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I have recently worked with 2 different digital temperature sensors and I want to discuss some of the pros and cons of both sensors in this article. The first one was the DS18B20 from Maxim (used to be Dallas Semiconductors). The package was the TO92 which is a small three terminal transistor type. The interesting part about this device is that it has a one wire interface. This means that apart from Gnd and Vcc, there is only one communications wire between the interrogating device, usually a microprocessor, and the sensor. Communications are therefore bidirectional and done using a set of strict timing rules. One can even take it a step further by removing Vcc and shorting it to the data pin. This means one can have a remote temperature sensor using only 2 wires. This gets even better because this structure has the ability to handle multiple sensors on the same pair of wires. In this way one can have a number of sensors in a room measuring its temperature and then make a decision on the average.All these advantages however do have a down side and that is that the complexity of the software increases quite dramatically. I say this because it is not a case anymore of asking a sensor, "what is your temperature". Now a certain procedure has to be followed. The address (internal unique fixed 48 bit) of each sensor has to be determined (see the application notes on the suppliers website), stored in EEPROM (so that we don't forget it) and then each sensor can be individually addressed and its data queried.The second digital temperature sensor that I have worked with recently is the TC74A2-5.0 from Microchip in the 5 pin SOT23 package. This is usually best mounted onto a pcb but soldering 4 wires onto it works just as well . The communications to this device works completely differently to the Maxim chip. The communications interface is the industry standard I2C bus which consists of 4 connections namely Gnd, Vcc, Clock and Data. The device is configured by default as a slave with a limited number of fixed addresses. These addresses are made known by the markings of the device eg A2 in the device under discussion has the fixed address 0b1001010. This a 7 bit address used within the I2C communications protocol to talk directly to a specific device. The problem here is that one can only hang so many devices onto the bus before the range of available addresses are exhausted (max 8). Secondly, it requires 4 wires. The advantages start appearing in the software especially if one already has an I2C bus running. All that is then required is one additional line of code to query the data from the new I2C device and the integration is complete.The choice is then ultimately yours. If one is new in writing software for microcontrollers then I would suggest the Microchip device but those that are comfortable with an increased level of software complexity may find it more convenient to use the 2 wire Maxim device .
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Marc Jarchow
46 year old entrepeneur providing MODular ElecTRONics solutions for the Professional (custom instruments for your business), Educator (can be used as a training aid), Entrepeneur (can be packaged with your logo) and Hobbyist (make your own home based products) using low cost modules to create whatever application you have in mind.
This is all done via modetron.com
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