This is a prototype / proof of concept for this project: https://github.com/faeisanxious/IoT-Weather-Station
This project is a simple internet-enabled Weather Station, built using Raspberry Pi Pico W microcontroller. The uC opens up an HTTP server, on port 80, to serve / display a simple web page with data from the temperature sensor.
(Pictures available in /doc directory)
Pico W uses the lwIP stack as the networking stack for its CYW43439 chip.
The lwIP stack uses HTTPd daemon to set up an HTTP server on port 80 on the chip's
network interface.
The webpage can be just any regular old HTML code with CSS styles and JavaScript
code, but it's important to remember that everything regarding any given webpage (like
CSS styles) needs to be embedded into that webpage (html file). That is because the html
files need to be "transpiled" from human-readable text into C's strings, i.e. char arrays,
put into a special .c source file and included into a project so that the web page's
raw data can be put onto the uC storage. The "makefsdata.py" (in src/helpers/) is the python script
that transpiles the .html files from the html directory into the html_data.c file (the
file is supposed to be ignored by .gitignore but was included for demonstration
purposes).
The HTTPd daemon uses CGI (Common Gate Interface) to process user's requests. In this
project the CGI is used to display 3 pages, like: the default-home page when accessing the
server with either no tag (just pure address of the uC) or with the "/home" tag. The
second tag used by the set up CGI is "/debug" tag, that tells the server to return a
simple web page with debug informations about the device. There is also a "/details"
page w/ summary of the info (temperature readings) that were taken by the uC.
The HTTPd daemon uses basic SSIs (Server-Side Includes) to fill out the HTML pages with
desired info. In this project the SSIs are used to fill out informations that can only be
obtained at run-time, e.g. temperature readings.
The software that manages the "hardware" functionalities can be grouped into different
functionalities:
- reading the temperature from sensor: Reading the temperature is as simple as reading an analog voltage value using on-board A/D converter on specific GPIO pin.
- reading the temperature from the on-board thermometer: same as above, but instead of using a GPIO pin, internal connections are used.
- reading the battery voltage level: Also uses A/D converter :)
- singalling LOC (Loss of Connection): When the uC observes that it lost connection to the Wifi network, it will light up an LED. Simple gpio_put(value) is used for driving the pin into high / low logical level
The circuit is a rather simple one. Between the battery and the rest of the circuit is a
switch that controls the power delivery (ON/ OFF).
The temperature sensor outputs an analog voltage value on its Vout pin that corresponds
to the read temperature, following this formula: T = (V - 0.5) * 100
The LED is not directly driven by the uC but rather a low-side switch made w/ a simple
NPN BJT, that is controlled by the uC. RPI Pico W is more than capable of delivering
enough current on its GPIO's output to light an LED, but it's not necesserily the most
safe nor recommended aproach. Thus: the low-side switch.
RPI Pico's connection are also rather self-explanatory:
- VSYS and GND are just: POWER IN and GND (common) inputs - "power bus" if you will :)
- ADC_REF pin is shorted to 3.3 V output as it is the A/D converters reference voltage level. It can't be connected to the "positive terminal" of the battery, because when the battery's output voltage would drop, the referenced value would also drop, therefore the readings regarding battery's voltage output would always equal 3.2 V (nominal value for my batteries)
- AGND - A/D converter's GND pin
- ADC1 / ADC0 - A/D converter's channel 1 and 0 inputs
The batteries for this device are: two parallel-connected two-slot battery baskets. Therefore
the device should have total capacity of around 2*2000mAh = 4000mAh @ 3.2 V.
The circuit draws an average of 84 mA while working. Considering an average AA battery w/
2000 mAh capacity, the device can work up to 47.6 hours.
As the "brain" of the project a Raspberry Pi Pico W was chosen. It is a rather inexpensive ARM Cortex M0+ microcontrolled with its CYW4339 chip that enables wireless connectivity. The uC was chosen over others as it is Raspberry Pi Foundation's device, thus it has a very expansive and well-done documentation as well as rather simple and powerful SDK.
For the temperature sensor TMP36 was used as it came with the starting kit that I've bought some time ago, and I don't have an oscilloscope at home to play around w/ I2C etc. devices.