1-Wire

(note, this client has been refactored, but not tested. Testing is welcome …)

1-Wire is a device communication bus that provides low-speed data over a single conductor. It is also possible to power some devices over the data signal as well, but often a third wire is run for power.

Simple IoT supports 1-wire buses controlled by the 1-wire (w1) subsystem in the Linux kernel.

To use a bus, add a 1-Wire node where you want it in the tree and set its Index to the number of the bus controller, which matches the w1_bus_master<index> directory the kernel creates in /sys/bus/w1/devices. The first controller is index 0. Simple IoT then detects the sensors on that bus and creates a node for each one.

1-wire nodes

Bus Controllers

Raspberry PI GPIO

There are a number of bus controllers available but one of the simplest is a GPIO on a Raspberry PI. To enable, add the following to the /boot/config.txt file:

dtoverlay=w1-gpio

This enables a 1-wire bus on GPIO 4.

To add a bus to a different pin:

dtoverlay=w1-gpio,gpiopin=x

A 4.7kΩ pull-up resistor is needed between the 1-wire signal and 3.3V. This can be wired to a 0.1 inch connector as shown in the following schematic:

1-wire schematic

See this page for more information.

1-Wire devices

`DS18B20` Temperature sensors

Simple IoT currently supports 1-wire temperature sensors such as the DS18B20. This is a very popular and practical digital temperature sensor. Each sensor has a unique address so you can address a number of them using a single 1-wire port. These devices are readily available at low cost from a number of places including eBay - search for DS18B20, and look for an image like the below:

DS18B20 DS18B20

Readings are in degrees Celsius by default. Set Units on a device node to F to report degrees Fahrenheit instead.

Schema

The configuration of a 1-wire bus and one of its devices:

nodes:
  - oneWire:
      debug: 0
      description: Tank sensors
      disabled: 0
      index: 0
      pollPeriod: 3000
      children:
        - oneWireIO:
            description: Tank top
            disabled: 0
            id: 28-0000073b6f4d
            units: F

index is the number of the bus controller, matching the w1_bus_master<index> directory in /sys/bus/w1/devices. pollPeriod is in milliseconds and defaults to 3000 when it is zero or missing.

id on a device is its 1-wire address rather than a node ID, which is why it is spelled like any other point. Simple IoT creates a device node for each sensor it detects on the bus, so these usually arrive on their own; what a file adds is a lasting description and, where wanted, units.

Leaving units out reports degrees Celsius. The readings and error counts are points the client maintains, so an export of a running bus carries them as well.

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