This section of the documentation is aimed at developers who wish to better understand and perhaps modify the Mini-Monitor software. The code is thoroughly commented, but the documents described below are meant to provide higher level documentation for the application. It is recommended that the <software> document be read as a prerequisite to this document.

Mini-Monitor is a Python 3 application and is currently running with Python 3.7.3 on the Raspberry Pi running a Raspbian Buster Lite Linux distribution, July 2019 version. On the Raspberry Pi, the Python code for the application is found in the pi user’s directory space in the folder /home/pi/pi_logger. References to file paths in the following text will all be relative to that directory.

Overall Structure of the Application

Starting with version 1.7, the Mini-Monitor software is designed to use an MQTT Broker to manage the transport of sensor readings from software that acquires the readings to software that consumes the readings, for example, software that posts the readings to a BMON server. The specific MQTT broker that is used and hosted on the Mini-Monitor is Mosquitto. The broker runs as a daemon on the Raspberry Pi and is started via the systemd script /etc/systemd/system/mosquitto.service.

As of version 1.7, there are a few different processes that acquire sensor readings and publish them to the MQTT broker:

  • This script instantiates “Readers” that are enabled in the settings file, these readers periodically read their associated sensors. After passage of a logging time interval (as specified in the Settings file), publishes reading summaries to the MQTT broker (average values for analog sensors, or state changes for sensors that determine the state of a device).
  • The Mini-Monitor has the ability to listen to meter reading radio broadcasts from utility meters. The utility meter is not “polled” by the Mini-Monitor but instead waits for readings to arrive. So, the style of sensor reading did not apply. Instead, a separate script is available that listens for these readings and publishes rates of change of the cumulative meter value to the MQTT broker.
  • The Mini-Monitor has the ability to read Acurite 592TXR Temperature/Humidity Wireless sensors by using a RTL2832U USB-connected software defined radio (the same device used by the meter reader script described above.)
  • The Mini-Monitor can read a Peacefair PZEM-016 Electric Power sensor through use of this script.

These sensor reading scripts are started by the standard rc.local script that the Raspberry Pi runs at the end of its boot up process. In order to include the rc.local script in the Mini-Monitor source code, a symlink is created from the standard /etc/rc.local file to an rc.local file located in the Mini-Monitor software folder at system_files/rc.local. It is valuable to look at this file for other details on the Mini-Monitor start-up process. is only started if at least one Reader file is enabled in the Settings file. The other scripts described above are started depending on flags found in the /boot/pi_logger/ file.

As of Version 1.7, there is only one possible consumer of sensor readings published to the MQTT broker: the script, which receives sensor readings and posts them to a BMON server on the Internet. Enabling and configuring that script occurs in the Settings file. If enabled, it runs in a separate process from and In the future, a basic log-to-disk sensor reading consumer is envisioned. subscribes to the readings/final/# topic on the MQTT broker. All sensor readings published to that topic set will be received by and posted to the BMON server. publishes to the topic readings/final/pi_logger and publishes to the topic readings/final/meter_reader.

Reader Files

The Mini-Monitor is designed so it is easy to mix different sensor types and data sources in the data acquisition process. As mentioned in the prior section, utilizes Reader classes to read different types of sensors. Each Reader class knows how to gather data from a particular data source, e.g. a 1-Wire sensor network or a particular boiler type. The READERS setting in the settings file holds that list of classes. The list items for the READERS setting are in the format of <file>.<class name>. All of these files are located in the readers directory of the application.

Each specific Reader class inherits from readers/base_reader.Reader. The specific Reader class only needs to implement a read() method. The requirements for that read() method are described in the base_reader.Reader base class. Basically, the read() method returns a list of sensor readings.

A number of equipment manuals are available that relate to the existing Reader classes. These are listed and are accessible from the Relevant Manuals page.

To add a new data source to the Mini-Monitor software, you only need to write a new Reader class and put that class in the readers directory. Then, add the class to the READERS variable found in the Settings file, described in the next section.

Alternatively, a separate script can be written, similar to, that can acquire sensor readings and publish them directly to the MQTT broker.

Settings File

In the Software document, the Settings file was described in detail. This file controls the types of data sources the deployed Mini-Monitor will collect data from and controls a number of other data collection parameters. The Settings file that is used by the deployed Mini-Monitor is found at /boot/pi_logger/ This file is not in source control because it contains a secret Store Key for the BMON system receiving the data, and it as setting specific to the particular application being deployed. There is a sample Settings file that is kept in source control and is present at system_files/

If you add a new Reader class to the Mini-Monitor software, you should add the <file>.<class> name to the READERS variable in both the sample Settings file (system_files/ and the operational Settings file for the Mini-Monitors you have deployed that need to use the new Reader.

Cron Tasks

The Mini-Monitor system has a Cron job that runs every 15 minutes and executes the scripts/ script. This script performs a number of health checks on the Mini-Monitor, records some some summary information in the application log file, and performs a few other tasks that should run in a process independent of the main Mini-Monitor software.

Raspbian OS Configuration

Some configuration was done to the Raspbian operating system for use in the Mini-Monitor. As a user of the Mini-Monitor, if you download the SD card image according to instructions on the Software page, you do not need to perform any of the following configuration changes. The SD card image already includes all of these configuration changes. The information in this section is meant for developers attempting to understand or modify the Mini-Monitor system.

The modifications to the base Raspbian Buster Lite image are described in this document