Research In Expanding The Carrying Capacity Of The 1-wire Temperature Sensor Circuit

1-wire digital temperature sensor DS18B20 made by DALLAS company is of wear & collision-resistant. With small size, easy to use, package in a variety types, specially adopting digital technology it is suitable to be used in a limited space as a sensor to measure normal temperature. Our department has used wireless transmission technology to detect the temperature in a granary firstly in our country, the used sensor just is DS18B20.However, in many temperature measurement systems, the problem about long-distance and multi-point is still not to be properly resolved. The key point is that the factors of the constraining transmission distance and the number of DS18B20 in 1-wire net are unclear, this thesis is dedicated to research and solve the problem.This paper illustrates characteristics and applications of DS18B20 in domestic and abroad, and points out that the temperature measurement in granary is the most widely used field in China. As the problem of long-distance and multi-point measurement is not solved satisfactorily, the electronic devices have to be added as a node for collecting-temperature data, and extended more cable for signal lines. The former will cause that the electronic components are easy to corrosion in the period of the insecticidal fumigation in granary if the devises is at the untight condition, the latter increases the chance of mice nibbling that may led to the cable damage. These problems are prevalent in the country's granary.Therefore this paper analyzes and researches different types of cable and different topological forms of DS18B20's distribution network with their reactance characteristics. A mathematical model is tried to build to simulate the circuit characteristics in order to identify the constraints factor and find a solution to the problem. For this purpose, a lot of tests and dynamic monitoring on the signal waveform have been done, combining with the detection of the capacitors and inductance for the static temperature measurement net. So then the mathematical models can be verified and revised to make it better. Based on the research results, the main effected factor, the distribution capacitance, is being found. So with a large current driving capability circuit, using reasonable pitch of twisted pair lines, and with a line topology, the signal transmission distance can be greatly extended and the 1-wire temperature sensor collection points can be increased more too.According to the analysis and tests, using 74ACT244 tri-state line driving chip and with a linear topological way, the 1-wire net can suspend 62 DS18B20 with a distance up to 280m. But it cannot be used with multiple T-joint branch topology. Branches could lead to signal reflections easy that results in communication failure. Moreover, the drive of 1-wire net to be further improved by using the higher qμΑlity cables, the load capacity still has the possibility to be much more improved. This research makes comprehensive verification of the DS18B20 application, and provides the temperature sensor a theoretical basis and feasible scheme in the agricultural greenhouses, bridges, road construction, mining, and the store field for needing measuring temperature. For example, although it cannot use a T-topology in grain depot, a multi-branch star topology can be instead. By putting the temperature-collected signal line directly outside the barn, the electronic components corrupted by phosphine can be avoided, the probability of damages by rat bite can be reduced obviously, and the reliability of temperature measurement system will be greatly enhanced.