The D in LED

Ugha

I was recently looking at a schematic on the net that I was considering building but I suddenly realized I didn't have anything that could be used as a flyback diode.

Well this disappointed me because the nearest radioshack is 150 miles away...

Then I thought... LEDs are diodes, can I use one of those?

I have been unable to find a clear answer online and I have no intrest in frying my stamp so... would one of you gurus mind enlightening a newbie?

Mike Green

As with most questions like this, read the datasheet and think about how things work.

Two significant parameters for any diode are the forward voltage and the reverse voltage. The first is the voltage across the diode when it first begins to conduct. For most red LEDs, this is 1.7V. For other LEDs, it's usually higher and depends mostly on the materials used to make the LED. The second is the voltage the diode will withstand in the reverse direction before it "breaks down". For LEDs it's often low, like 3V to 5V. A standard power diode will often have a forward voltage of 0.6V and a reverse voltage of 50V or more, sometimes as high as 1000V.

Flyback diodes are usually used to effectively "short" a winding in one direction and to withstand a bit of voltage in the other direction. An LED has a little too high a forward voltage to efficiently "short" the winding and is unlikely to have a high enough reverse voltage to withstand the voltage in the other direction. There may be some circumstances where an LED would work, but most of the time it won't work well or at all.

Post Edited (Mike Green) : 5/3/2008 9:52:52 PM GMT

Ugha

Thanks for the info Mike [noparse]:)[/noparse]

Ok, LEDs as flyback diodes is out... are there any other interesting applications that take advantage of the fact that an LED is a
diode?

I've seen a mono-stable vibrator using LEDs to control the flow before... but that's about it.

kjennejohn

Hi. I saw a thread recently, maybe even here, that discusses using LEDs as communications devices. It seems exposing LEDs to various levels of external light changes current flow or impedance or some such. There was even a link to a paper that discussed using LEDs in this function. They're very slow though.

Later!
kenjj

pwillard

I remember seeing a document where Forrest Mims had used an LED as a light detector.

Here is someone doing some experiments based on that work.

http://laser.physics.sunysb.edu/~tanya/report2/

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔


There's nothing like a new idea and a warm soldering iron.

whicker

led's can work in reverse, in that they generate current from light. But they're not optimized for that purpose. That's what photodiodes and phototransistors are for.

That being said, the high brightness stuff (>4000 rated mcd) work a LOT better than the old radioshack standbys.

LoopyByteloose

You can have a red and a green LED wired parallel - but opposite - indicate different polarity in a circuit. Regarding flyback diodes, there are actually high amperage, high speed, and high voltage ones that allow motors to roll more efficently. when off.

LEDs can be stacked in series to make a predictable regulated output or voltage drop. Since the current is so low, I don't see why LEDs cannot be used in the same manner. Similarly, you can put them on one input of a comparator or an op amp to have a set voltage level.

But at the end of the day, I suspect a conventional diode will cost one tenth as much as the LED and more predictable. After all, when exposed to ambient light, the LED may actually create an unwanted electrical current..

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
PLEASE CONSIDER the following:

Do you want a quickly operational black box solution or the knowledge included therein?······

···················· Tropically,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan

Post Edited (Kramer) : 5/5/2008 6:12:32 PM GMT

Humanoido

The thread you are talking about is posted at this tech group

tech.groups.yahoo.com/group/toddler_robot/
tech.groups.yahoo.com/group/toddler_robot/message/1569

It includes some experiments by David Buckley and a Basic
Stamp program to read a red or green LED as a light sensor.

humanoido

Ugha

This is all extremely interesting.

I especially like the info about the photovoltaic properties of LEDs.

What about using LEDs for homemade optoisolators? Anyone have any experience with this?

If so did you use a photo resistor or a reciever?

Has anyone used the photovoltaic properties of an LED for optoisolation?

I understand speed is an issue, but I'm just curious if anyone's put them to such a use.

Humanoido

For over a decade, designing and experimenting with optoelectronic interfaces was fascinating. "Yours truly" built an addition onto the home for a limited cybernetics laboratory where these were homebuilt from common components and leftovers - many of the parts were recycled and scavenged from junked machines. You can make good optoisolators using diodes. For the robot, the multiple channel unit evolved into a device for communicating with light waves and this led to the creation of a new language using light. The first multiple channel unit was about a square foot in size, however a few years later it had evolved into a tiny portable unit carried by my first contest autonomous robot. Some of the parts on that robot were small tractor parts from a farm surplus store, and their power line spikes had to be isolated from the controlling microprocessor.

Experiments with phototransistors, photocells, and LEDs concluded that the principle concern is relative component recovery speed. This determines how fast the system can respond to commands. With the small spacing distances of homebrew components, light sensitivity remained sufficient and was not a concern. Cadmium sulfide (CdS) cells are the most easy to use, and rely on the material's ability to vary its resistance according to the amount of light striking the cell. This led to other fascinating inventions, especially with a simple solar cell. It could also isolate light signals across space and time on its photo-voltaic surface. For example, as a demo of the techniques, the sound was removed from a tv channel by turning off the volume, and the video was offset tuned to the sound carrier - showing only the changing sound in a changing video image, and the cell with an amplifier could reproduce the original sound from the light, across the room, in total isolation from any audible sound. While these parts can ideally isolate signals, sound, voltage, spikes, noise, and interference, they can also provide signal communications in various ways.

The unit evolved once again into a product that controlled a 28,000 pound robot, and a 2-ton robotics positioning system, where error was not tolerated, so you can see there can be considerable reliability in a homemade component. Of course years later, companies began manufacturing optoisolator chips and now we have some great choices like the 4N25 and 4N35 for example. However, nothing can compare with the thrill of creating your own!

humanoido