How to measure on/off LED timing ?

Compare what? Brightness?

How will that help?

wha wtf??? I'm going to try to translate the questions..

Sorry..need more info .

I cringe at how nonenglish the details will be. :( D from BC

Did we fail to read the subject before we pounced on this poor fella?

If you want to measure the on/off time of an LED you might try using a moderately high-speed photodiode; at least 2X faster than the LED datasheet specification (some Nyquist thingy or something ?). Make sure that the spectral response of your photodiode is appropriate for the LED you are testing (i.e. the photon energy of your LED is greater than the band gap of your photodetector). Silicon photodiodes will work out to about 1.1 microns, InGaAs will work between 900 nm and 1.7 microns. Attach the output of your photodiode to the input of an oscilloscope (again, the analog bandwidth of your o-scope must be at least 2X the frequency you are trying to measure) and measure the voltage drop across 50 ohm input impedance. You might need to use a lens to focus the LED emission onto the photodiode in order to get enough signal to make this measurement across 50 ohms. If you have a high speed scope probe with small capacitance (~10 pf for a passive probe these days) you can also make this measurement across a 1MegOhm input impedance for a larger signal.

I know nothing about the subject matter at hand, but I hope this helps :)

It's in Bokish.

Best regards, Spehro Pefhany

There is the old equation

bandwidth=0.35/(rise time)

Yep, I agree for the subject matter at hand. Not related to the current topic, but you might find the following (dated) article of interest.

lol :) D from BC

Maybe he just needed to check Vd at 20mADC... D from BC

usually it's a good idea to put the entire question in the message

That'd work, hook it to a scope and once you select the correct scales and trigger settings etc the answer should be fairly obvious.

Bye. Jasen