I want to take some LEDs off some circuit boards and use them in other projects. How do I find out their value and polarity?
Thanks

 

Use about 4 or 5 volts and a 390 ohm resistor to light them up. Then make a judgement call about whether they were designed for 10ma to 20 ma.

 

Easiest way is with a breadboard using a three terminal adjustable voltage regulator such as the LM-317, a .1 uf ceramic bypass cap on the input pin, and a 100 ohm trimpot on the output to adjust pin. Typical procedure is set the resistance of the trim pot to around 62 ohms before putting it into the breadboard and then fire it up and adjust the current using the trim pot to the desired level using a multimeter. You will need 6+ volts on the input to ensure that the regulator does not "drop out". Most LEDs are rated at 20ma of current, and 62 ohms will get you pretty close to that. Once you got your circuit set up it is as easy as putting the positive lead of the LED on the adjust pin and the negative to ground and take a voltage measurement across the pins of the LED to determine how many volts the LED drops. Once you know that number you can determine how much resistance is needed to put the LED into another circuit at a desired level of current using a plain old resistor for current limiting.

 

LEDs cost like pennies on eBay. Not worth desoldering old leds for that price.

To know the polarity, you uses google.

Assuming they are standard LEDs the flat side will be the negative side.

Again assuming standard LED, you can google for typical forward voltage and current for a given color. Worse that can happen is you burn them out.

 

Its not recommended, since the plastic is brittle and easily the metal pins break off. As well they dont like heat very much.

The chance to remove a LED from a 2layer PCB isnt great.

 

LEDs cost like pennies on eBay. Not worth desoldering old leds for that price.

I tend to agree. I don't scavenge plain resistors or small, common capacitors either for the same reason. When you go to build something, it's just so much better to have a nice array of new components on hand. It only costs a few dollars to set yourself up that way.

Modern LEDs are sooooo much brighter at a given current than the old ones. The only LED I might scavenge would be one with an unusual shape or some other property that would save me placing an order.

 

I agree about the small stuff, when I first started learning to desolder a couple of weeks ago I ruined a few real small caps because they got too hot and I didn't have them quite
desoldered enough. I am building up my stash of parts, one ebay purchase a day or so it seems. It's getting fun opening up the mail box to see what little package of goodies came today. (None today dangit.) Well time to go order some LEDs.
Thanks

 

I bought something like 300 all at once, in 3 or 4 colors. They came from China and they're fine. Very bright.

 

I just ordered 130, 100 in four colors and 30 ultra bright white. I would have gotten them from China for a little cheaper but I have a couple of projects going that I want to finish so I paid a little more and got them here in the US.

 

I just had an idea. For the heck of it I took a 3 volt button cell battery, taped a wire to each side and touched them to one of the LEDs on a junk circuit board and what do you know, it lit up. I switched the wires on the LED and it didn't light up. I tried a few more and the same thing happened. Hmmm, well I guess that's one way of doing it. I don't know if that's as bright as they could get with the low volts but they still work.

 

I just had an idea. For the heck of it I took a 3 volt button cell battery, taped a wire to each side and touched them to one of the LEDs on a junk circuit board and what do you know, it lit up. I switched the wires on the LED and it didn't light up. I tried a few more and the same thing happened. Hmmm, well I guess that's one way of doing it. I don't know if that's as bright as they could get with the low volts but they still work.

As long as the voltage exceeds the forward voltage, the amount of voltage applied has no effect on brightness. What matters is the current. But if you exceed the rated current then you can blow out the LED or at least limit its life.

 

If you have enough time then go for desoldering otherwise its better to purchse

 

Its not good since it affects relieability, the LED may fail later.

If the leads are long, you can clip off the LED. Really, dont desolder LEDs, except they are on long leads.

 

As long as the voltage exceeds the forward voltage, the amount of voltage applied has no effect on brightness. What matters is the current. But if you exceed the rated current then you can blow out the LED or at least limit its life.

When resistance stays the same, doesn't current and voltage increase and decrease proportionately??? If so, voltage does matter.

I just had an idea. For the heck of it I took a 3 volt button cell battery, taped a wire to each side and touched them to one of the LEDs on a junk circuit board and what do you know, it lit up. I switched the wires on the LED and it didn't light up. I tried a few more and the same thing happened. Hmmm, well I guess that's one way of doing it. I don't know if that's as bright as they could get with the low volts but they still work.

LED's are polarity sensitive as shown in the diagram above. They will only light up with current flowing in one direction.

 

My Fluke multimeter on the diode scale will make most LEDs glow softly. Not so sure if it works with white LEDs, but the other colors all work.

Note the LED is out of circuit when I do this.

 

My free Harbor Freight meters do the same.

 

When resistance stays the same, doesn't current and voltage increase and decrease proportionately??? If so, voltage does matter.

Yes and no. Yes, the current driven through an LED varies with voltage. The problem is that the voltage range between <turn-on, very dim>, and <out-of-spec, exploding bright> is only about 1/2 volt. Worse, this range will vary from LED to LED, and will even vary with temperature for a particular LED.

In this narrow range, the current rises roughly linearly with voltage, from <1mA to 20mA typically. So if you have a precision voltage source, you can use it to vary the brightness of an LED. But it's much easier and more reliable to control the current over the 0-20mA range. You won't have a problem switching from on LED to another, and temperature variation won't matter.

 

I just had an idea. For the heck of it I took a 3 volt button cell battery, taped a wire to each side and touched them to one of the LEDs on a junk circuit board and what do you know, it lit up. I switched the wires on the LED and it didn't light up. I tried a few more and the same thing happened. Hmmm, well I guess that's one way of doing it. I don't know if that's as bright as they could get with the low volts but they still work.

Some of the cheap novelty key fob lights are literally just a LED and coin cell.

White, blue and sometimes green have a stacked pair of 2016 cells, all the other colours have a single 2032.

They rely on the cell internal resistance to limit the LED current - it doesn't do the cell or the LED any good!

 

Some of the cheap novelty key fob lights are literally just a LED and coin cell.

White, blue and sometimes green have a stacked pair of 2016 cells, all the other colours have a single 2032.

They rely on the cell internal resistance to limit the LED current - it doesn't do the cell or the LED any good!

they are not meant to be for permanent fobbing or professional use

 

they are not meant to be for permanent fobbing or professional use

I guess you must've missed the words "cheap novelty" in my post.