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
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.
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
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.