Add dht11 w/ timer as an introduction to timers

Author: ColinKerr

05_interrupts_and_timers/05_04_dht11_timer.md

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+# DHT11 Temperature and Humidity sensor using a timer
+
+In [Project 3.6](../03_sensors/03_06_hygrothermograph_dht11.md) a DHT11 is used to measure temperature and humidity every 2 seconds.  To do so the program contains an infinite loop that checks the temperature then sleeps for two seconds.  This is inconvenient when your program is doing something else, an alternative solution is to use a timer to check the temperature at a regular time interval without sleeping.
+
+## New Concepts
+
+- Timer
+
+### Timer
+
+A MicroPython class that controls the hardware timers of the microcontroller.  They can execute a callback once after a given amount of time has passed or repeatedly once every period of time.  
+
+Use `mode=Timer.PERIODIC` to run callback multiple times and `mode=Timer.ONE_SHOT` to run the callback once.
+
+For more info see [MicroPython Timer documentation](https://docs.micropython.org/en/latest/library/machine.Timer.html)
+
+> NOTE: The ESP32-S3 has 4 timers, so only 4 can be active at a time.
+
+*Circuit and components are identical to Project 3.6*
+
+## Component List
+
+![Components](../images/03_06_components.png)
+
+## Circuit
+
+> Disconnect all power before building the circuit. Reconnect once verified.
+
+### Wiring Diagram
+
+![Wiring Diagram](../images/03_06_wiring_diagram.png)
+
+**Connections:**
+- DHT11 VCC → 3.3V (through a 10kΩ pull-up to the SDA line)
+- DHT11 SDA → GPIO21
+- DHT11 NC → not connected
+- DHT11 GND → GND
+
+### Schematic Diagram
+
+![Schematic Diagram](../images/03_06_schematic_diagram.png)
+
+## Code
+
+```python
+from machine import Pin, Timer
+import dht
+
+DHT = dht.DHT11(Pin(21))
+
+def getTemperature(t):
+    DHT.measure()
+    print('temperature:',DHT.temperature(),'humidity:',DHT.humidity())
+
+timer = Timer(0)
+timer.init(mode=Timer.PERIODIC, period=2000, callback=getTemperature)
+```
+
+## Code Explanation
+
+This code is very similar to the code from [Project 3.6](../03_sensors/03_06_hygrothermograph_dht11.md#code-explanation).
+
+The code to take a measurement from the DHT11 sensor is moved from the main loop in Project 3.6 to a function that can be used as a callback.
+
+```python
+def getTemperature(t):
+    DHT.measure()
+    print('temperature:',DHT.temperature(),'humidity:',DHT.humidity())
+```
+
+Create a timer and initialize it as a periodic timer with a period of 2000ms and `getTemperature` as the callback function.  This configures the timer to run for ever and call the `getTemperature` callback every 2000ms.
+
+```python
+timer = Timer(0)
+timer.init(mode=Timer.PERIODIC, period=2000, callback=getTemperature)
+```
+
+> Notice there is no `while True:` loop, it is not necessary because the timer runs until `deinit` is called on the timer or the microcontroller is reset.
+
+## Key Concepts
+
+- **Hardware Timers**: Timers run in hardware on the microcontroller providing a small number of high precision timers that have low or no runtime overhead to other code running on the microcontroller
+- **Periodic**: Something that happens over and over again at a fixed time interval.
+
+## Further Exploration
+
+- Adjust the period to make temperature measurements more frequent.  Can you make it so frequent that it fails?

05_interrupts_and_timers/code/Hygrothermograph_timer.py

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+from machine import Pin, Timer
+import dht
+
+DHT = dht.DHT11(Pin(21))
+
+def getTemperature(t):
+    DHT.measure()
+    print('temperature:',DHT.temperature(),'humidity:',DHT.humidity())
+
+timer = Timer(0)
+timer.init(mode=Timer.PERIODIC, period=2000, callback=getTemperature)
+
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