Noob's LED Tester

Introduction: Noob's LED Tester

I find myself, my friends and many other people trying to use mind blowing methods to test an LED.
Many people think that applying 2V or 3V to an LED is easiest way to test it. Perhaps, but not the safest way !

We have, have to limit the current through LED. LED is current controlled device for Electronic's sake !

If you have an insight of Digital Multimeter, you know that it has a Constant Current source when its on LED test function.
This is the best way to test an LED, provide a constant current low enough not to blow it.

Now, sometimes I can't think of a simple solution. I initially designed a "Constant Current Source using LM7805", which can be also made with LM317. And then I halted for a moment, all the 7805 or 317 would do is provide a constant DC voltage. Then its the RESISTOR who limits the current, duh.

So why not to go for a simple way ? Two 1.5V AA batteries (Full Stop).
And that's it, this is not rocket science ! Hence I named it as Noob's LED Tester.

But, seriously, I find it very very handy to test LEDs, even IR LEDs.
Let's begin and have this thing done in less than 10 minutes.

Step 1: Remembering Ohm's Law and Grocery List

The whole and sole of this or any LED circuit is a Resistor.

R = V / I

(Courtesy: George Ohm.)

Now, typically LEDs draw 15 to 20 mA of current. Bright LEDs may draw more but they show indication of life with low current as well.

So I decided to have current limited to 10 mA, the safer way.

Now as I am using two 1.5 V AA batteries in series, I have 3 V of supply voltage.
For 10mA,

R = 3 V / 10 mA = 300 Ohm.

This R is in series with the LED under test. When LED is on, since its a diode we can consider it as a Closed Switch with R = 0.
So Total R will be always 300 +/- a few Ohms.

" This circuit will never provide current more than 10 mA (Full Stop). "

330 Ohm is the resistor I have used. Quarter watt would go without a hitch but for again on the safer side, I have added 1 W.

Blueprint attached is a top level secret. Please respect the secrecy !

Here's what we need:
1. 2 AA Battery Holder
2. Two small banana plug terminal mounts
3. A 330 Ohm, 1W resistor. Carbon Composition.
4. A box to house everything.

Total is less than 2$.

Plus we need common tools: Soldering gun, solder, wires and wire cutter, etc.

Step 2: First Step = Last Step

What should I put in here ? Just see the pictures !

Yes, since we are using this for testing and that too with maximum 10 mA current I think batteries will last until their expiray date probably :)

I hope this comes handy for you too.

Tip: While checking an Infrared LED, check if it shows violet/blue light in a any kind of Digital camera. IR is not visible to human eyes.

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    5 Discussions


    8 years ago on Introduction

    I think you need to take into account the voltage drop across the LED!

    When using a red LED, it drops about 1.5-1.8V. That leaves about 1.5V to drop over your resistor, giving you a 5mA current. That should be just enough to light it.

    If you use a white LED with a 3-3.5V fwd voltage you have hardly any voltage drop left for your resistor. At most, with new batteries you might get 0.5V allowing less than 2mA. With anything sort of brand new batteries you are not going to light the LED with this.

    Oddly, in order to test all colours of LED you want a high voltage and a big resistor so that the difference between a 1.2V IR LED and a 3.8 V UV LED is comparatively insignificant. You need to watch the power rating, of the resistor if you get too high, however.

    If we stick to 1/4 W resistors, and want 10 mA max, we can allow a voltage drop of 0.25/0.01 = 25V. OK, no issue there. So let's use the highest common DC source - 12V DC.

    Allowing for the smallest LED forward voltage of say 1V (IR LED) we want or resistor to drop 11V at 10 mA. 11/0.01 = 1100 Ohms. 1.1K, which we can get. In practice 1K would give 11 mA, which is fine too.

    Checking the current for a nearly 4V UV LED, we would have 8V left for our resistor, giving a healthy 8mA through a 1K or 7.2 mA through a 1K1.

    It's pretty inefficient but it's safe and it's going to light whatever LED you put in its way.


    Reply 7 years ago on Introduction

    I completely agree with you.
    But when it is about testing LEDs, a series resistor is the simple solution.
    I am sorry that my instructable isn't a universal LED tester.


    Reply 7 years ago on Introduction

    I wasn't saying that a series resistor was not a good solution. Just that you don't get 10mA with your device because you assume no voltage drop across the LED.

    Also for a more universal tester you would be better with a higher voltage & higher resistor.

    Even so, it's a nice project. Just that it could be more broadly applicable with only a minor change (e.g. a 9v battery and a about a 500-800 ohm resistor).


    8 years ago on Introduction

    You can make a constant current source with a LM317 ...
    Have a look at this:
    (awesome website btw !)


    Reply 7 years ago on Introduction

    Yes, constant current source is a good option. But its not really required, series resistor is a simple solution.