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Ultraviolet Business Card Flashlight

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Step 6: Attach the LED

Picture of Attach the LED
step6b.jpg
You will need to pull apart the legs of the LED a little, but trying to keep them roughly parallel.  They need to be about 5mm (1/4") apart, but this is not super critical.

Now push one leg underneath the cell (will go between the card and foam), and the other leg should go above the cell.

If you have done it correct, when you press down on the leg which is on top of the cell, the LED should glow purple.

If it doesn't glow at all, pull it out, rotate it 180 degrees (so the opposite leg is on the bottom), and put it back in.

If it glows all the time (without pushing), you will need to pull it out, spread the legs further apart, and put it back in again.
 
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Draeyoc2 years ago
Shouldn't there be a current limiting resistor in the circuit?
tomward (author)  Draeyoc2 years ago
Well spotted! Actually we are bending the rules here but it is quite justified for a couple of reasons:

1) Typical lithium cells of this size can't source enough current for long enough to kill the LEDs (usually quite a bit over 25mA)

2) Minimal current flows through an LED anyway until you overcome their 'forward voltage drop' which is usually a little over 3V for white, blue or UV LEDs. As this is less than the cell voltage (around 3V) then we get only a small amount of current (but then again, less light than we could otherwise)