Table of Contents
STM_4: 1-Wire Temperature Sensor
The temperature-only sensor DS18B20 uses a 1-wire protocol. “1-wire” applies only to the bidirectional bus; power and GND are on separate pins. The sensor is connected to the MCU using GPIO D0 only (PA_3 in Nucleo numbering). Many devices can be connected on a single 1-wire bus, each with a unique ID. Except plastic version, which we have in our laboratory (enclosure TO-92) DS18B20 also comes in a water-proof metal enclosure version that enables easy monitoring of the liquid's temperature.
Prerequisites
To handle operations with DS18B20, we will use a dedicated library that uses a 1-wire library on a low level:
lib_deps = milesburton/DallasTemperature@^3.11.0
Sensor readings can be sent over the network or presented on one of the node's displays (e.g. LCD), so understanding how to handle at least one of the displays is essential:
Suggested Readings and Knowledge Resources
Hands-on Lab Scenario
In this scenario, we present how to interface the 1-wire sensor, DS18B20 (temperature sensor).
Task to be implemented
Read the temperature from the sensor and present it on the display of your choice. Show the reading in Celsius degrees. Note that the scenario below presents only how to use the DS18B20 sensor. How to display the data is present in other scenarios, as listed above. We suggest using an LCD (scenario STM_5: Using LCD Display).
Update reading every 10s. Too frequent readings may cause incorrect readings or faulty communication with the sensor. Remember, the remote video channel has its limits, even if the sensor can be read much more frequently.
Start
Check if your display of choice is visible in the FOV of the camera once the device is booked.
Steps
The steps below present only interaction with the sensor. Those steps should be supplied to present the data (or send it over the network) using other scenarios accordingly.
Step 1
Include Dallas sensor library and 1-Wire protocol implementation library:
#include <OneWire.h> #include <DallasTemperature.h>
Step 2
Declare the 1-Wire GPIO bus pin, 1-Wire communication handling object, sensor proxy and a variable to store readings:
#define ONE_WIRE_BUS D0 static OneWire oneWire(ONE_WIRE_BUS); static DallasTemperature sensors(&oneWire); static float tempDS;
sensors class represents a list of all sensors available on the 1-Wire bus. There is just a single one in each of our laboratory nodes.
Step 3
Initialise sensors' proxy class:
sensors.begin();
Step 4
Read the data:
sensors.requestTemperatures(); if (sensors.getDeviceCount()>0) { tempDS = sensors.getTempCByIndex(0); }
Remember not to read the sensor too frequently. 10s between consecutive readings is just fine.
Devices in the 1-Wire bus are addressable either by index (as in the example above) or by their address.
Some useful functions are present below:
DallasTemperature::toFahrenheit(tempDS)- converts temperature in C to F,sensors.getAddress(sensorAddress, index)- reads device address given byuint8_t indexand stores it inDeviceAddress sensorAddress.
Result validation
The observable temperature is usually within the range of 19-24C. If you find the temperature much higher, check your code, and if that is okay, please contact our administrator to inform us about the faulty AC.
FAQ
I've got constant readings of value 85.0. What to do?: Check if GPIO is OK (should be D0), check if you initialised controller class and call the function sensors.requestTemperatures();. Give the OneWire bus and the sensor some recovery time (at least 250ms) between consecutive readings.
