LED Glows Without Input, How?

No way,light emitting diodes need to be connected in forward bias.

Did you tried to figure out that what is inside or under the bread board?

Were there any large (i.e. high-value) capacitors in the circuit?

I agree with JohnDG, there must be a capacitor in the circuit. The capacitor must have to discharge first its energy contents after unplugging. There is no such thing as magic.

Perhaps the Professor was demonstrating that the wire you had connected and he had disconnected, had no purpose in the function of the circuit and may have been redundant or parallel to another conductor in the circuit path.

Leds may glow without "input" because of induced voltage. (On that reason are very unreliable as signal lamps - it is better to use the incandescent ones instead)

LED need significant amount of voltage to glow, Now I am not able to understand how this much voltage can be induced in circumstances suggested by OP?

Single LEDs run with only a few volts applied. The typical range is 1.5 to 5V DC.

I agree with Unredundant in Reply #4. I think the professor saw that the circuit was laid out incorrectly and the LED was being powered even when the supposed input wire was disconnected. The other idea mentioned here, that a capacitor might have provided some current to drive it, is possible, but LEDs require a lot of current and the LED would probably have discharged the capacitor very quickly.

This is one of those classic cases where eye witness testimony falls flat on its face, and also why illusionists and magicians can make a decent living. There is no way an LED can produce light without current going through the junction. Now if this was an illusionist or magician demonstrating an apparent LED being lit I'd have a very large collection of suspicions how they made it glow. Since this was instead a professor demonstrating to a student I expect that either a large capacitor or almost any battery was providing the current. But many magicians and professors consult with each other.

Now I don't want to go through all of the possible meanings of your undefined acronym FF for obvious reasons but while a flip flop does store data it does not store energy. Many flip flops today successfully store their data using the energy stored in the femto-farad (another FF) capacitors that make part of the circuit that is a flip flop but using this amount of stored energy to light an LED will be difficult to detect with a photomultiplier tube, impossible with just a human eye.

"There is no way an LED can produce light without current going through the junction..."

That's not entirely true. Some LEDs -- especially the white phosphor type -- will glow when UV or blue light shines on them. Depending on the amount of UV or blue light the glow can be fairly bright. I've seen it happen and have measured it. Even took pictures.

I carefully phrased my comment for precisely this scenario. The Light Emitting Diode (LED) is not providing the excitation energy to make the phosphor glow, the external UV or blue light provides the excitation energy. Even when the LED is providing the excitation energy, very little if any of the light the eye perceives actually comes from the LED in this case.

Oh, this is one method that I would expect an illusionist could make an LED appear to be ON.

I've seen non-phosphor green ones glow, too. Yes, I said 'especially' the phosphor type, but I've seen the non-phosphor ones glow too. For the non-phosphor type the glow is of course much fainter, but it will happen.

For the phosphor type, (nearly) all of the light in the shaded area in the graph below comes from the blue LED; the light in the unshaded area comes from a yellow phosphor. The blue light is about 15 % of the total light.

Since we have no idea what the "board" consisted of, the simplest possibility is that there was a capacitor somewhere, i.e. they had an SMPS powering the circuit. With a very small bleed-off resistor, the LED could glow for a long time.

May be there is "SMALL SOLAR CELL" in it !

Which reminds me... I feel there needs to be lots more attention (here) placed on those people claiming to make 'overunity' machines (magnet based)... & energy fielded from radio/etc. waves, energy from galvanized steel (in tandem with copper) pipes stuck in the ground, all kinds of stuff! These folks look mighty serious. The fact is we (good ol' U-S-of-A) need to get with it. BIG TIME! Carlos (for example: I saw one video where a fellow used two plain wax candles, two nails, connected to a flashlight bulb... the nails (briefly) rubbed with a magnet - then the lamp turns on when the candles are lit! Don't take my word for it - see for yourself!)

About a million years ago when I was in college my electricity professor had a little demonstration. (500,000 volts, almost no amperage) Like a sucker I offered to participate. He had a large Tesla coil set up. The "ball" was actually an old copper toilet tank float. Even with the rooms lights on you could see mini lightning seeking ground from all over the ball. It really looked cool. He had me stand on a small wooden stool about 6 feet from the ball (the stool added resistance to ground). Once I was on the stool he handed me a typical 4 foot florescent bulb. He instructed me to hold the bulb perpendicular to the floor. Then I was to slowly move in the direction of the ball. As I began to get closer the bulb started to glow weakly. The closer I inched the more the bulbs brightness increased. Once I got to within 18 inches or so of the ball a bolt of mini blue lightning hooked up to the end of the bulb and it lit up to full brightness. At the same time my forearm muscles contracted strongly (sort of like the shock machines you see at carnivals). It was really an exciting experiment.

The purpose of the demonstration was to demonstrate the Tesla theory that electricity could be transferred through air without any type of conductor (wires). It was also to demonstrate that if you have a high enough voltage electricity will come out and get you, you don't have to actually touch a wire to get killed. Ask a lineman if you don't believe me.

As for the original question, as usual there is no picture or diagram and too little information to make a solid suggestion. I would however say if he was in close range of a big enough field it's entirely possible the LED showed some life.

If he disconnected the low side of the latch, it makes perfect sense. "A" wire doesn't a circuit make.

John DG

If there was a capacitor, what sort of circuit could it have been? Possibly the only circuits I remember (in that time) with capacitors in it is the Integrator and differentiator using IC 741. I am quiet sure it wasn't a capacitor. But if there was a capacitor, under what criteria/condition would the LED continue to glow? Its till the capacitor discharges if I am not mistaken and if the value is large the time taken to discharge is also large. Would that be correct?

Fredski

That was an interesting story. But this circuit didn't deal with voltages more than 5 to 12 V for sure and the LED to was an ordinary 1 V red LED. Unfortunately, I don't have a picture because I don't remember what I was working on then, I am quiet sure it was something like logic gates, flip flops etc.

Unredundant

"Perhaps the Professor was demonstrating that the wire you had connected and he had disconnected, had no purpose in the function of the circuit and may have been redundant or parallel to another conductor in the circuit path."

That could have been the case.

Cuba Pete

Explain what you are saying. I didn't understand the term "low side of the latch" . Its possible what you are saying is what most likely happened. If what you are saying is wrong, then I guess Unredundant could be right - that I got the circuit wrong and his question was more rhetorical. If my memory doesn't serve me right, even John DG could be right that there was a capacitor.

Regards to all,

Jay

Could have just been a high value electrolytic capacitor straight across the power rails. This is often done to keep a circuit running during disturbances to the input supply.

With a low-current LED (say 1mA nominal current), a 10V circuit (so a series resistance of about 8,500Ω to limit the current) and a 10,000μF capacitor, the current would drop to about 0.36mA after about 85 seconds, so the LED would continue to glow for quite some time.

Let's not forget the great Engineering acronym KISS. One AAA carbon zinc battery can light an LED for days with the right circuit, the simplest circuit will easily last though any lecture.

True enough. I was just addressing Jay_'s questions in #22.

Also, Jay_ said in the OP that he assembled the circuit. Why would a bread-boarded circuit have an external power supply and a battery?

But as the original details of the circuit are lost in the mists of time, all this is just speculation.

I did use an external power supply (5 V dc regulated) from the mains, which I connected to wires from the breadboard. I didn't use a battery.

I shouldn't jump in here (because I'm not Cuba Pete), but his suggestion about the latch problem is one plausible example of Unredundant's answer, a redundant, parallel, or in some other way, wrong connection.

Perfectly fine...but I am just guessing as to the circuit anyway.

There seems to be several ways of reading the OP

"disconnected a wire", or 'a power supply wire', or 'a leg of the LED'

"A wire", I think, points to Unredundent's redundancy conclusion.

_{[i think, but i could be wrong, nar]}

GA to #4

Cuba Pete

I didn't understand your answer exactly. Could you or anyone (explain it). I feel I was working with a logic circuit (gates, flip-flops that kind of stuff).

I'm not absolutely certain what Cuba Pete meant but I suspect he was referring to a S-R latch or flip flop circuit. His "low side" of the circuit comment likely meant that that wire was no longer needed to continue setting or resetting the circuit. Think of an elevator call button. Pressing the button just once turns on the call light, every person that shows up and presses the button again do not change the call light at all.

Exactly.

Redfred, Cuba Pete

That could have been it, because if I recall correctly I was working with a logic circuit, it would have had a Flip-flop. In that case, any idea how would the circuit have been?

I not only had an LED that glowed without input, but the images of various icons would appear.

thats a simple oscillator circuit. google it u'll find it. i know a similar ckt dat works has no i/p but gives d o/p. {Actually the i/p is the nearby noise signals

Welcome to the mad house.

I warn you, you really should put a little more effort to write complete English sentences here and not texting gibberish. The imprecision of texting will often just add confusion to a discussion. It also promotes the attitude that you know something others do not but you want others to know this without any proof. This will usually promote a challenge by one of us for your proof. Allow me to demonstrate.

Can you draw a schematic of this circuit that makes an LED to light without a wire? A block diagram with an explanation of the blocks will work. I also do not understand how an oscillator fits into your claim. Please explain.

Hi Guys. Thanks for this info on LEDs glowing after being disconnected from power. I am investigating this now and your input is helpfull. I can conform this for you.

If you are running on an existing lighting circuit, change the LED feed to a seprate circuit only for the LED lights.

Do not have parallel wires, i.e. lamp wire,

Have a twsited pair of wires for the lighting circuit. Even if you have to twist the wires by hand (Drilling machine) One does not need a tight twist on the live and neutral, even a couple of twists just before you connect to the mains input of the driver, if a driver is used.

I have tested capcitance and inductance so far, I cannot get the LEDS to glow, I have tried 1.3 Tesla magnetic field, no light. 69kV inductance from a HV capacitor, no glow. I have even tried inductance from 863Amps running on a MV 3 core cable. no glow.

To date I can confirm, put a twsit in the feeder wires and it will eliminate the after glow.

If you have any further info you can offer me to further research this issue let me know, as I am busy trying to solve this problem. Once I have the answer you shall be made aware of the answer.

Thanks

IQ