Matching LED's with a FLUKE

[Ned Flanger]

I searched the forums and found nothing specific.  I am trying to match LED's for a Nurse Quacky project.  My DMM can test diodes, but for some reason it does register LED's.  What am I doing wrong?  Is the voltage needed for the LED too high for the meter?  It can only test up to 1.0V.

[deadastronaut]

er..i dont know which dmm you have but im pretty sure it should read more than 1v...!!

anyhow i built the quacky a couple of weeks ago..i used any two 3mm red leds..

and it quacked ok...

the only problem (which i never got sorted)...was that it wouldnt quack on the b and e strings...
and apparently this is a common problem for the nurse quacky...

i gave up..it sounded really good..but the b and e  issue put me off boxing it..

it now lays in my circuit graveyard waiting to be revived..(properly)..

just thought i'd share that..rob.

[Ned Flanger]

THanks for the info.  I'll check the user manual again. 

Now I am testing 5mm LEDs, is this a problem?

[Ned Flanger]

My mistake.  THe manual reads that it only measures diodes up to 2.0V.  Would the voltage drop method be the best way to match?

[R.G.]

My mistake.  THe manual reads that it only measures diodes up to 2.0V.  Would the voltage drop method be the best way to match?

What you do is arrange a constant(ish) current, and then measure the voltage across them at that current.

One way is to use a resistor from, say, the ubiquitous 9V to the LED. You measure the voltage across the LED. Then you change LEDs, measure the new voltage and subtract, looking for a zero difference. That may be enough. Probably will be for the purposes of the circuit it's going in, which is not by any means a precision device.

[Ned Flanger]

Great.  Thanks for the info.

[CynicalMan]

I'm not sure how precise this is, but you can also set up a 9V source to a resistor to two LEDs in parallel and try to find two LED that are at the same brightness when they're in parallel.