Use platformio to compile and upload firmware to the device. The firmware periodically takes measurements using sensors of the device (see The measurements below), and every few measurements connects to the local Wifi access point to broadcast them to an MQTT broker at address ttgo-server.local.
See the README in mqtt-server for how to set up and run the server.
- Long press the BOOT button to enter smartconfig mode
- Long press the User button to enter deepsleep mode
- Double press the User button to enter smartconfig mode
- Press the Reset button to reset the device
This is a measure of the moisture content of the soil.
A TLC555DR timer generates a square wave which is output through a 10k resistor, then through the large copper "pill" shaped pour (labelled "bytton" on the schematic), through a diode, to the Humi net which is connected to pin IO32 on the ESP32, in parrallel with a 1uF capacitor to ground (itself in parrallel with a resistor of 1M).
Using equations (8) from the datasheet and seeing that Ra = R34 = 300, Rb = R35 = 1600 and Ct = C60 = 470pF (and using some estimated values for the other parameters) we find the frequency is around 760kHz.
Treating the "bytton" pad as a capacitor to ground whose capacitance changes with the moisture content of the surroundings, the voltage at pin IO32 decreases from ~2.7V (at 0.1pF) to 0V (at ~55uF):
(See the LTSpice model in \electronics\)
The dielectric constant of water is 80.4 [2] and dry soil is around 4 [1], so presumably, as the moisture content of the soil increases, the dielectric constant, and thus capacitance of the "bytton" increases, and so the voltage drops.
- [1] Dean et. al, 1987, SOIL MOISTURE MEASUREMENT BY AN IMPROVED CAPACITANCE TECHNIQUE, PART I. SENSOR DESIGN AND PERFORMANCE
- [2] Sears, F. W., Zemansky, M. W., Young, H. D., University Physics, 6th Ed., Addison-Wesley, 1982.
