esp8266-temperature-humidity

Problem statement:

As a home-owner/renter I need to control multiple heating systems with room-by-room thermostats that I can move around for optimum measurements.

Solution statement:

Hardware (ESP12F with Si7021 sensor) and software (Arduino) implementation of a WiFi MQTT temperature/humidity sensor with battery voltage monitoring and 9+ months rechargeable battery life. Designed for integration with Home Assistant.

See the product website for a summary.

Prerequisites

1. An ESP8266 implementation like ESP-01, ESP-12F or NodeMCU
2. A DHT22 or Si7021 sensor
3. An MQTT server like Mosquitto
4. 2.4GHz WiFi network with the ability to assign static IP addresses
5. Arduino IDE for flashing

Hardware

Tested on:

1. ESP-01 lacks easy GPIO16 access for deep sleep
2. ESP-12F current preferred module
3. NodeMCU has power-hungry onboard components

Tested with:

1. DHT22 inefficient and low precision
2. Si7021 current preferred sensor

Tested with:

1. 2xAA batteries fine but unsustainable
2. Generic JST Li-Ion battery current preferred battery

Tested with:

1. MCP1700 current preferred regulator
2. TPS62203 high frequency switching too fiddly to implement stably

End goal is to print the smallest PCB + battery package and 3D print a case for discrete deployment.

Software

Goal is to make the device last as long as possible on batteries.

Energy performance

Current best energy usage is using the Si7021 sensor, measured using a currentRanger.

Startup: 0.0789mAh

This includes initial connection to WiFi, sensor reading, publication, writing to RTC memory.

Reading (no change): 0.0012mAh

This shows the initial reading of RTC memory, sensor measurement, sleep and wake up to publish result.

Reading (change): 0.0285mAh

This shows the initial reading of RTC memory, sensor measurement, sleep and wake up to publish result.

Energy estimates

• Worst case (a reading every 10 minutes): 4.75mAh/day this should last ~9 months on a 1200mAh battery.
• Better case (heuristically 50 readings a day): 2.19mAh/day this should last over a year on a 1200mAh battery.

Credits

• Many thanks to Andy Bennett for all the support and assistance throughout the projet
• Thanks to the OpenEnergyMonitor Project for the tip on Si7021 sensor and low power inspirations.
• Thanks to the LowPowerLab team for making the CurrentRanger and low power inspirations.