# Assignment: MQTT with button and LED

## Goals

* Get aquainted with MQTT.
* Publish a message when a button is pressed.
* Subscribe to a topic to control the LED

## Preparation

* Install the *PubSubClient* library. Use the library manager under
  *Sketch -> Include library -> Library Manager* to install it.

* Open http://www.hivemq.com/demos/websocket-client/ and subscribe to
  `ndc/#` or `ndc/$deviceId/#`.

## Step 1

Wire up this schematic on the bread board:

![](schematic/mqtt-with-button_schem.pdf)

![](schematic/mqtt-with-button_bb.pdf)

# Step 2

* Read button's value inside `loop()`. If the button's state changes,
  print a message.

*Note:* reading the button in a busy loop is not really a best
practice as it uses lots of energy. Instead use the `attachInterrupt`.

# Step 3

* Connect to the Wi-Fi network
* Connect to MQTT broker. Use `broker.hivemq.com:1883` as host and
  port.

See the slides for example code.

# Step 4

* Create a subscription for `ndc/$device-id/#` with HiveMQ's web ui:
  http://www.hivemq.com/demos/websocket-client/
* Publish a message when the button is pressed on the topic
  `ndc/$device-id/button`

# Step 5 (Bonus)

*Feel free to be creative here.*

The other useful half of MQTT is subscribing to topics and reacting to
messages. Here are some example things you can do:

* Subscribe to the topic you're publishing to and change the state of
  the LED when the button is pressed. This will give you a very
  complicated and brittle way of toggling a LED.

* In `loop` blink the led. Subscribe to another topic (for example
  `ndc/$device-id/frequency`), and use the value as a way to control
  the blink rate.

# Step 6 (Strech goal)

Use a last will message to indicate if the device is online or not.

* When connecting, include a last will message with topic
  `ndc/$device-id/online` and payload `0`.
* After a successfull connection, publish a similar message with the
  payload `1`. Observe what happens when you unplug the device.