RPi::Device::DS18B20

Interface to the DS18B20 digital thermometer from Maxim Integrated

Synopsis

use RPi::Device::DS18B20;

my $t = RPi::Device::DS18B20.new;

for $t.thermometers -> $thermometer {
    say $thermometer.name, ":\t", $thermometer.temperature;
}

Or asynchronously:

use RPi::Device::DS18B20;

my $t = RPi::Device::DS18B20.new;

react {
    whenever $t -> $reading {
        say $reading.when, "\t", $reading.name, "\t", $reading.temperature;
    }
}

The source is in the examples directory of the distribution.

Description

The DS18B20 is a handy and inexpensive digital thermometer that uses the Dallas 1-Wire bus.

The Linux kernel has support for 1-Wire which can be enabled with raspi-config (via Interface Options -> 1-Wire,) which by default enables the 1-Wire interface on GPIO 4 as per:

The minimum viable circuit will be something like:

The 4.7K resistor is required as a pull up on the data wire, only one appears to be needed for multiple sensors on the same bus.

It can be configured on another pin (or pins,) as described here

Each thermometer has a unique identifer (in common with all 1-Wire sensors,) which means that multiple thermometers can be used on the same bus at once, however there is no way of distinguishing the devices without querying them, so a programme may need to provide its own mapping of identifier to thermometer (location etc,) this can be achieved by either wiring them in one by one, running something like the synopsis code and noting the identifier (and presumably labelling the sensor,) or by applying some heat source to each one in turn (if you have one of the encapsulated "waterproof" sensors a cup of hot water is ideal, but holding the sensor in your hand should work if the ambient temperature is lower than body temparature,) and similarly noting the identifier.

The module provides for the enumeration of the thermometers detected on the 1-Wire bus providing a list of Thermometer objects, having a name attribute and a temperature method that returns the degrees Celcius with a precision of a thousandth of a degree (though the device is commonly stated as having ±0.5⁰ accuracy so this precision may or may not be useful to you.) The default "conversion time" for the device is 750 milliseconds so requesting the temperature more frequently than that is likely to be fruitless.

Alternatively the RPi::Device::DS18B20 object provides a Supply "coercion" method which allows it be used anywhere a Supply can be used (such as a whenever in a react block,) this will emit a Reading object with a name attribute of the sensor id, a temperature attribute with the measured temperature and a when attribute, for every sensor detected at minimum frequency determined by the supply-interval attribute as supplied to the constructor (the default is 30 seconds.) The readings may not be emitted in a predictable order at each interval as each sensor may take a different length of time to produce a reading, plus the bus protocol will, by necessity, serialise the readings.

Install

Assuming you have a working copy of Rakudo you can install with zef :

zef install RPi::Device::DS18B20

It is unlikely to work on anything else than a Raspberry Pi.

Support

This is difficult to test in a completely automated fashion without the actual device attached and knowing what the temparature should be so there may be bugs which I haven't noticed.

Please send any patches/suggestions/issues via Github.

Ideally any reports should include the Raspberry Pi and OS versions and some indication of how the device was wired up.

This may work with other 1-Wire thermometer sensors supported by the Linux kernel by adjusting the device-class passed to the constructor, but as I don't have any to test with right now I can't make any guarantees, it also appears that DS18B20 is by far and away the most commonly used.

Copyright & Licence

This is free software, please see the LICENCE in the distribution.

© Jonathan Stowe 2022