diff --git a/gpiozero/__init__.py b/gpiozero/__init__.py
index 255a83e..e4c0d48 100644
--- a/gpiozero/__init__.py
+++ b/gpiozero/__init__.py
@@ -19,6 +19,7 @@ from .exc import (
     BadEventHandler,
     CompositeDeviceError,
     CompositeDeviceBadName,
+    CompositeDeviceBadOrder,
     SPIError,
     SPIBadArgs,
     EnergenieSocketMissing,
diff --git a/gpiozero/compat.py b/gpiozero/compat.py
index b21d0d3..8387038 100644
--- a/gpiozero/compat.py
+++ b/gpiozero/compat.py
@@ -49,5 +49,5 @@ def median(data):
         return data[n // 2]
     else:
         i = n // 2
-        return (data[n - 1] + data[n]) / 2
+        return (data[i - 1] + data[i]) / 2
 
diff --git a/gpiozero/devices.py b/gpiozero/devices.py
index 5493664..c4cbd90 100644
--- a/gpiozero/devices.py
+++ b/gpiozero/devices.py
@@ -21,6 +21,8 @@ from .mixins import (
     )
 from .exc import (
     DeviceClosed,
+    CompositeDeviceBadName,
+    CompositeDeviceBadOrder,
     GPIOPinMissing,
     GPIOPinInUse,
     GPIODeviceClosed,
@@ -191,7 +193,7 @@ class GPIOBase(GPIOMeta(nstr('GPIOBase'), (), {})):
         method). Once a device is closed you can no longer use any other
         methods or properties to control or query the device.
         """
-        return False
+        raise NotImplementedError
 
     def _check_open(self):
         if self.closed:
@@ -223,7 +225,7 @@ class Device(ValuesMixin, GPIOBase):
         ranges (e.g. -1 to +1) and composite devices usually use tuples to
         return the states of all their subordinate components.
         """
-        return 0
+        raise NotImplementedError
 
     @property
     def is_active(self):
@@ -261,8 +263,8 @@ class CompositeDevice(Device):
         if self._order is None:
             self._order = kwargs.keys()
         self._order = tuple(self._order)
-        for missing_name in set(self._order) - set(kwargs.keys()):
-            raise ValueError('%s missing from _order' % missing_name)
+        for missing_name in set(kwargs.keys()) - set(self._order):
+            raise CompositeDeviceBadOrder('%s missing from _order' % missing_name)
         super(CompositeDevice, self).__init__()
         for name in set(self._order) & set(dir(self)):
             raise CompositeDeviceBadName('%s is a reserved name' % name)
@@ -372,9 +374,6 @@ class GPIODevice(Device):
             self._check_open()
             raise
 
-    def _fire_events(self):
-        pass
-
     def close(self):
         super(GPIODevice, self).close()
         with _PINS_LOCK:
diff --git a/gpiozero/exc.py b/gpiozero/exc.py
index 96ab9a3..77e8e6d 100644
--- a/gpiozero/exc.py
+++ b/gpiozero/exc.py
@@ -22,6 +22,9 @@ class CompositeDeviceError(GPIOZeroError):
 class CompositeDeviceBadName(CompositeDeviceError, ValueError):
     "Error raised when a composite device is constructed with a reserved name"
 
+class CompositeDeviceBadOrder(CompositeDeviceError, ValueError):
+    "Error raised when a composite device is constructed with an incomplete order"
+
 class EnergenieSocketMissing(CompositeDeviceError, ValueError):
     "Error raised when socket number is not specified"
 
diff --git a/gpiozero/input_devices.py b/gpiozero/input_devices.py
index 11c325c..fc08de5 100644
--- a/gpiozero/input_devices.py
+++ b/gpiozero/input_devices.py
@@ -11,7 +11,7 @@ import warnings
 from time import sleep, time
 from threading import Event
 
-from .exc import InputDeviceError, GPIODeviceError, GPIODeviceClosed
+from .exc import InputDeviceError, GPIODeviceError, DeviceClosed
 from .devices import GPIODevice, CompositeDevice
 from .mixins import GPIOQueue, EventsMixin
 
@@ -161,7 +161,7 @@ class SmoothedInputDevice(EventsMixin, InputDevice):
     def __repr__(self):
         try:
             self._check_open()
-        except GPIODeviceClosed:
+        except DeviceClosed:
             return super(SmoothedInputDevice, self).__repr__()
         else:
             if self.partial or self._queue.full.wait(0):
@@ -649,7 +649,9 @@ class DistanceSensor(SmoothedInputDevice):
         return self.pin
 
     def _read(self):
-        # Make sure the echo event is clear
+        # Make sure the echo pin is low then ensure the echo event is clear
+        while self.pin.state:
+            sleep(0.00001)
         self._echo.clear()
         # Fire the trigger
         self._trigger.pin.state = True
diff --git a/gpiozero/pins/mock.py b/gpiozero/pins/mock.py
index 6cb0a0b..53b1023 100644
--- a/gpiozero/pins/mock.py
+++ b/gpiozero/pins/mock.py
@@ -8,7 +8,7 @@ str = type('')
 
 
 from collections import namedtuple
-from time import time
+from time import time, sleep
 from threading import Thread, Event
 try:
     from math import isclose
@@ -182,7 +182,7 @@ class MockChargingPin(MockPin):
     """
     def __init__(self, number):
         super(MockChargingPin, self).__init__()
-        self.charge_time = 0.01
+        self.charge_time = 0.01 # dark charging time
         self._charge_stop = Event()
         self._charge_thread = None
 
@@ -193,16 +193,51 @@ class MockChargingPin(MockPin):
                 self._charge_stop.set()
                 self._charge_thread.join()
             self._charge_stop.clear()
-            self._charge_thread = Thread(target=lambda: self._charged)
+            self._charge_thread = Thread(target=self._charge)
             self._charge_thread.start()
         elif value == 'output':
-            if self.charge_thread:
+            if self._charge_thread:
                 self._charge_stop.set()
                 self._charge_thread.join()
 
-    def _charged(self):
+    def _charge(self):
         if not self._charge_stop.wait(self.charge_time):
-            self.drive_high()
+            try:
+                self.drive_high()
+            except AssertionError:
+                # Charging pins are typically flipped between input and output
+                # repeatedly; if another thread has already flipped us to
+                # output ignore the assertion-error resulting from attempting
+                # to drive the pin high
+                pass
+
+
+class MockTriggerPin(MockPin):
+    """
+    This derivative of :class:`MockPin` is intended to be used with another
+    :class:`MockPin` to emulate a distance sensor. Set :attr:`echo_pin` to the
+    corresponding pin instance. When this pin is driven high it will trigger
+    the echo pin to drive high for the echo time.
+    """
+    def __init__(self, number):
+        super(MockTriggerPin, self).__init__()
+        self.echo_pin = None
+        self.echo_time = 0.04 # longest echo time
+        self._echo_thread = None
+
+    def _set_state(self, value):
+        super(MockTriggerPin, self)._set_state(value)
+        if value:
+            if self._echo_thread:
+                self._echo_thread.join()
+            self._echo_thread = Thread(target=self._echo)
+            self._echo_thread.start()
+
+    def _echo(self):
+        sleep(0.001)
+        self.echo_pin.drive_high()
+        sleep(self.echo_time)
+        self.echo_pin.drive_low()
 
 
 class MockPWMPin(MockPin):
diff --git a/gpiozero/source_tools.py b/gpiozero/source_tools.py
index 4aa1b51..c7f6847 100644
--- a/gpiozero/source_tools.py
+++ b/gpiozero/source_tools.py
@@ -16,7 +16,7 @@ try:
 except ImportError:
     pass
 from itertools import count, cycle
-from math import sin, cos, floor
+from math import sin, cos, floor, radians
 try:
     from statistics import mean
 except ImportError:
@@ -270,7 +270,7 @@ def sin_values():
     If you require a wider range than 0 to 1, see :func:`scaled`.
     """
     for d in cycle(range(360)):
-        yield sin(d)
+        yield sin(radians(d))
 
 
 def cos_values():
@@ -292,5 +292,5 @@ def cos_values():
     If you require a wider range than 0 to 1, see :func:`scaled`.
     """
     for d in cycle(range(360)):
-        yield cos(d)
+        yield cos(radians(d))
 
diff --git a/gpiozero/threads.py b/gpiozero/threads.py
index 805212d..8a07b8a 100644
--- a/gpiozero/threads.py
+++ b/gpiozero/threads.py
@@ -23,8 +23,8 @@ def _threads_shutdown():
 
 class GPIOThread(Thread):
     def __init__(self, group=None, target=None, name=None, args=(), kwargs={}):
-        super(GPIOThread, self).__init__(group, target, name, args, kwargs)
         self.stopping = Event()
+        super(GPIOThread, self).__init__(group, target, name, args, kwargs)
         self.daemon = True
 
     def start(self):
diff --git a/tests/test_compat.py b/tests/test_compat.py
new file mode 100644
index 0000000..a3e6e28
--- /dev/null
+++ b/tests/test_compat.py
@@ -0,0 +1,148 @@
+from __future__ import (
+    unicode_literals,
+    absolute_import,
+    print_function,
+    division,
+    )
+str = type('')
+
+
+import pytest
+import random
+
+from gpiozero.compat import *
+
+
+# ported from the official test cases; see
+# https://github.com/python/cpython/blob/master/Lib/test/test_math.py for original
+
+NAN = float('nan')
+INF = float('inf')
+NINF = float('-inf')
+
+def test_isclose_negative_tolerances():
+    with pytest.raises(ValueError):
+        isclose(1, 1, rel_tol=-1e-100)
+    with pytest.raises(ValueError):
+        isclose(1, 1, rel_tol=1e-100, abs_tol=-1e10)
+
+def test_isclose_identical():
+    examples = [
+        (2.0, 2.0),
+        (0.1e200, 0.1e200),
+        (1.123e-300, 1.123e-300),
+        (12345, 12345.0),
+        (0.0, -0.0),
+        (345678, 345678),
+        ]
+    for a, b in examples:
+        assert isclose(a, b, rel_tol=0.0, abs_tol=0.0)
+
+def test_isclose_eight_decimals():
+    examples = [
+        (1e8, 1e8 + 1),
+        (-1e-8, -1.000000009e-8),
+        (1.12345678, 1.12345679),
+        ]
+    for a, b in examples:
+        assert isclose(a, b, rel_tol=1e-8)
+        assert not isclose(a, b, rel_tol=1e-9)
+
+def test_isclose_near_zero():
+    examples = [1e-9, -1e-9, -1e-150]
+    for a in examples:
+        assert not isclose(a, 0.0, rel_tol=0.9)
+        assert isclose(a, 0.0, abs_tol=1e-8)
+
+def test_isclose_inf():
+    assert isclose(INF, INF)
+    assert isclose(INF, INF, abs_tol=0.0)
+    assert isclose(NINF, NINF)
+    assert isclose(NINF, NINF, abs_tol=0.0)
+
+def test_isclose_inf_ninf_nan():
+    examples = [
+        (NAN, NAN),
+        (NAN, 1e-100),
+        (1e-100, NAN),
+        (INF, NAN),
+        (NAN, INF),
+        (INF, NINF),
+        (INF, 1.0),
+        (1.0, INF),
+        (INF, 1e308),
+        (1e308, INF),
+        ]
+    for a, b in examples:
+        assert not isclose(a, b, abs_tol=0.999999999999999)
+
+def test_isclose_zero_tolerance():
+    examples = [
+        (1.0, 1.0),
+        (-3.4, -3.4),
+        (-1e-300, -1e-300),
+        ]
+    for a, b in examples:
+        assert isclose(a, b, rel_tol=0.0)
+    examples = [
+        (1.0, 1.000000000000001),
+        (0.99999999999999, 1.0),
+        (1.0e200, .999999999999999e200),
+        ]
+    for a, b in examples:
+        assert not isclose(a, b, rel_tol=0.0)
+
+def test_isclose_assymetry():
+    assert isclose(9, 10, rel_tol=0.1)
+    assert isclose(10, 9, rel_tol=0.1)
+
+def test_isclose_integers():
+    examples = [
+        (100000001, 100000000),
+        (123456789, 123456788),
+        ]
+    for a, b in examples:
+        assert isclose(a, b, rel_tol=1e-8)
+        assert not isclose(a, b, rel_tol=1e-9)
+
+# ported from the official test cases; see
+# https://github.com/python/cpython/blob/master/Lib/test/test_statistics.py for
+# original
+
+def test_mean():
+    examples = [
+        (4.8125, (0, 1, 2, 3, 3, 3, 4, 5, 5, 6, 7, 7, 7, 7, 8, 9)),
+        (22.015625, (17.25, 19.75, 20.0, 21.5, 21.75, 23.25, 25.125, 27.5)),
+        (INF, (1, 3, 5, 7, 9, INF)),
+        (NINF, (1, 3, 5, 7, 9, NINF)),
+        ]
+    for result, values in examples:
+        values = list(values)
+        random.shuffle(values)
+        assert mean(values) == result
+
+def test_mean_big_data():
+    c = 1e9
+    data = [3.4, 4.5, 4.9, 6.7, 6.8, 7.2, 8.0, 8.1, 9.4]
+    expected = mean(data) + c
+    assert expected != c
+    assert mean([x + c for x in data]) == expected
+
+def test_mean_doubled_data():
+    data = [random.uniform(-3, 5) for _ in range(1000)]
+    expected = mean(data)
+    actual = mean(data * 2)
+    assert isclose(expected, actual)
+
+def test_mean_empty():
+    with pytest.raises(ValueError):
+        mean(())
+
+def test_median():
+    assert median([1, 2, 3, 4, 5, 6]) == 3.5
+    assert median([1, 2, 3, 4, 5, 6, 9]) == 4
+
+def test_median_empty():
+    with pytest.raises(ValueError):
+        median(())
+
diff --git a/tests/test_devices.py b/tests/test_devices.py
index 43b3b92..1db7fab 100644
--- a/tests/test_devices.py
+++ b/tests/test_devices.py
@@ -68,3 +68,60 @@ def test_device_repr_after_close():
     device.close()
     assert repr(device) == '<gpiozero.GPIODevice object closed>'
 
+def test_device_unknown_attr():
+    pin = MockPin(2)
+    device = GPIODevice(pin)
+    with pytest.raises(AttributeError):
+        device.foo = 1
+
+def test_device_context_manager():
+    pin = MockPin(2)
+    with GPIODevice(pin) as device:
+        assert not device.closed
+    assert device.closed
+
+def test_composite_device_sequence():
+    device = CompositeDevice(
+        InputDevice(MockPin(2)),
+        InputDevice(MockPin(3))
+        )
+    assert len(device) == 2
+    assert device[0].pin.number == 2
+    assert device[1].pin.number == 3
+    assert device.tuple._fields == ('_0', '_1')
+
+def test_composite_device_values():
+    device = CompositeDevice(
+        InputDevice(MockPin(2)),
+        InputDevice(MockPin(3))
+        )
+    assert device.value == (0, 0)
+    assert not device.is_active
+    device[0].pin.drive_high()
+    assert device.value == (1, 0)
+    assert device.is_active
+
+def test_composite_device_named():
+    device = CompositeDevice(
+        foo=InputDevice(MockPin(2)),
+        bar=InputDevice(MockPin(3)),
+        _order=('foo', 'bar')
+        )
+    assert device.tuple._fields == ('foo', 'bar')
+    assert device.value == (0, 0)
+    assert not device.is_active
+
+def test_composite_device_bad_init():
+    with pytest.raises(ValueError):
+        CompositeDevice(foo=1, bar=2, _order=('foo',))
+    with pytest.raises(ValueError):
+        CompositeDevice(close=1)
+
+def test_composite_device_read_only():
+    device = CompositeDevice(
+        foo=InputDevice(MockPin(2)),
+        bar=InputDevice(MockPin(3))
+        )
+    with pytest.raises(AttributeError):
+        device.foo = 1
+
diff --git a/tests/test_inputs.py b/tests/test_inputs.py
index ec01e56..1e92879 100644
--- a/tests/test_inputs.py
+++ b/tests/test_inputs.py
@@ -7,11 +7,16 @@ from __future__ import (
 str = type('')
 
 
+import sys
 import pytest
-import mock
 from threading import Event
 
-from gpiozero.pins.mock import MockPin, MockPulledUpPin, MockChargingPin
+from gpiozero.pins.mock import (
+    MockPin,
+    MockPulledUpPin,
+    MockChargingPin,
+    MockTriggerPin,
+    )
 from gpiozero import *
 
 
@@ -136,12 +141,43 @@ def test_input_motion_sensor():
     assert sensor.wait_for_no_motion(1)
     assert not sensor.motion_detected
 
-@pytest.mark.skipif(True, reason='Freezes')
+@pytest.mark.skipif(hasattr(sys, 'pypy_version_info'),
+                    reason='timing is too random on pypy')
 def test_input_light_sensor():
     pin = MockChargingPin(2)
     sensor = LightSensor(pin)
-    pin.charge_time = 1
-    assert not sensor.light_detected
-    pin.charge_time = 0
-    assert sensor.light_detected
+    pin.charge_time = 0.1
+    assert sensor.wait_for_dark(1)
+    pin.charge_time = 0.0
+    assert sensor.wait_for_light(1)
+
+@pytest.mark.skipif(hasattr(sys, 'pypy_version_info'),
+                    reason='timing is too random on pypy')
+def test_input_distance_sensor():
+    echo_pin = MockPin(2)
+    trig_pin = MockTriggerPin(3)
+    trig_pin.echo_pin = echo_pin
+    trig_pin.echo_time = 0.02
+    with pytest.raises(ValueError):
+        DistanceSensor(echo_pin, trig_pin, max_distance=-1)
+    # normal queue len is large (because the sensor is *really* jittery) but
+    # we want quick tests and we've got precisely controlled pins :)
+    sensor = DistanceSensor(echo_pin, trig_pin, queue_len=5, max_distance=1)
+    assert sensor.max_distance == 1
+    assert sensor.trigger is trig_pin
+    assert sensor.echo is echo_pin
+    assert sensor.wait_for_out_of_range(1)
+    assert not sensor.in_range
+    assert sensor.distance == 1.0 # should be waay before max-distance so this should work
+    trig_pin.echo_time = 0.0
+    assert sensor.wait_for_in_range(1)
+    assert sensor.in_range
+    assert sensor.distance < sensor.threshold_distance # depending on speed of machine, may not reach 0 here
+    sensor.threshold_distance = 0.1
+    assert sensor.threshold_distance == 0.1
+    with pytest.raises(ValueError):
+        sensor.max_distance = -1
+    sensor.max_distance = 20
+    assert sensor.max_distance == 20
+    assert sensor.threshold_distance == 0.1
 
diff --git a/tests/test_source_tools.py b/tests/test_source_tools.py
new file mode 100644
index 0000000..9ba32cb
--- /dev/null
+++ b/tests/test_source_tools.py
@@ -0,0 +1,88 @@
+from __future__ import (
+    unicode_literals,
+    absolute_import,
+    print_function,
+    division,
+    )
+str = type('')
+
+
+import pytest
+from math import sin, cos, radians
+from time import time
+
+from gpiozero import *
+try:
+    from math import isclose
+except ImportError:
+    from gpiozero.compat import isclose
+
+
+def test_negated():
+    assert list(negated(())) == []
+    assert list(negated((True, True, False, False))) == [False, False, True, True]
+
+def test_inverted():
+    assert list(inverted(())) == []
+    assert list(inverted((1, 0, 0.1, 0.5))) == [0, 1, 0.9, 0.5]
+
+def test_scaled():
+    assert list(scaled((0, 1, 0.5, 0.1), 0, 1)) == [0, 1, 0.5, 0.1]
+    assert list(scaled((0, 1, 0.5, 0.1), -1, 1)) == [-1, 1, 0.0, -0.8]
+
+def test_clamped():
+    assert list(clamped((-1, 0, 1, 2))) == [0, 0, 1, 1]
+
+def test_quantized():
+    assert list(quantized((0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1), 4)) == [
+            0.0, 0.0, 0.0, 0.25, 0.25, 0.5, 0.5, 0.5, 0.75, 0.75, 1.0]
+    assert list(quantized((0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1), 5)) == [
+            0.0, 0.0, 0.2, 0.2, 0.4, 0.4, 0.6, 0.6, 0.8, 0.8, 1.0]
+
+def test_conjunction():
+    assert list(conjunction(())) == []
+    assert list(conjunction((False, True))) == [False, True]
+    assert list(conjunction((0, 1, 0, 1), (0, 0, 0, 1))) == [0, 0, 0, 1]
+
+def test_disjunction():
+    assert list(disjunction(())) == []
+    assert list(disjunction((False, True))) == [False, True]
+    assert list(disjunction((0, 1, 0, 1), (0, 0, 0, 1))) == [0, 1, 0, 1]
+
+def test_averaged():
+    assert list(averaged(())) == []
+    assert list(averaged((0, 0.5, 1), (1, 1, 1))) == [0.5, 0.75, 1]
+
+def test_queued():
+    assert list(queued((1, 2, 3, 4, 5), 5)) == [1]
+    assert list(queued((1, 2, 3, 4, 5, 6), 5)) == [1, 2]
+
+def test_pre_delayed():
+    start = time()
+    for v in pre_delayed((0, 0, 0), 0.01):
+        assert v == 0
+        assert time() - start >= 0.01
+        start = time()
+
+def test_post_delayed():
+    start = time()
+    for v in post_delayed((1, 2, 2), 0.01):
+        if v == 1:
+            assert time() - start < 0.01
+        else:
+            assert time() - start >= 0.01
+        start = time()
+    assert time() - start >= 0.01
+
+def test_random_values():
+    for i, v in zip(range(1000), random_values()):
+        assert 0 <= v <= 1
+
+def test_sin_values():
+    for i, v in zip(range(1000), sin_values()):
+        assert isclose(v, sin(radians(i)), abs_tol=1e-9)
+
+def test_cos_values():
+    for i, v in zip(range(1000), cos_values()):
+        assert isclose(v, cos(radians(i)), abs_tol=1e-9)
+