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I have a single AA battery, I used duck-tape to attach wires to - and +. I tried to attach wires directly to the legs of the LED (without resistor) to see if it burns out, just for playing around. But I didn't see any light. I changed plus minus on legs, still nothing.

I have few questions:

  • Is this because voltage of 1 AA battery is not enough? (I doubt this).
  • Is this because by attaching two thin wires with duck-tape, there is less current passing through? (I doubt this too).
  • Is LED dead? I tested few and none worked (how do I test LED, if it is alive or not? with multimeter? Is led like bulb, if current passes through it means it is fine?)

So, I know... this is the dumbest question ever asked. I have little experience with electronics, mostly from childhood, when I was taking toys apart instead of enjoying them :)

But I want to learn how to do things myself, I'm a software guy, and as great minds once said.. if you want to make great software you have to make hardware too :D:D But probably this is the future, I hope in a year I will get hold of things... plus this site is awesome.

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    \$\begingroup\$ most multimeters have a "diode testing" mode, which also works for LEDs and even makes them light up \$\endgroup\$ Apr 19, 2012 at 7:23
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    \$\begingroup\$ Depending on the type of LED, you may need 2 or 3 AA cells to turn it on. The voltage of a single AA cell is too low. Also in general, it's not a bad idea to get the habit of reading the datasheets for components. Doing that will solve problems such as this and make it possible for you to use more complex components. \$\endgroup\$
    – AndrejaKo
    Apr 19, 2012 at 7:28
  • \$\begingroup\$ Even if the LED was within voltage spec with one AA, it can burn out within ms and you'd never see it. \$\endgroup\$
    – tyblu
    Apr 19, 2012 at 11:45
  • \$\begingroup\$ Where do I find datasheet? it is yellow led, but have no other info. Just bought it from ebay HK and came without any description. \$\endgroup\$
    – user8539
    Apr 19, 2012 at 12:52

2 Answers 2

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Voltage required to operate an LED varies with colour and technology used but is typically around

  • 2 - 2.5 Volt for a red LED and
  • 3.0 - 3.7 Volt for a white LED.

As 1.5V is below the rated operating voltage at normal operating currents of any visible light LED, not seeing light when using 1.5V is very expected. If you use a red LED and apply 1.5V at correct polarity and look at it in total darkness you may see very low level light output. Turn it on and off and look for any change in output. I have found that White LEDs that are rated for operation at say 20 mA maximum will produce visible light "in the dark" at well under 0.1 mA.
[Infrared LEDs may operate at about 1.5V at rated current but unless you have IR vision or an IR sensor then not seeing light is also very expected :-). ]

If you had used 3V and a RED LED and no series resistor it is possible for it to die so fast that you see no light at all.
Operation without a resistor makes very little sense in almost all cases.

Here is a TLHE510 - a typical 5mm red LED

It has the advantage for comparison purposes of having Red, Green and Yellow versions with some parameters in common.Red & Yellow versions have apparently the same voltage-current curves, whicle green has a higher voltage drop at a given current. The green current curve has been extended to 1000 mA. As can be seen, operating any of these at 1.5V will not produce much light. If you'd used two batteries to try to get light out and not used a resistor the green LED would conduct about 90 mA and the red and yellow "rather more" (off the graph but fairly obviously excessive). Note that the green LED is rated to carry up to about 800 mA - BUT only for 10 us and only with a duty cycle of not more than 1:1000.

Absolute maximum forward current is 30 mA and recommended maximum operating current is 20 mA.

enter image description here Applying reverse polarity above the rated reverse breakdown voltage will kill most LEDs very rapidly even at very low currents - <= 100 uA is often given as max allowed reverse current and many power LEDs simply say "not rated for reverse voltage operation." . You need to check the relevant data sheet, but reverse breakdown voltage is generally greater than the rated forward operating voltage in continuous operation. Typically in the 5V - 7V range BUT check for your LED.



Information only:

The tape you refer to is generally called "duct tape", not "duck tape" although "duck tape" is a legitimate name as it originally used "cotton-duck cloth". BUT there is a Duck Tape brand of duct tape :-). Duct tape was originally designed in 1942 for quick repair of miltary equipment (its reputation is deserved ! :-) ) . Wikipedia - duct tape. Since then it has been used for every imaginable purspose and a few unimaginable ones - even things as diverse as helping solve the lives of the crew of lunar missions. Incidentally it is NOT recommended by its makers for patching air leaks in air conditioning ducts.

Yee Ha!:

enter image description here


How to save a lunar mission:

Wikipedia - Apollo 13 & duct tape

The real thing - not quite how it looked in the movie :

enter image description here

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"Is this because voltage of 1 AA battery is not enough? (I doubt this)."

Why? An AA battery will give you 1.5V for an Alkaline battery, 1.2V for a rechargeable NiMH. The voltage a LED takes depends largely on the color. A typical red LED will require around 2V. The only LEDs which may work at 1.5V are infrared.

Don't test a LED by using it out of spec. ALWAYS have a series resistor to limit the current. Otherwise the test may be a destructive test and then in the end you only know you have a defective LED.

LEDs are very sensitive to reverse polarization, a reverse voltage of 5V may kill it. In your case where you only have 1.5V maximum not a problem.

The duct-tape is not a problem, either the wires make contact and then you'll have milliohm contact resistance, or they don't make contact at all. If you're serious about playing with electronics I suggest you buy a soldering iron ASAP.

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