# LED Resistor Finder

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This is a small resistor array I made so I can figure out what resistor to use with various LEDs at different voltages. They start at 2.2 ohms (at the top) and go up to 2.7 kilo-ohms (at the bottom) .Plug the LED in across the board and work your way up till you see a usable brightness for whatever you happen to be doing. For a group of LEDs you can arrange them at the base of the board and use jumper wires to probe the different resistors. You can see the different brightness of these two LEDs at different points on the array. If I try any lower resistance values I would probably end up destroying them. You don't test two points on the board at a time. This is just so you can see how they vary their output. These are all resistors that I have on hand so when I find what works I just grab another out of my parts pile. Not shown here is that I wrote all the values on the back of this breadboard in the order they are on the board. So here the 16th one down the board is very bright. On the back of the board I can see I have written that the 16th resistor is 82 ohms. Way too bright for an indicator light with 9 volts but suitable for a flashlight or toy.

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## Step 1: Bend

Bend your LEDs over something appropriate and snip off the legs. I use a small pair of pliers.

Ta Da!

## Step 3: Build the Array

The jumpers are the leftover resistor leg cut offs. Five band resistors are a real pain to read. If you read them left to right or right to left you can end up with two different values. Always check these stupid things with a meter even if you think you know what they are. Doesn't help that the colors are pretty close in shade to each other too. Is it yellow? orange? silver? grey? It's enough to drive a guy crazy. But for a few dollars I got a bunch of values so I can't complain too much. Also note that I skip every fifth row on the resistor side... which works out pretty good since my resistors are too big to line up row for row and the rail on the right skips one every five anyway.

## Step 4: Plug in Your Power

Here I am using 9 volts but plug in whatever suits your needs. This particular breadboard has continuous + and - rails on the sides of the board but some are split. Keep that in mind if you use more than the first half of the board. If you use more voltage you will also probably have to add higher resistor values. With a little poking you can find a resistor that lights up your LED an acceptable amount. Handy for salvaged LEDs or ones that you have no info on. LED calculators are great but if you don't have values to plug into them they are kinda useless. I hope you find this useful.

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## 14 Discussions

Leds are defined by their nominal current, which is in the data sheet with lots of other things such as zener voltage also called direct voltage. As you know both of them, the resistor "r" is equal to supply voltage, lets call it "V" minus zener voltage "z" divided par the nominal current. r = (V-z)/i where r is in ohm, V and z in volt and i in ampere. This is a sure way to calculate a resistor without risking any component failure.

Are you worried about short LED life (i.e., over voltage)?

I was always warned to avoid exceeding the nominal voltage... which I know is a big problem for unknown LEDs

Not really. You can see where the LED reaches it's maximum brightness then after that point the color degrades, for example yellow will start turning orange. So after it hits it's brightest point you back off a couple values and just use that. Honestly I think me de-soldering them and overheating them shortens their lives more than anything else. I have tested a few new LEDs to destruction as well just to see how they change as they get lower and lower values.

Thanks! You can use an adjustable potientiometer to find an appropriate resistance but you still have to measure the resistance and find a matching resistor. I think this is easier.

Thank you. Feel free to hit that vote button. Who knows what I could do with a laser!

Just make sure you sort your resistors by decending value and to start at the highest resistance. If you start at the low end you blow up your LEDs.

This beats using a potentiometer since you still have to measure the final resistance and try again with the nearest standard value to confirm it is the desired effect, with your idea, you will know this data.

Nice idea!

3 replies

Yes, I like this because it leads you right to a part you already have in stock since you build it with what you have. This is made with the lowest 25 values that came in a 50 value assortment. As you work your way up the values you begin to see the LED start glowing and stop when it is as bright as you like. With a 9 volt almost every value will make them glow. I will have to add a few more values. This is not the case with lower voltage power sources like 5 volts from USB or 6 volts from a 4 AA battery pack.

If i worked a lot with led's - which I don't- I'd probably make a dedicated p.c. board of this, it's just so common sense. ☺

Well it doesn't have to be just for LEDs. How many times have you read "Change the value of R1 to adjust the circut." ? A couple jumper wires and you're all set. I would make one out of capacitors too if I had a better assortment of parts.