Spoiler: All you need to do is connect a potentiometer in series with the LED and measure the current. Adjust the potentiometer until the light intensity and current are as you want them – say 20mA.

Then measure the voltage dropped on the LED pins and you’ll have the LED’s forward voltage.

Now you have all the necessary information to calculate the limiting resistors and other parameters of your LED circuit.

All the details are explained in the article below. Happy reading!


There may be times when you want to use one or more LEDs in your project, but even though you have a ton of LEDs on your hands, there’s no way to find a datasheet because you perhaps purchased them from a local store that didn’t have a datasheet to give you, or you got them from a friend, or you’ve just had them for a very long time, and now you can’t remember their original source. And if you don’t know where you got them from, there’s no other way to find their datasheet because usually there are no markings on most LEDs. A datasheet would be helpful if you want to find out the following:

  • Forward Voltage
  • Forward Current
  • Reverse Voltage
  • Reverse Current

I’ve covered the first two in the LED Basics article, and these two are the ones that you need in order to actually use the LED.

The Reverse Voltage and Current are helpful to know when you’re designing more advanced circuits where in some cases there may be a reverse voltage applied on the LED.

You don’t want to apply a reverse voltage higher than the specified one to a Light Emitting Diode because that’s the voltage at which the LED will be destroyed. This is sometimes called the Breakdown Voltage. And if you reach this breakdown voltage, a small current will flow through the LED in reverse, and this is – you got it – the Reverse Current.

Now what this means is that you need to know the Forward Voltage and Forward Current in order to use a LED in your project, and you need the Reverse ones only if there’s a chance that in your circuit there will, at some point, be a possibility for a reverse voltage being over the LED. This may happen, for example, in some multiplexing LED arrays, depending on the arrangement. Anyway, back to the two pieces of information that you actually need: Forward Voltage and Forward Current.



LEDs in a box

A good way to organize your LEDs is in a small box with multiple compartments.

This is how I store most of my LEDs.

I try to group them by specifications and when I buy another batch of LEDs, if their specs are not provided, I just test them and put them in the right compartment, with other similar LEDs.

Finding the Voltage and Current for a LED without a datasheet

It turns out that it’s actually very easy to find out these two pieces of information of an unknown LED that you have and want to use. Therefore, even if you don’t have a datasheet, there’s no problem, you can still use your LEDs in the way that they were designed. The bare minimum tools that you need are:

  1. A multimeter that can measure volts and milliamps.
  2. A potentiometer (variable resistor)

For this article, I’m using a few more tools. You can see all of them, including the LEDs in the image at the top, but I’ll enumerate all items here anyway:

  1. Multimeter
  2. Breadboard
  3. The LEDs that I want to test, of course.
  4. Some wires to make the connections on the breadboard
  5. Potentiometer (10KOhm variable resistor). 10K is just a random one that I had close by. A lower value would be easier to use, so if you have an 1K or 5K one, use that one.
  6. Step-up boost converter. This is a small assembly that boosts the voltage up, so I can have, for example, 20V even though my battery can only provide 9V. This is an optional item, and is only useful in some very specific cases, so if you don’t have one or don’t want to build one, don’t worry.


How to actually use these items to find the LED’s specs

On a breadboard, here’s what you will need to do:

Find LED specs using a simple potentiometer

A LED’s Forward Voltage and Current can be found out using a simple potentiometer on a breadboard

  1. Connect a battery to the breadboard’s positive and negative rails.
  2. Place a potentiometer on the breadboard and connect it as shown in the image:
    1. Connect the two outermost pins to the positive and respectively negative rails.
    2. Connect the center pin of the potentiometer to another row on the breadboard.
    3. Turn the knob all the way to the maximum resistance (towards the most negative pin).
      1. If unsure, measure the resistance between the center pin and the one that is connected to the positive rail. It should be at the maximum value possible on this resistor.
  3. Place a LED on the breadboard as shown in the image:
    1. The LED’s Anode (+) should be connected on the same row that the potentiometer’s center pin is connected to.
    2. The LED’s Cathode (-) should be connected through a wire to the negative rail
  4. Now slowly turn the potentiometer’s knob until the LED starts to light up, to verify that the circuit is working.

Ammeter measuring LED current

An Ammeter needs to be inserted in series with the component that you’re trying to measure



Multimeter set to measure DC current on the 200mA range

Typical labels for the 200mA DC range on a multimeter.

The Ammeter

The Ammeter is a device that measures electrical current. If you’ve never heard of an ammeter before, don’t worry. If you have a multimeter, set it to the 20mA or 200mA range for measuring DC (Direct Current) and insert it in your circuit as shown by the image.

To measure the current that passes through the LED, the multimeter needs to be connected in series with it.

Once you’ve made all the connections and set your multimeter to the correct range, start turning the potentiometer’s know, but very slow. You’ll notice the current slowly increasing as you keep turning the knob and at the same time you’ll also notice that the intensity of the light emitted by the LED increases.

Keep observing the intensity of the LED until you are happy with it. There’s really no point in going to the absolute limit of the LED after you’ve reached the desired intensity for the light.

For example, you may only want a low intensity light for a simple indicator LED, and you can achieve this with a few milliamps, no need to go up to 20 for example.

Also, you can figure out the approximate max current that you can run through the LED by observing when the light actually starts to dim or change color, although the LED that you are testing will probably be damaged at that point.

For example, if you LED starts to behave erratically at around 27mA, you can be sure that you have a 20mA LED.

Important: The LED will get quite hot when running it with more current than it was designed for, so make sure you keep your fingers away from it while doing this.

Congratulations! If you’ve done everything as described above, you now know the current that is needed by the LED in order to light up in the way that you want it to for your project. All you have to do is read the value from your multimeter’s display and you have it.

After you’ve carefully found out the current, you now have to find one more thing, and that is the forward voltage drop that the LED has when this current passes through it. this is quite simple to do.

First, disconnect the multimeter from the breadboard.

A multimeter set to the 20V DC range

A multimeter set to the 20V DC range

Voltmeter connected on the breadboard on the same lines as the L

A voltmeter has to be connected in parallel with the LED in order to measure the voltage drop across its terminals.

Now, with the multimeter disconnected, set it to its DC voltage range, 20V should be a good starting point.

After you’ve set this connect it to the breadboard in a way that its leads is connected directly to the LED pins – have a look at the image to the left, it will show you exactly how you have to connect the multimeter.

Now, as can be seen in the image, you also need to connect the center pin of the potentiometer to the Anode (+) of the LED.

Important: Make sure you don’t touch the potentiometer knob while doing this, otherwise you’ll have to start again from the beginning.

If all went well, you should now have a value displayed on your voltmeter screen. You can adjust the multimeter to a lower range to give you more precision, if the displayed value allows it – that is if you don’t have an autorange multimeter, which will happily do it for you automatically.

Well, the value that you have now is the forward voltage drop on the LED at the current that you measured at the previous step.

Now you can use these values to calculate your circuits, including the limiting resistors that you need for the LEDs that you’ve just tested.

This was easy, right?

I hope that his has helped you understand how to find out the specs of a LED that you have and most importantly, how and why it works.

Have fun with your projects!

Remember the other thing? The DC-DC Boost Converter.

For the purposes of finding LED specs, you might find at some point that you have some LEDs that don’t even get to a decent level of light by using your 9V battery or 5V supply alone. This is because there are LEDs that have an integrated resistor. These are called – of course – resistor LEDs.

The reason why a resistor is integrated inside a LED is so that it makes it very easy to use that LED at a specific voltage, for example 5V or 12V or maybe more.

So basically with a 12V resistor LED, what you would do is just connect it to a 12V supply and that’s it, it will work and limit the current correctly by itself, without the need of an external resistor.

So in the case of you having a 12V LED for example, you probably won’t be able to reach 10mA or 20mA of current, just because the LED can’t adjust is forward voltage higher than your battery is able to supply.

The Boost Converter to the rescue.

Step-up boost converter connected between battery and breadboard

In order to increase the voltage available to your breadboard circuit, use a step-up boost converter between the battery and the breadboard.

Well, where could you possibly get 12V if you don’t own an actual power supply or don’t have it available? Well, it turns out this is also simple. All you will have to do is use a boost converter, and insert it between your battery and the breadboard.

These devices usually have a + and – input and a + and – output, so all you have to do is connect your battery’s plus and minus to the boost converter’s plus and minus inputs and then the boost converter’s outputs – plus and minus – to your breadboard’s rails.

The below steps have to be taken once the multimeter is connected in the circuit to measure the current, exactly as described above.

Now, once you turn the potentiometer all the way to the minimum and still can’t get a decent amount of light from your LED, you will have to do the following things:

  1. Turn the potentiometer all the way to maximum resistance
  2. Connect the Boost Converter to your breadboard and make sure it’s on the lowest setting. They usually have a tiny potentiometer on them that adjusts the output voltage. Place your voltmeter’s leads on the boost converter’s outputs and turn the potentiometer on it all the way until you reach the lowest possible voltage.
  3. Turn your main potentiometer (the one on the breadboard) slowly until you reach somewhere around the center point.
  4. Slowly adjust the potentiometer on the Boost converter up in small increments until the light output is as desired  – be careful not to reach a point where the LED starts to get hot or changes its color, as that signals the fact that you’re over the LED’s current limit.
  5. Read the value of the current intensity displayed on your multimeter and proceed as described above to find the voltage drop using the DC Volt range on your multimeter.