[Intro]


# Problem

The best humidity for an indoor space is 35--50%. But when indoor heating
is used, the air gets dry.  In the winter it can easily drop below 30%.
This can be pretty uncomfortable.  It irritates your skin and eyes,
and can lead to respiratory infections.


# Solution

The solution is a humidifier system.  It can be installed in campus'
HVAC systems to regulate the humidity.

Ideally it should be user-programmable, energy-efficient.  It should be
able to control humidity dynamically, so it uses a closed-loop control
system.


# Schematic

Installed in each room of the building is a microcontroller that
periodically measures the humidity of the room and sends it to a web
server.  It also sends the target humidity which is controlled by the
user.  There are two buttons, one to increase the target humidity and one
to lower it.  This whole enclosure with the microcontroller, humidity
sensor, and buttons, will be mounted on the wall like a thermostat.
It also has an OLED screen which shows the target humidity and the
measured humidity.

The server keeps track of the humidity measurements from each room and
calculates an average for the whole building.  It has a web control panel
that displays the current conditions in the building and a graph of the
humidity of each room over time.

The humidifier itself is stored in a back room somewhere, connected to the
HVAC ductwork.  It's a _fog type_ humidifier---it uses a high-pressure
water pump and a nozzle to inject tiny water droplets into the air.
The flow of water is controlled by a solenoid which is driven by PWM
from a microcontroller. It receives the measured and target humidity
from the server and uses a PID algorithm to determine how much water
to inject.  If the humidity is too low, it will increase the duty cycle
of the solenoid to bring the humidity back on-target.


# Assessment

Overall, I'm happy with the outcome of the project.  The system satisfies
all of the original goals and criteria.

It regulates humidity dynamically using a closed-loop PID algorithm with
feedback from the sensor in each room.

It's user-programmable with the up and down buttons.

It's easy to use.  The screen shows the target and actual humidity,
and the web dashboard can be used to monitor the system.

It's easy to install into an existing HVAC system on campus.

It's energy-efficient which also means it's cheap to run.

- Security [HTTPS?]

It's reliable.  The ESP32s are programmed to reconnect to WiFi if they
lose their connection.  The server can handle requests concurrently and
the internals are synchronized properly.  It does input validation on
all of the POST requests from the Arduinos.