LED Help

Discussion in 'FAQs' started by Play-Doh, Dec 14, 2006.

  1. Play-Doh

    Play-Doh Member

    Im currently scratchbuilding a very cool gas station. I bought a couple of light up signs and put one aside for the parking lot. They look like this:


    For the other one, I removed the sign from the post and removed the LED. What im doing with it is fixing it to the front of the gas station so I can have two light up signs.

    Now for my probelm.

    I dont know anything about LCDs. I mean, I could hooke them up to my transformer, but its packed up in storage right now. It went into storage with the 4x8 layout.

    Is there a way for me to test this LED with a battery? I dont have alot of tools or anything, but if there is a way for me to test this at home id like to know. Im willing to take the risk of shocking myself!

  2. ezdays

    ezdays Out AZ way

    Is there a current-limiting resistor is series (attached to one leg) with the LED? if not, you should be able to test it using just a single flashlight battery. They normally take between 2 and 3 volts, the battery is 1.5 volts and should light it, but not quite as bright. If there's a resistor in series, you might need more than one battery to get the voltage up, so try a 9 volt cell. You shouldn't have to worry in either case about shock since they can't deliver that much current.

    Remember that if the LED doesn't light, you might have the polarity reversed, so switch leads going to the LED. Also, if you put too much voltage across the LED, it will burn out very quickly.:rolleyes:
  3. hooknlad

    hooknlad Member

    AA Battery

    Hi ya TJ - To test a LED with a battery, simply get a "AA" battery, 1.5 volts and connect the correct polarity to the anode and cathode of the LED.. LED's are polarity dependant. If the polarity if wrong, the LED will not light, unless its a bi-color LED. OK i hit ya there with some terms, The Anode , + terminal of the LED would be the terminal closest to the rounded part of the LED, and the Cathode "-" terminal of the Led would be the terminal with the "flat edge" of the LED.
    Also the shortest lead is the Cathode, "-", negative terminal ( black) and the longer lead is the Anode "+" terminal ( red)...
    LEDs usually run nicely on 1.5 volts. If you plan on using the AA battery, 1.5 volts, a current limiting resistor isn't necessary, but suggested with all other batteries.. A button battery would be idea, not the one the size of a quarter, but the ones the size of a pencil eraser would work as well. It does not matter which side of the LED you place the resistor.
    I hope I got this right for you. Good Luck.

    Don - you beat me again - How are you doing these days my friend?
  4. Play-Doh

    Play-Doh Member

    Hey Guys

    THanks for your help but im still having no luck.

    Heres what ive done

    I tried your advice on two different LEDs, now these are both brand new. I tried it on both a AA and D cell with no luck, I switched the polarity on both of em and no luck there.

    Now, im dealing with two wires, one red, and one yellow. Im not suer who these are made by.

    I held one end of the bare wire to one end and the other bare wire to the other end, and vice versa.

    Am I doing something wrong? These LEDs are brand new out of the package so I dont think they are damaged.

    Sorry for the confusion, I apprecieate all your help, im just retarted when it comes to this stuff!

  5. hooknlad

    hooknlad Member

    TJ - do you have a ohm meter available? The + lead of the meter will go to the anode and the black lead will go to the Cathode... The ohm meter will only read in one direction, the other will not. If there a resistor inline with the Led, the ohm meter will tell you what the resistance is.
    Another idea - if you believe that the LED is dead on arrival, then it also could not hurt to connect a 9 volt battery to the LED. It will get bright quick if it works, it may / may not work after you do this.
    Just trying to help out. if it does lite, you still have yet one more good one.. I have yet to come across a bad LED out of the package.
    Are the AA or 1.5 volt batteries fresh???????
  6. Play-Doh

    Play-Doh Member

    Sadly I dont have an OHM meter right now. Im gonna go try a few things...Ill report back!

    Thanks for the quick reply, i really do apprecieate it!
  7. ezdays

    ezdays Out AZ way

    I suspect that there's a resistor in series with the LED. In that case, a single 1.5 volt battery won't light it. Try a 9 volt battery and see what that does. I'm guessing there's probably around 1000 to 2200 ohms in series to allow it to light at 12 volts DC. That's just a guess now.
  8. Play-Doh

    Play-Doh Member

    Thanks guys! Got it figured out thanks to you!

    And im an idiot...I should have read your posts more clearly...the AA is only throwin out 1.5 volts...I hooked up three and it works!

    And heres a preview of a scratchbuild to come! This is the roof of the storefront! Its a fuzzy camera photo but it is backed against styrene clapboard and inlayed inside the trim. I simply removed it from the post, filed it down, glued it in place and drilled a hole through the back. I will hide the pole and wires when the roof is built.

    Thanks again guys! As always, your the best!

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