Build your own (cheap) LED tester

[peteski]

Since this subject comes up from time to time I thought I would show the tester I built about 40 years ago, and which I still use.

All you need is a fresh 9V battery, a dead 9V battery, a 1000 ohm (1k ohm) resistor, a piece of copper clad PC board, and (optional) 4" piece of wire.

Take the dead battery apart and extract the top piece with the terminals.

Cut a piece of PC board, about 0.45" x 0.4" and using a hobby knife, or a triangular needle file cut/file a gap in the copper cladding in the center of the PC board.  In this example I used a file

Using 330 or 400 grit sandpaper sand the copper areas slightly to flatten the cladding raised slightly when cutting/filing the gap, and to clean the entire copper clad area.

Put some flux on the copper areas and then using a soldering iron tin them completely.  This is done because bare copper oxidizes easily, but tin stays cleaner.

Solder one side of the PC board to one of the battery terminals, then solder the 1k resistor between the other area of copper and the other battery terminal.  See photos to see how it should be done.

Optionally add 1 or 2 wire leads for when it is not convenient to touch the LED to the PC board.  I just added a single wire lead.\

That's it - you have a simple LED tester for all types of LEDs  It will show you if the LED works and identify its polarity.







Here it is shown used on a 3mm LED.


And tiny SMD LEDs can also be tested.  Just place the LED (using tweezers) across the gap in the copper clad areas.

[peteski]

Sir, your posts like this are why you are such a valuable asset to this community.
Thanks !
ed

Thank you Ed!

Hey Pete, I see you made the tester 40 years ago.  Have you had to change the battery yet?

Actually I have - multiple times. But not because the battery get completely discharged, but because it is so old that I'm afraid it will start leaking. 
When I replace batteries (like in smoke alarms) which still have some life left in them, I use them in the LED tester. The battery voltage is not critical, and the current used is negligible, so they easily last until the next batch of half-used batteries is available.

[davefoxx]

Dumb question of the day: "How does it show the polarity of the LED?" 

DFF

[MK]

Dumb question of the day: "How does it show the polarity of the LED?" 

DFF

LEDs are essentially diodes so if you reversed the polarity it just won't light up.

[peteski]

LEDs are essentially diodes so if you reversed the polarity it just won't light up.

What he said. 

Since you know the battery's polarity, when the LED lights up, you know which of its leads or pads are positive and negative.  If you hook it up in reverse, the voltage is too low to damage the LED, so you're safe.

[davefoxx]

I told you that it was a dumb question, but thanks, fellas!

DFF

[Lemosteam]

Wait till you see the doozy LED brightness tester @peteski had me make one year.  I still have not figured out how to measure the resulting resistance to know what resistor to use to achieve the brightness, but it does work... 

I add some pics tomorrow...

[Sokramiketes]

Add a dollar and you can also clamp it to 20mA no matter the input voltage.  (Well, up to 100v anyway...)

https://www.ngineering.com/CL2.pdf

If you want to blow $11, Ngineering has one that clips onto a 9V battery as well.  They call it intelligent. 
https://www.ngineering.com/other_cool_tools.htm

[peteski]

No need for a fancy current limiter.  Besides, 20mA is way too much for most of our applications.  Many white SMD LEDs have operating current of 5mA. It would make the White LEDs way too bright!! The 1k resistor resistor already limits the current to much lower value anyway.

Even if short circuited at the LED testing terminal (and with fresh battery @ 9V) the current will be 9V /1000 ohms = 0.009A (or 9mA).
With white, blue or true green LEDs (Vf ~ 3V) the current will be (9V - 3V) / 1000 ohms = 0.006A (or 6mA).
With other color LEDs (Vf ~ 2V) the current will be (9V - 2V) / 1000 ohms = 0.007A (or 7mA).
These currents are closer to the currents that will be used in real-world applications.  Then as the battery voltage goes down, the current twill be even lower.

As shows above, figuring out the LED current is not rocket science - just basic math.

[RBrodzinsky]

As shows above, figuring out the LED current is not rocket science - just basic math.

But isn’t rocket science just basic math (ok, I admit, I consider anything below Advanced Diff Eq and Boundary Value Problems “basic math”). 

[davefoxx]

But isn’t rocket science just basic math (ok, I admit, I consider anything below Advanced Diff Eq and Boundary Value Problems “basic math”). 

And just think what @GaryHinshaw thinks is "just basic math." 

[DKS]

If you'd like something a bit more sophisticated, consider: http://davidksmith.com/modeling/project-3.htm

The advantage of this design is that there's no math needed at all. Just dial up the voltage and resistance until the LED is at the desired brightness, and read them off the label.

And you can easily add one of those test pads Peteski made and just glue it to the box.

It's also useful for testing motors as well...

 

[Joetrain59]

Ooh, I like that David. Any chance of making some to sell here?
 Joe D

[mighalpern]

David:
Sweet and simple

I would buy a few so I could give these out to my math /LED challenged Round Robin group, And definitely one for Me 

Happy Holidays
Miguel
 

[peteski]

David:
Sweet and simple

As I see it mine is sweet and simple. Simple go-no-go and polarity check.    David's is also sweet, but a wee more complex.