It comes up now and then that I need to find LED in a display, or something similar. The problem is, LEDs are delicate if you back voltage them, they can blow much above 4-5V! Here is a simple scheme I came up with for a quick and dirty breadboard tester. The first circuit is what you might use in a box, the second is the bread board with the calculated voltages.

 

Nice, I designed a led tester which tested polarity and what current would be best.

 

I don't understand why you included R2 in the circuit.

 

Hello,

R2 will reduce the voltage, when the led is connected reversed.
As known the leds can not have more then about 5 Volts in reversed connection.

Bertus

 

I don't understand why you included R2 in the circuit.

no? what would drain the battery?

i think concern was with exceeding LED voltage limits.
the LEDs have low breakdown (reverse) voltage and some may be worse than others.
when testing it is likely that intentionally or not, led is connected backwards.
but i don't recall blowing LEDs from 9V even when they ware reverse biased.
i would be surprised if one finds models that actually does blow up with 9V source.

 

The series diode in the upper schematic protects the LEDs.

 

Not if the LED is connected backwards. Matter of fact, if I were putting it in a box with a battery clip I would eliminate that diode.

Like I said, quick and dirty. Most of my projects like this I simply disconnect the battery. It could have been done other ways, like 8 diodes in series, or a zener supply, but the point is to make it as simple as possible.

 

Sorry. I was thinking of a backwards battery.
I agree that it's a simple, useful tool. I was just thinking that the current vs fwd voltage would be more nearly constant if R2 were eliminated. You could replace R2 with a 4.3V or 4.7V zener, and change R1 to 680Ω.
Just a thought.

 

I usually just slap the leads on either side of a 2032 coin battery. Anvil/big part inside LED is - (usually), goes on the buttoned side.

Quick and easy for sorting colors when you dump out a box of water clear LEDs.

 

I was answering a thread asking about LEDs, and referred back to this one. If I was to use the concept and wire it into a box I think I would install a simple button, similar to this one.

http://www.bgmicro.com/LED1063.aspx

I usually just slap the leads on either side of a 2032 coin battery. Anvil/big part inside LED is - (usually), goes on the buttoned side.

Quick and easy for sorting colors when you dump out a box of water clear LEDs.

Actually I like that suggestion, simple. Exactly the way I like my testers.