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 layout: project
 title: S.O.S.
 description: How do you know if the showers or toilet stalls down the hall are occupied? S.O.S., a Shower/Stall Occupancy System, allowed anyone on the local network to check a website when they wanted to know occupancy status. 
-start_date: 2016-01
+start_date: 2016-01-01
+end_date: 2018-06-01
 tags: arduino c++ python flask
 ---
 
-Description coming soon!
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+## Introduction
+
+S.O.S. was born out of a small, recurring issue I ran into while living in a dorm at MIT. Oftentimes, just when I stopped what I was doing to take a shower, I'd walk into the bathroom and both shower stalls were occupied. Sometimes, I'd even walk down the hall to the next bathroom and those showers, too, were occupied. I talked to a few hall mates about this and they also encountered the same problem frequently enough to mention it. 
+
+Inspired by the laundry machine server my dorm had (which showed which washers and dryers were in use) and bathroom occupancy signs on planes, I thought to myself: How could I build a system that would allow us to know whether the showers and toilets were in use?
+
+I got to work.
+
+## System design
+
+To design the system, I started with thinking about how I wanted people to access occupancy data. A few choices were:
+1. looking at signs on top of the bathroom door that can be seen when walking down the hall
+2. viewing an icon that sits either in your menu bar (on Macs) or task bar (on Windows) that shows status of favorited stalls
+3. going to a website
+
+Although I wanted to build all of them, I chose #3 for its simplicity. Additionally, the system I'd build for #3 was the most extensible and would allow me to build #1 and #2 down the road.
+
+My choice meant I needed a central node that receives occupancy data from all stalls and hosts a web server with a page displaying occupancy status, as well as several sensor nodes that gather occupancy data.
+
+## Electronics
+
+First, I needed to determine what sensors to use. While choosing the toilet stall sensors was straightforward (magnetic contact sensors), I recall having some trouble figuring out which sensors to use for the shower. I had a conversation with one particular hall mate about this, and she mentioned thinking about building a system like this once and told me to consider humidity sensors. We talked about possibly using some kind of water sensor, but
+1. I knew I wanted to try and avoid the electronics touching water, and
+2. noticed that a stall can still be occupied without water being present. (What if someone is putting on or taking off clothes?)
+
+Relying on sensor knowledge from high school robotics, I thought my best bet was to use motion sensors – specifically, passive infrared sensors.
+
+I then needed to determine what microcontroller would drive the sensor node. I was primarily looking for something cheap (just in case I needed to replace electronics due to water vapor seeping in) and something with Wi-Fi onboard. Luckily, I found the WeMos D1 Mini, a cheap ESP8266 board (at the time, around $2-$3 per board). Additionally, I decided I was going to use an MIT server to host the central node's web server.
+
+The rest of the components included water-tight boxes for shower sensor nodes, power cables, and basic circuit components (wire, resistors, etc).
+
+## Software
+
+Next, I needed to decide how to build the software layer. 
+
+### Central node
+
+I knew I wanted bi-directional communication between the central node and the sensor nodes, so I chose to use MQTT, a lightweight messaging protocol for Internet-of-Things devices. The web server itself was a Flask server. I used SocketIO for real-time updates between the web client and web server.
+
+### Sensor nodes
+
+The WeMos D1 Mini can be programmed using Arduino, so that's what I wrote C++ code in. The code was pretty simple. For setup, the sensor nodes just needed to connect to the local Wi-Fi network and MQTT daemon; in the main loop, it publishes occupancy status when the last sampled status is different from the current status.
+
+## Reflection
+
+- The hardest part was figuring out how long the power cables needed to be.
+- I think a fully built system would include the other two status access options I described in the beginning (a sign on top of the bathroom door and a small, out-of-the-way program that would show you the occupancy status of a favorited shower and toilet stall). Sadly, I didn't get to build these.
+
+## Links
+- Sensor node code: [https://github.com/njerig/sos-sensor-node](https://github.com/njerig/sos-sensor-node)
+- Boilerplate central node code: [https://github.com/njerig/flask-socketio-mqtt-server](https://github.com/njerig/flask-socketio-mqtt-server)
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