Merge pull request #124 from morganstanley/accelerometer

Add accelerometer exercise

site/content/exercises/circuitpython/robotics/E10/index.md

@@ -1,6 +1,6 @@
 ---
 template: exercise
-title: Robot Assembly
+title: Crickit and Motors
 level: 4
 exercise: 10
 category: Robotics

site/content/exercises/circuitpython/robotics/E11/index.md

@@ -0,0 +1,10 @@
+---
+template: exercise
+title: Robot Assembly
+level: 4
+exercise: 11
+category: Robotics
+tags: ['CircuitPython']
+---
+
+Coming soon!

site/content/exercises/circuitpython/robotics/E9/index.md

@@ -1,10 +1,86 @@
 ---
 template: exercise
-title: Crickit and Motors
+title: Accelerometer
 level: 4
 exercise: 9
 category: Robotics
 tags: ['CircuitPython']
 ---
 
-Coming soon!
+Both the Circuit Playground Express and Bluefruit contain an accelerometer, so the device can detect when it's moving and its orientation in three dimensions.
+
+The follow program reads acceleration along the x, y, and z axis, prints the values to the Serial console, and sets the red/green/blue values of the LEDs based on the x/y/z readings of the accelerometer.
+
+```python
+from adafruit_circuitplayground import cp
+import time
+
+while True:
+    if cp.switch:
+        print("Slide switch off!")
+        cp.pixels.fill((0, 0, 0))
+        time.sleep(1)
+    else:
+        R = 0
+        G = 0
+        B = 0
+        x, y, z = cp.acceleration
+
+        print((x, y, z))
+
+        R = R + abs(int(x))
+        G = G + abs(int(y))
+        B = B + abs(int(z))
+        cp.pixels.fill((R, G, B))
+        time.sleep(0.25)
+```
+
+### Challenge Problem
+
+With the above program running, tilt the device so that the value of x is 0 while the values of y and z are approximately equal. What degree angle is the device being held at?
+
+### Bluetooth Connection
+
+Our phones have accelerometers too - let's send the readings from our phone to the Bluefruit device via bluetooth. This program is almost identical to the previous bluethooth exercise, except now the Bluefruit device will receive packets of type `AccelerometerPacket` instead of `ColorPacket`.
+
+```python
+from adafruit_circuitplayground import cp
+
+from adafruit_bluefruit_connect.packet import Packet
+from adafruit_bluefruit_connect.accelerometer_packet import AccelerometerPacket
+
+from adafruit_ble import BLERadio
+from adafruit_ble.advertising.standard import ProvideServicesAdvertisement
+from adafruit_ble.services.nordic import UARTService
+
+ble = BLERadio()
+uart_service = UARTService()
+advertisement = ProvideServicesAdvertisement(uart_service)
+
+while True:
+    # Keep advertising until another device connects
+    ble.start_advertising(advertisement)
+    while not ble.connected:
+        pass
+    ble.stop_advertising()
+
+    # Keep listening for packets while connected
+    while ble.connected:
+        if uart_service.in_waiting:
+            packet = Packet.from_stream(uart_service)
+            if isinstance(packet, AccelerometerPacket):
+                x, y, z = packet.x, packet.y, packet.z
+                print((x, y, z))
+                R = 10 * abs(int(x))
+                G = 10 * abs(int(y))
+                B = 10 * abs(int(z))
+                cp.pixels.fill((R, G, B))
+```
+
+### References
+
+- [cp.acceleration](https://docs.circuitpython.org/projects/circuitplayground/en/latest/api.html#adafruit_circuitplayground.circuit_playground_base.CircuitPlaygroundBase.acceleration)
+
+- [Python: abs function](https://www.w3schools.com/python/ref_func_abs.asp)
+
+- [AccelerometerPacket](https://docs.circuitpython.org/projects/bluefruitconnect/en/latest/api.html#adafruit-bluefruit-connect-accelerometer-packet)