Hardware Reference
In-Depth Information
Figure 8-2:
How to use
an LED.
You want to have 20mA lowing through the LED, and when you do there will be a voltage of
3.2V across the LED. his voltage is said to be
dropped across
the LED. So irst of, how do you
know the voltage dropped will be 3.2V? Well, you look it up in the LED's data sheet. If you
haven't got a data sheet, you guess because the voltage drop of an LED is determined mainly
by the material it is made of and the material determines the colour. So most LEDs with the
same colour have the same sort of voltage drop; 3.2V is typical for a green one and the newer
type of blue LEDs. Red LEDs have a much lower drop of typically 1.8V. So look at Figure 8-2:
If there is 3.2V across the LED, there has to be 5 - 3.2 = 1.8V across the resistor. If you know
