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Electronics => Electronics => Topic started by: hazzer123 on June 21, 2007, 09:36:02 AM

Title: LED question
Post by: hazzer123 on June 21, 2007, 09:36:02 AM

Pretty straight forward question :)

How long, after I apply voltage to an LED, does it take to light up? How long after i take it away will the light stop? is it a gradual increase in brightness when powered on?

Thanks

Title: Re: LED question
Post by: ed1380 on June 21, 2007, 10:36:40 AM

fast enough so that our eyes think it's instant.

Title: Re: LED question
Post by: hazzer123 on June 21, 2007, 10:58:40 AM

Yeh but not exactly instant thought :D

Any ideas how long?

Title: Re: LED question
Post by: dunk on June 21, 2007, 12:01:34 PM

so i don't know this for a fact but i've always presumed very very quickly.

infared LEDs are capable of operating at a frequency of 40kHz (in TV remote controlls for example) so an IR LED can switch on and off again within 1/40,000th of a second.

i don't know if standard visual spectrum LEDs are optimised to operate quite that quickly but i think it's safe to assume they light up in a few thousandths of a second.

dunk.

Title: Re: LED question
Post by: Brandon121233 on June 21, 2007, 12:43:45 PM

the human eye has trouble distinguishing things at about 20 hertz, or frames per second. So if you mean how fast the eye could detect it flashing, then the LED can go a lot faster than what you can detect, it will look like it is constantly on to you.

Title: Re: LED question
Post by: hazzer123 on June 21, 2007, 01:24:27 PM

Yeh i just had a thought about using a light based "sonar" type sensor. Im not planning on using it :) Too advanced haha.

Basically if you had a PIC at 40MHz, light would travel about 30 meters for every instruction cycle. Now if you had a 600Mhz gumstix (not sure how many MIPS) the distance traveled in an instruction cycle would be siginificantly less and maybe usable.

Hmm just a thought. Is this the basis of a laser distance detection system?

Title: Re: LED question
Post by: Jeff on June 21, 2007, 02:25:19 PM

I think typical led rise time is 20ns, but the data sheet definitely will tell you.

Title: Re: LED question
Post by: megaman935 on June 22, 2007, 09:54:30 AM

*blink*
"Aaawwww... I missed it..."
Yes- that fast...

/Mega/

Title: Re: LED question
Post by: stinkerbomber on June 22, 2007, 03:52:37 PM

Speed would be fast, very fast otherwise fibre-optic data transfer would be slow  ;)

Title: Re: LED question
Post by: hazzer123 on June 22, 2007, 04:53:41 PM

But laser diodes are faster yes?