Hardware Reference
In-Depth Information
Although the GPIO library is now installed in Python, it won't be loaded by default. Like
pygame
, the library needs to be explicitly imported into your program. To use the library,
start your program with
import RPi.GPIO as GPIO
at the top. You'll learn more about
this in the following examples.
The Raspberry Pi's GPIO port does not provide any protection against voltage spikes or electrical
shorts. Always make sure you've checked that your circuit is sound before connecting it to the
Pi. If possible, use an isolation board such as the Gertboard (discussed in Chapter 13, “Add-on
Boards”) to provide protection.
WARNING
Calculating Limiting Resistor Values
An LED needs a
current limiting resistor
to protect it from burning out. Without a resistor,
an LED will likely only work for a short time before failing and needing to be replaced.
Knowing a resistor is required is one thing, but it's also important to pick the right resistor
for the job. Too high a value and the LED will be extremely dim or fail to light at all; too low
a value and it will burn out.
To calculate the resistor value required, you will need to know the
forward current
of your
LED. This is the maximum current the LED can draw before being damaged, and is measured
in milliamps (mA). You'll also need to know the
forward voltage
of the LED. This latter value,
measured in volts, should be 3.3 V or lower—any higher, and the LED will require an external
power supply and a switching device known as a
transistor
before it will work with the Pi.
The easiest way to work out how large a resistor is required is with the formula
R=(V-
F)/I
, where
R
is resistance in ohms,
V
is the voltage applied to the LED,
F
is the forward
voltage of the LED and
I
is the maximum forward current of the LED in amps (with a thou-
sand mA to the amp).
Taking a typical red LED with a forward current of 25 mA and a forward voltage of 1.7 V, and
powering it using the 3.3 V supplied by the Pi's GPIO port, you can calculate the resistor needed
as
(3.3 - 1.7) / 0.025 = 64
. Thus, a resistor of 64
Ω
or higher will protect the LED.
These igures rarely come out to match the common resistor values as sold, so when you're
choosing a resistor, always round up to ensure the LED is protected. The nearest commonly
available value is 68
Ω
, which will adequately protect the LED.
If you don't know the forward voltage and forward current of your LEDs (for example, if the
LEDs did not come with documentation or were salvaged from scrap electronics), err on the
side of caution and it a reasonably large resistor. If the LED is too dim, you can revise
downwards—but it's impossible to repair an LED that has been blown.
