ESP8266 Call Button

It's NDN Community Meeting again and this time I'm officially presenting HomeCam at the demo session. I'm the sole presenter of this project. Normally, I have to stay at my table to show my project to the audience. However, I don't want to miss the chance of seeing what others have been doing. To solve this dilemma, I come up with an idea: a call button.

I put a big button on my table. It is labelled as: if I'm not here, press the button to send an Interest. Then, I wear a battery powered light ring on my body. Whenever someone presses the button, it lights up for 15 seconds.

me wearing a light ring at NDNcomm demo session

How It Works

Both the button and the light ring are based on ESP8266. The light ring unit acts as WiFi access point and NDN producer. The button unit acts as WiFi station and NDN consumer. When the button is pressed, the consumer transmits a signed Interest, and the producer turns on the light for 15 seconds after verifying the signature. Since the light ring unit is battery-powered, it enters deep sleep mode if there's no connected WiFi client.

Get Online in McDonald's with ESP8266

McDonald's used to be the largest chain of free restrooms in the world. Nowadays, they are the largest chain of free WiFi across the United States. Every McDonald's I've been to offers AT&T WiFi services. I can walk in, connect to "attwifi" on my phone, accept the license agreement, and I'm online.

attwifi captive portal in McDonald's

The license agreement page is called a captive portal, a web page displayed to newly connected users before they are granted broader access to network resource. While it's trivial to click through the captive portal on a smartphone, if you are wearing an ESP8266 as a connected jewelry, it would not connect successfully. When I encountered a captive portal in my apartment WiFi, I made an Arduino sketch to send the packets I found through Fiddler, but McDonald's WiFi is different:

  • Each McDonald's store hosts their captive portal on a different domain.
  • The form submission step needs several parameters from the HTML, but does not use cookies.

attwifi captive portal form HTML

How to Print uint64_t in Arduino

The Arduino programming language looks like C++, but it is weird in its own way: there's no C++ standard library. In C++, you can print a variable to the debug console with std::cerr << x;. In Arduino, you have to write a function call: Serial.print(x);. I love that the Streaming library brings back the familiar syntax Serial << x; through some template and macro magic. But when it comes to a uint64_t variable, nothing works!

error: call of overloaded 'print(uint64_t&)' is ambiguous

note: candidates are:
note: size_t Print::print(const __FlashStringHelper*) <near match>
note:   no known conversion for argument 1 from 'uint64_t {aka long long unsigned int}' to 'const __FlashStringHelper*'
note: size_t Print::print(const String&) <near match>
note:   no known conversion for argument 1 from 'uint64_t {aka long long unsigned int}' to 'const String&'

Simple Temperature and Humidity Indicator with ESP8266 and HTU21D Sensor

I decide to mix up a few existing toys to make a simple temperature and humidity indicator. ESP8266 reads temperature and humidity from an HTU21D sensor, and displays the numbers on a 4-digit 7-segment LED display. Since a 4-digit display isn't wide enough for both temperature and humidity, the display cycles between temperature, humidity, and a blank state.

temperature and humidity on LED display


Bill of materials

Moving Dot: How Many Displays Can You Fit on an ESP8266?

In's toy vault, there is an assortment of LED displays. I'm wondering, how many LED displays can I fit on an ESP8266? So I built this "moving dot" demonstration, with two LED displays and a buzzer.

moving dot demo

The LED matrix serves as the game board. A dot appears on the matrix. In each time step, the dot randomly moves by one pixel or stays in the same position. The 4-digit displays current time step number. Whenever the dot reach any of the four corners, the buzzer plays a piano note selected between C3 and B5.


Bill of materials

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)!