Add MCP3008 and potentiometer example

docs/recipes.md

@@ -343,3 +343,36 @@ sleep(5)
 left_motor.off()
 right_motor.off()
 ```
+
+## Potentiometer
+
+Continually print the value of a potentiometer (values between 0 and 1023):
+
+```python
+from gpiozero import MCP3008
+
+while True:
+    with MCP3008(channel=0) as pot:
+        print(pot.read())
+```
+
+## Full Colour LED controlled by 3 Potentiometers
+
+Wire up three potentiometers (for red, green and blue) and use each of their values to make up the colour of the LED:
+
+```python
+from gpiozero import RGBLED, MCP3008
+
+def read_pot(channel):
+    with MCP3008(channel=channel) as pot:
+        return 100 * pot.read() / 1023
+
+led = RGBLED(red=2, green=3, blue=4)
+
+while True:
+    red = read_pot(0)
+    green = read_pot(1)
+    blue = read_pot(2)
+    led.rgb = (red, green, blue)
+    print(red, green, blue)
+```

gpiozero/__init__.py

@@ -11,6 +11,7 @@ from .input_devices import (
     MotionSensor,
     LightSensor,
     TemperatureSensor,
+    MCP3008,
 )
 from .output_devices import (
     OutputDevice,
@@ -29,4 +30,3 @@ from .boards import (
     FishDish,
     TrafficHat,
 )
-

gpiozero/input_devices.py

@@ -8,6 +8,7 @@ from threading import Event
 
 from RPi import GPIO
 from w1thermsensor import W1ThermSensor
+from spidev import SpiDev
 
 from .devices import GPIODeviceError, GPIODeviceClosed, GPIODevice, GPIOQueue
 
@@ -346,3 +347,32 @@ class TemperatureSensor(W1ThermSensor):
     @property
     def value(self):
         return self.get_temperature()
+
+
+class MCP3008(object):
+    """
+    MCP3008 ADC (Analogue-to-Digital converter).
+    """
+    def __init__(self, bus=0, device=0, channel=0):
+        self.bus = bus
+        self.device = device
+        self.channel = channel
+        self.spi = SpiDev()
+
+    def __enter__(self):
+        self.open()
+        return self
+
+    def open(self):
+        self.spi.open(self.bus, self.device)
+
+    def read(self):
+        adc = self.spi.xfer2([1, (8 + self.channel) << 4, 0])
+        data = ((adc[1] & 3) << 8) + adc[2]
+        return data
+
+    def __exit__(self, type, value, traceback):
+        self.close()
+
+    def close(self):
+        self.spi.close()

setup.py

@@ -21,6 +21,7 @@ setup(
     install_requires=[
         "RPi.GPIO",
         "w1thermsensor",
+        "spidev",
     ],
     long_description=read('README.rst'),
     classifiers=[