PicoInk

PicoInk was developed to measure physical quantities in locations where frequent measurements are not necessary, and a single measurement every 15 minutes is sufficient. For example, temperature, water level, CO2 concentration, soil moisture, air humidity, and similar parameters. Due to this nature, the design emphasizes long battery life. Unlike similar devices that use the deep-sleep mode of the microcontroller, periodic measurements here are managed by an external timing circuit, the TPL5110, which has an almost negligible consumption (35 nA) during its countdown phase.

Another feature contributing to long battery life is the e-ink display. By its technical nature, an e-ink display retains its content even when the power supply is disconnected. Normally, the display must undergo a so-called full refresh when powered on. A full refresh is not aesthetically pleasing and is also energy-intensive. These reasons led to the development of a display driver library capable of partial updates without requiring a full refresh.

The horizontal resolution of the e-ink display is 250 pixels. When using the display to show a graph of measured values, with each value occupying one pixel, it is possible to display a history of nearly three days, specifically 62.5 hours. This makes PicoInk a unique battery-powered thermometer capable of continuously displaying a graph of historical data.

Since the device's brain is a Raspberry Pi Pico W microcontroller, it offers connectivity options in the form of Bluetooth Low Energy and 2.4 GHz WiFi. For transmitting measured values via Bluetooth, the open BTHome protocol was chosen, using passive advertisement messages. This choice was made due to excellent compatibility and quick integration with Home Assistant. For WiFi connectivity, the MQTT protocol was chosen, which can also be easily integrated into Home Assistant.

The battery life of a Liitokala 3500mAh battery, when using an SHT41 temperature and humidity sensor with heating disabled and a WiFi connection with a strong signal, was measured at 6 months, corresponding to approximately 10,000 measurements. The battery is a rechargeable 18650 cell that can be charged via a USB-C connector. For quick replacement, the battery can be removed and swapped with a freshly charged one.

PicoInk consists of two physical parts:

The main unit: Contains the electronics, battery, display, and circuit board.
A detachable module: Depending on the module type, the device fulfills its purpose — the module houses the sensor.

The connection between these parts is ensured by a connector that allows the module to be powered either from the 3.3V microcontroller supply or directly from the battery. The connector also includes pins for analog measurements or communication buses such as UART, SPI, I2C, and OneWire (see documentation). PicoInk can also be used without the module, with software modifications, for tasks such as displaying online data.

Currently, two fully developed modules are available:

A temperature sensor for horizontal and vertical water heaters (bojlers).

PicoInk has also been tested and is software-ready for the following:

CO2 sensor SCD41: Battery life drops to one month (with WiFi).
Analog soil moisture sensor: Soil moisture changes very slowly, resulting in a short graph.
Temperature and humidity sensor DHT22: Measurement times vary and can sometimes be very long.
Temperature and humidity sensor SHT41: Everything works fine.
Temperature sensor Dallas DS18B20: Original sensors must be used; otherwise, everything works fine.