5 volt or 12 volt LED
Assuming we are using a LED which does
NOT require the current limit resistor (one of the 5 volt or 12 volt versions)
the other leg can be connected to a + POSITIVE voltage source, not exceeding
that required by the LED, when the LED will light as there is a "built in
" limit resistor. However connecting such an LED round the wrong way is likely
to lead to failure as they do not like what is called a reverse voltage.
A standard LED requires a limit resistor.
Without having to go into the maths you can assume that a 1000 ohm resistor
(1K) will be ok for most voltage up to a maximum or 22 volts and not less
than 12 volts. On 22 volts a 1k resistor gives you a current flowing of 22mA
or for a 12 volt supply 12mA.
As a generalization 22mA is considered
a maximum that the LED likes but you can go into things more technically
and look up the data sheet for the LED in question.
There is a minimum voltage required else
the LED will not light which is about 3 volts.
The current limit resistor can be fitted
in series with either lead of the LED which should then be insulate with
sleeving to prevent a short circuit in use.
The flashing Led has not only a built
in resistor but also the circuitry to make it flash at about 1.5 flashes
per second. Minimum voltage about 5 volts and a current capability of about
22mA is required to operate the flashing LED.