METESP


A measurement station by Cedric Hölzl

Goal

Build an environment quality monitor.

Powered by FreeRTOS, a low power IOT device

Global System Design

ESP32

  • Low power, dual-core processor
  • C/C++ & FreeRTOS support
  • Wifi & BLE support

Sensors

  • BM280: temperature, humidity & pressure
  • SGP40: Volatile Organic Compounds (VOC)
  • LTR390-UV-1: UV
  • TSL25911: ambient light (infrared & visible)
  • PMSA003I : particle concentration by size

Touch E-ink Display

  • GT1151: Touch
  • Multiple views of live data
  • Low power consumption
  • User interaction

Open Weather Map

  • Exterior realtime data source
  • Free API access
  • +200'000 cities

Time-Series Database

  • Influx DB, open source database system
  • Time-Series optimized storage
  • Lightweight HTTP client

Network Time Protocol

  • Get time from the network
  • Compute & display local time
  • Keep it up-to-date using internal RTC

I2C

  • 2 wires: clock & data
  • Device have unique adress
  • Bi-directional, half-duplex
  • Simple protocol

I2C protocol

  • Select device with R/W capability
  • Select register
  • Send/Read data

Technical specs

  • 100kbps usually, up to multiple Mbps
  • 120 slave devices in 7 bit mode
  • Some devices have 10 bit adresses
  • Some devices have multi-byte register adresses

SPI

  • 4 wires: clock, master-data, slave-data, chip-select
  • Device have unique adress
  • Bi-directional, full duplex
  • More complex protocol

SPI protocol

  • Multiple configurations (direct or chained, data lines)
  • For the EPD, library is provided by constructor

Technical specs

  • 1Mbps usually, ~2x the speed of I2C, up to hundreads of Mbps
  • Possibility for multiple data lines to increase bandwith further
  • Slave number limited by IO in direct configuration
  • Slave number limited by delay in chained configuration

HTTPS

  • Manually imported certificates
  • GET from OWM (JSON)
  • POST to Telegram & DB (String)
  • JSON handled with arduinojson library

ESP32 Configuration

Internal Timer

  • Base clock runs at 80Mhz
  • We set the scaler to run at 1Mhz
  • We trigger IRQ every 1 second

Core Configuration

3 FreeRTOS core pinned tasks
  • Sensor read, network get & post (Core 0)
  • Touch screen IRQ polling & handling (Core 1)
  • E-Ink Display refresh & drawing (Core 1)

Sleep Mode

  • After inactivity the device goes into sleep mode
  • Wakes up periodicaly to run measurements
  • Returns to standard mode on touch IRQ [WIP]

Task Timing

  • List of functions with their run frequency
  • On minute tick check if any should have run since last tick
  • Not affected by function execution time

Power Consumption

Device Default Sleep
ESP32 20-200 mA 10-800 uA
EPD & Touch 26 mA 122 uA
Sensors 104.7 mA 245.4 uA
~330 mA (33mAh) ~1.1 mA (1mAh)

Case Design

Case Design

Future Features

  • Sleep between measurements (wake from touch/timer interupt)
  • Improved "Air quality" metric
  • More views & layouts to choose from
  • Support to controll IOT heating/cooling/ventilation

Questions