunk themselves. They say they use a Cree XM L2 LED. So let's just look up what that thing can output. Datasheet XM L2 LED and we see: even if it is driven with 2000 mA - which is quite the stress on the battery - it outputs 600 lm. And they kindly provided a picture showing that they only use ...


41

I'm going to go out on a limb and say this question is valuable from the point of view of electronic design, as it pertains to some fundamental understanding on how fluorescent lights work. Fluorescent lights work by accelerating electrons from the cathode to the anode in an almost-vacuum environment. In this vacuum is mercury vapour, and when the electron ...


40

Okay, let's set up a simple simulation: According to the Wiki page on incandescent bulbs, for a 100W, 120V bulb, the cold resistance is ~9.5Ω, and the hot resistance ~144Ω. It takes around 100ms for the bulb to reach the hot resistance on turning on. So armed with this info, we can simulate and prove the initial surge would be absolutely ...


28

An answer to this question consists of two parts. The first is a question itself. What is the use of a multichip LED in a common housing? The purpose of incorporating red, green and blue chips in a common LED housing is to generate a light source capable of producing any colour from apparently one pixel. This is necessary in two cases: To generate pixels ...


25

There are a couple of answers to this, but pertaining to optical smoke detectors: it doesn't. It responds to anything that reflects enough light of that particular wavelength. Now, there is some method to the madness of course. You can make a list of things you expect to be in the air in a home and tune your detector to ignore these things, increasing the ...


24

Can I do this circuit practically o_O? Is there any transistor or IC that can oscillate 300 THz? Can I find an inductance (coil) of 0.0025 pH and capacitor of 1 pF? Not quite, no, and no. But this is an area of active research: The Truth About Terahertz. The basic principle of the tuned LC radio emitter is resonance. The techniques for producing high ...


23

These LEDs are not a single radial die, they are made with a transparent substrate with many LED dies in series (probably 25) placed on it. The whole thing is then coated in phosphor. The light isn't completely uniform but it's good enough. There's a spec sheet here: http://www.runlite.cn/en/product-detail-145.html


22

Nobody answered why were they designed that way? I recall it was for practicality and cost of manufacturing. I heard about it indirectly, on a TV show about Tesla. He was contracted to provide lights as Edison's competitor, but could not use Edison's patented sealing/base method and had to use his own which lacked the advantages. Trying to find more about ...


22

For some cases, you can: If you have a large directional antenna, it might, from very far away, simply look like a beam-generating "flashlight" for radio waves. That breaks down very quickly if the wavelengths are not much, much smaller than all physical objects interacting with them. We even use specific terms: If wavelengths are very small compared to all ...


20

There are two possibilities the switch is inserted: Switch switching the voltage line. Switch switching the GND line. simulate this circuit – Schematic created using CircuitLab The capacitors shown in the circuit are the capacities the (more or less) long lines form to GND. If the 2nd circuit is what you have the lamp always is connected to ...


20

You are right, antennas and light sources are equivalent constructs. But the mathematics of light sources is not as simple as you seem to think. The reason why most of the answers so far see them as different is just a matter of scale. While we would commonly call "RF" wavelengths of 1mm or above (300GHz) and "light" wavelengths of 1µm and below (300THz), ...


19

According to a Mythbusters episode summary on Wikipedia: " The MythBusters calculated that the power surge from turning on a light would only consume as much power as leaving it on for a fraction of a second (except for fluorescent tube lights; the startup consumed about 23 seconds' worth of power)". So in fact it is possible that on/off would consume more ...


19

LEDs will emit steady light if given steady current. The question then becomes what kind of current waveform LEDs are driven with in a light assembly. The exact answer depends on the circuit in the light. LEDs run on just a few volts. The much high line voltage has to be converted to the lower LED voltage somehow. Most likely, this will include the ...


18

In the drawn position the lamp will be on. Switching any switch will turn it off, and again any switch will turn it on again. If you need more than two switching points you have to add "cross-switches", like this: You have two A-type switches (for A and B) and the others are C-type. edit Any manufacturer of residential switching material has those cross-...


16

300THz transmitter? (the band between infra red and microwaves) - with a lot of technology and know how perhaps. See http://www.rpi.edu/terahertz/about_us.html 300THz transistor/IC - no. Use discrete inductors and capacitors at these frequencies? No. At very high frequencies conventional capacitors and inductors are replaced by other devices (see resonant ...


16

TL/DR: It's a scam. Long answer: Current LEDs that work well in flashlights have luminous efficacies in the 100-200 lumen/Watt (roughly, not factoring in driver losses etc). Thus a 900 000 lm flashlight would require 4500-9000W of power... thus a huge battery and about the same cooling fan as a hairdryer. Also LEDs for flashlights are usually rated ...


15

Not all recombinations result in the emission of a visible-light photon. Only ones that occur within the P-N junction of the LED itself have the energy for that, and this volume can become "saturated" at high current levels. When this happens, some of the electrons and holes pass all the way through the junction before recombining in the bulk material on ...


14

Yes you can! And no specialistic knowledge is required. Check out this site: Building a DIY LED from SiC Michael Lippert writes: The green glow of the DIY LED! At the contact point of pin and crystal greenish light is emitted. The setup is very simple. I took a crystal of SiC and attached a clamp to it to supply it with about 20 Volts. It is ...


14

You've got the general idea down already, so start documenting what you know - make a block diagram. Draw the blocks (processor, battery, light switchy thingy), determine where you have to draw lines that connect them together. Put a lot of '?'s all over the place for everything you don't know. Above all figure out specific questions! For example, you ...


14

None of the above. All you want is Deep Red (650nm) and Deep (Royal) Blue (450nm). You do NOT want Full Spectrum, UV, IR, Far Red, Only White or anything besides Deep Red and Deep Blue. Red White and Blue is always a safe bet. Red and Blue come way before white. White (aka Full Spectrum) is more like supplemental. I do some consulting work regarding ...


14

The transition from visible wavelengths to invisible is not infinitely abrupt. Your eye's sensitivity falls off in the IR range. But in the near IR, it may not be zero sensitivity. And the emission spectrum of LED's is not infinitely narrow. So not all of the photons coming off of an LED have the exact same wavelength. The net effect of these two things is ...


14

Telling us what you wish to do in much more detail would be helpful. I need that LED to be omnidirectional without using a reflector. Ideally it should be only one (Maybe 2 as suggested) because its power source will be very tiny (A capacitor constantly being charged by electrostatic induction), it should light an led for a brief moment. You need to ...


13

Assume the power input to the bulb is 10 Watts. Assume for now 100% efficiency from battery output to bulb input. Efficiency of energy storage by the battery of energy supplied to it will vary with battery chemistry and how well the charger is designed. Best case using a Lithium battery of some sort, over 90% efficiency may be achievable. Lower or much ...


13

When the lamps fails, it is often the case that the delicate filament collapes on itself, causing a short circuit. This causes a momentary peak of current. So much current that the short almost immediately blows itself open circuit again, due to a teeny explosion. The lamp usually fails when switched on because the resistance of most materials, including ...


12

The LDR and a 10 k\$\Omega\$ resistor together form a voltage divider, whose output depends on the LDR's resistance. If you connect the output to a low impedance circuit that will get parallel to one of the resistors and distort the reading. edit (re Sauron's question for further explanation) "Impedance" is the general word for any type of load, but ...


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