How I Put a Temperature Sensor on the NDN Testbed

The frontpage of my recently renovated website shows a room temperature reading of my apartment. This is not your average IoT project, but a showcase of how an IoT device can communicate directly over Named Data Networking (NDN). This article explains where does this temperature reading come from, how is it turned into NDN Data packets, how the sensor appears on the NDN testbed, and how the temperature reading shows up on the website.

room temperature display on frontpage

Sensor Hardware

If you have been reading my posts, you may have guessed that the temperature reading comes from an ESP8266. Of course! It is actually my very first ESP8266, which I received from Losant IoT Inc. It comes with a TMP36 analog temperature sensor, and has been reporting my room temperature to Losant platform since May 2016.

Losant Builder Kit with all the additions

How to Change the MAC Address of ESP8266?

Each ESP8266, like every other WiFi network interface card, comes with a MAC address that identifies itself to the network. Sometimes you want to change the MAC address of an ESP8266. How to do that?

ESP8266 has the built-in MAC address

ESP8266 Arduino core does not provide an API to change ESP8266's WiFi MAC address. While there is a WiFi.macAddress function, it actually retrieves the current WiFi MAC address, instead of setting it. However, Espressif SDK offers an API to change the WiFi STA MAC address:

Show Temperature and Humidity as WiFi SSID with ESP8266 and HTU21D Sensor

Since I started playing with ESP8266 WiFi microcontroller in 2016, I had a TMP36 temperature sensor. TMP36 is an analog sensor: it uses a voltage between 0.00V and 1.75V to represent a temperature reading between -50℃ and 125℃. The ESP8266 has an analog-to-digital converter (ADC) capable of reading voltages up to 1.00V. To read a temperature from TMP36 into ESP8266, I need to use a pair of resistors as a voltage divider, so that the resulting voltage does not exceed ADC's limit. However, I feel the temperature reading is very inaccurate: is my room really 19℃ when @TheTucsonHeat is in town? I once replaced the carbon resistors with metal film resistors, and the temperature reading instantly changed by as much as 8℃.

I need an upgrade to the temperature sensor! The new sensor must output digital signal, so that my lousy resistors wouldn't affect its accuracy. After comparing DHT11, DHT22, DS18B20, and several others, I eventually chose HTU21D-F because it has an I2C interface, which can be added directly to my LCD kit.

The HTU21D sensor arrives in the mail a few weeks later. It needs a bit of soldering to add the pin headers onto the breakout board; ImmodderNation has a soldering video to get you started. Afterwards, wiring is simple: just add it into the existing I2C bus! As long as you don't short the wires, you won't burn down the office.

HTU21D on I2C bus of ESP8266

A quick test with Adafruit's library confirms the HTU21D sensor is working correctly. Temperature and humidity readings are showing up on the serial console. However, I don't want to tug around the laptop to measure temperature around the house, and don't want to program the LCD or connect to Losant or NDN right away. I thought up a quick and easy way: let's create a Wi-Fi hotspot from the ESP8266, and show temperature and humidity as the WiFi network name (SSID)!

"freewifi" via ESP8266 Captive Portal

Many outdoor places do not have permanent Wi-Fi access points. Occasionally I can get a weak unencrypted WiFi signal from a nearby shop; otherwise, I'll have to face the fact of not having WiFi, and resort to my slow and expensive SIM card for cellular Internet access.

Since I learned that the ESP8266 can serve as a WiFi hotspot, I got an idea. I can make the ESP8266 as a Wi-Fi access point (AP), and provide free WiFi to everyone at the outdoor venue. Except that, this is a freewifi prank: I am providing free WiFi, but my WiFi does not offer Internet access.


ESP8266 makes a freewifi WiFi access point SSID:

freewifi SSID

ndnping Jewelry on ESP8266

I was wearing a unique piece of jewelry at NDN community meeting, Mar 2017: a pair of ESP8266 units that communicate with each other over the NDN testbed. They are ugly, but it is a nice way to demonstrate my creation in a Named Data Networking community meeting.

Two Witty Cloud boards are tied to my wrists, and powered by a USB powerbank in my pocket. One of them runs a ndnping client, and the other runs a ndnping server. The client sends Interests to a router in Arizona, the Interests (under a multicast prefix) are flooded through the testbed, and reach the server which is connected to a router in Memphis.

Arduino Code

Library: esp8266ndn

Access C.H.I.P UART Wirelessly with ESP8266

A year ago, a Kickstarter campaign CHIP - The World's First Nine Dollar Computer caught my attention: it's a $9 computer smaller than a banana. Unlikely the Raspberry Pi, it comes with onboard storage so I don't need to buy a separate SD card, it has WiFi instead of wired Ethernet so I don't have to run wires everywhere, and it is compatible with my existing VGA monitor through a $10 adaptor so I don't have to buy another HDMI monitor. Therefore, I snapped two of these little computer along with one VGA adapter during the campaign.

During the whole year of waiting, Next Thing Co sends me regular email updates on the development progress, with each email ending with mmmtc (much much more to come) and a lot of hearts. NTC also clarified that C.H.I.P is strictly B.Y.O.B. Finally, my pair of CHIPs and a VGA DIP arrived in my mailbox on Jun 16. An hour later, homepage is displayed on its Debian desktop.

A few more hours later, I start to discover a limitation of C.H.I.P software: The Linux kernel comes with CHIP operating system has very limited features.

$ sudo modprobe fuse
modprobe: FATAL: Module fuse not found.

Obviously, the solution to this problem is to compile my own Linux kernel with more features. The compilation can be done on the C.H.I.P itself. I managed to do that when the CHIP is powered by a 5V 1A phone charger plus a 1500mAh LiPo battery. I had the compilation running under screen(1) and attended to it intermittently, and finished in a day.

Do Evil with ESP8266: Slow Down the WiFi

While hackers do good most of the time, we occasionally do evil and play a prank. The ESP8266, unlike JSON, allows me to do evil. Thus, I programmed the microcontroller for an evil purpose: slow down the WiFi.

How it Works

802.11 WiFi typically operates in infrastructure mode, where a router acts as an access point, and other hosts (stations) connect to the router on a wireless frequency (a channel).

One property of the wireless channel is that, at any moment, only one party (station or access point) can be transmitting. If multiple senders are transmitting at the same time, the wireless signal will be jammed, and the recipient is unlikely to receive the packet correctly. In this case, the sender would have to transmit the packet again at a later time.

Packets can be transmitted at different speeds on the wireless channel. With 802.11g standard, the maximum speed is 54Mbps, and the minimum is as slow as 1Mbps. The sender (station or access point) dynamically chooses a speed for every packet depending on its perception of wireless channel quality. Usually, we prefer to transmit at a higher speed, so that the wireless channel can be freed as soon as possible for other senders to use. However, if the sender and recipient are far apart, high speed transmission is less likely to succeed because signals can be faded, and a slower speed is necessary to increase the chance of a successful transmission.