Question 1

Do these lights need a electronical source to turn on?  I'm wanting to use these on a cake without wires.

Accepted Answer

Yes they will need a Batteronical thing (Loopacell CR1225) on the side of the cake and wires connecting to the LEDs.  If you want to turn them off remove the batteronical source. If the button shaped CR1225 gets lost inside the cake then at least one of the party people will po*p it out pretty soon.

Question 2

what size resistor should be used with 1 LED and a supply voltage of 5V?

Accepted Answer

How to choose a resistor for your LED:

First you need to know two things about your LED.
1. The Forward Voltage
2. The operating current

A typical LED will have a forward voltage of 2-3 Volts and an operating current of about 20mA (or 0.02A)

Next, you need to know the voltage of your power source.

Once you know these three things, it is just a little bit of simple math to identify the value of the resistor that you need.

Step 1, subtract the forward voltage of your LED from the voltage of your power source.
Step 2, divide the value you derived in step 1 by the operating current of your LED.  This value that you get is the value of the resistor that you need.

Lets walk through an example:

Here are the values of our hypothetical LED and voltage source.

LED:
Forward Voltage: 2V
Operating Current: 20mA

Power Source:
5V

Here is the math:

Step 1, subtract the forward voltage of the LED (2V) from the voltage of the power source (5V).

5V - 2V = 3V

Step 2, divide the value you derived in step 1 (3V) by the operating current of your LED (20mA).

Note, you need to convert mA to A for this equation.  To convert from mA to A, just move the decimal place three places to the left.  20mA converts to 0.02A

3/0.02 = 150

This means that you need a 150 Ohm resistor for this LED.

Question 3

Are the "purple" ones actually ultra-violet? They seem very dim but I notice the security strip of a $20 bill starts glowing under them...

Accepted Answer

Ultra-violet is not visible. Wavelengths spans 10 nanometers (nm) to 400 nm. The strips are tuned to  UV but near frequencies, e.g. violet, also work but not as well.

Question 4

what size resistor do I need with a 16v power supply?

Accepted Answer

LED's in parallel need resistors for each parallel circuit since the voltage drop for a given current is slightly different for each diode--one of the diodes would be stressed much more than the other diodes if they are directly in parallel.  The maximum current should not be greater than 20 ma, so 10 ma might be a more reasonable design goal. 
Depending on which color diode is selected, voltage drops may be between 1.8 Volts and 3.0 volts. 
Four 3.0 volt diodes in series would drop 12 volts leaving 4 volts for the resistor.  10 ma through a 480 ohm resistor would be 4.8 volts, so that should keep the current through the 3-volt diodes slightly under 10 ma.  
Four 1.8 volt diodes would drop 7.2 volts leaving 8.8 volts for the resistor.  10 ma through a 1 kohm resistor would be 10 volts, so that would keep the current through the 1.8-volt diodes under 10 ma.

Brand	EBOOT
Manufacturer	eBoot
Part Number	LN0986
Item Weight	0.634 ounces
Package Dimensions	5.43 x 3.11 x 0.59 inches
Country of Origin	China
Item model number	LN0986
Color	Blue
Material	copper
Batteries Included?	No
Batteries Required?	No

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EBOOT 100 Pieces Clear LED Light Emitting Diodes Bulb LED Lamp, 5 mm (Blue)

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