Seedling Control

A bang-bang temperature controller for seedling heat mats.

Implements four independent channels of control.
Each channel uses a DS18B20 1-wire temperature sensor for feedback.
Additional DS18B20 sensors can be configured to monitor auxiliary temperatures.
A web interface is provided to monitor and adjust temperatures.
The top level monitor process runs as a SYSTEMD(1) service.

Hardware

The system runs on a Raspberry Pi Zero. The Pi’s I2C port controls an MCP23008 8-bit I/O expander
and a DS2482-100 1-wire master. The I/O expander in turn drives a quad opto-coupled relay module.
The 1-wire master controls four DS18B20 temperature sensors running parasitic power mode.
Additional DS18B20 sensors can be added. The I/O expander and I2C master both run on +5v power
to reduce load on the Pi’s 3v3 supply. An Adafruit BSS138 4-channel bi-directional level shifter
is used to convert the Pi’s 3v3 I2C bus to run with the 5v I/O.

Control Process

A simple control loop reads the DS18B20 temperature sensors and controls the relay ports.
A command queue allows control channels to be enabled/disabled and the setpoint adjusted.
A response queue acknowledges commands and provides system status.

Web User Interface

A Flask web app displays the system status and allows control channels to be adjusted.