Brigadoon - An Occasional Corner on the Internet
Circuit for Ground Temperature Sensor
Designed by Mark Little
The digital temperature sensor is contained in a sealed metal tube. Because each sensor has a unique digital address, all three lines of the sensors (power, earth and data ) are connected in parallel with the corresponding lead of each sensor.
The DS18B20 sensors in parallel require a pull-up resistor on the data line. The standard voltage to the 8-pin DIN connector is +5V which means if the pull-up resistor is connected to the +5V rail, it may overload the Raspberry Pi GPIO input bit, causing damage to the Raspberry Pi.
When using multiple sensors, in parallel for a series of ground thermometers, that some units exhibited a higher level of noise on the readings than others, as can be seen in the plots below. In the upper plot, the temperature slowing decreasing and the only noise like components are where the reading transitions slowly between two adjacent level. The graph below, however shows much larger and more random variations in temperature.
At first glance, it would not be unreasonable to assume that the lower sensor has a fault, but comparing the size of the fluctuations to the device's specifications (±0.5°C), the variations are within specifications. This characteristic has been reported by other uses of this device, so to get best results, it ma be necessary to place a selection of temperature sensors in the same environment and see which ones do not exhibit more noise than the others, and use them in a set.