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.
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.