Before we get too excited about the "obvious" candidates for temperature sensing, a word about the DS2438 Smart Battery Monitor. Don't forget that it has a temperature sensor built into it. If your system already has one of those, you may not need a DS18xx as well! Do note, however, that as the DS2438 is a SMT device, it may be unduly affected by heating of your PCB by the operation of the circuits on it.
Be sure that you at least check out the little "gotcha" explained at the bottom of this page, if other matters here are not what you were looking for.
Dallas (part of Maxim) has various chips for temperature sensing. Some are old designs, now replaced by better ones. There is more than one current design, as you can choose different levels of accuracy, price and extra features, like thermostat functions. I found details by using Maxim's "Lookup by part number" page. It has at times in the past been temperamental. If "DS(space)1820" gives you no result, try using "ds(no space)1820", etc... the usual search engine-friendly tricks.
DS18S20 (aka DS1820S?)- Recommended replacement for DS1820:
The DS18S20 Digital thermometer provides 9-bit centigrade temperature measurements and has an alarm function with nonvolatile user-programmable upper and lower trigger points. The DS18S20 communicates over a 1-Wire® bus that by definition requires only one data line (and ground) for communication with a central microprocessor. It has an operating temperature range of -55°C to +125°C and is accurate to +/-0.5°C over the range of -10°C to +85°C. In addition, the DS18S20 can derive power directly from the data line ("parasite power"), eliminating the need for an external power supply.
DS18B20: Datasheet easy to miss on Dallas site.
The DS18B20 Digital Thermometer provides 9 to 12bit centigrade temperature measurements and has an alarm function with nonvolatile user-programmable upper and lower trigger points. The DS18B20 communicates over a 1-Wire bus that by definition requires only one data line (and ground) for communication with a central microprocessor. It has an operating temperature range of 55°C to +125°C and is accurate to +/-0.5°C over the range of 10°C to +85°C. In addition, the DS18B20 can derive power directly from the data line ("parasite power"), eliminating the need for an external power supply.
The DS1822 is a digital thermometer with +/-2°C accuracy over a -10°C to +85°C range. Data is read out over a 1-Wire® serial bus in 2's complement format with 9 to 12 bits of resolution (user-programmable).
The DS1822 offers thermostatic functionality with over-temperature (TH) and under-temperature (TL) user-programmable setpoints stored in on-chip EEPROM. An internal flag is set when the measured temperature is greater than TH or less than TL. If thermostatic operation is not required, the two bytes of EEPROM reserved for TH and TL may be used for general-purpose nonvolatile storage.
=== end from site...
BUT! Closer reading of at least one of these device's sheets in the past speaks of reading tture to more precision... not "accurate" that far? Bah! And old datasheets (DS1820) not immediately, if at all, accessible at site. Bah!
DS18S20 has RESOLUTION of up to 0.0625 °C... but ACCURACY of only 0.5°C., over range -10 -+85°C. Op range: -55-125°C.
DS1822: Same, EXCEPT accuracy only 2° (But has extra feature already mentioned.) In DS1822 datasheet: "Also available is the high-precision DS18B20, which provides +/-0.5°C accuracy and is software-compatible with the DS1822."... so... did the DS18B20 also offer the flags?
18B20: There was a bad batch... and poor customer support.
Note: DS1822 has internal flags for "there HAS BEEN an under/ over tture event since you last visited". Others can alarm WHILE under/ over exists.
Search site for DS1820, and you get three hits: DS1820, DS1820B, DS1820S. Click on link for DS1820B, and you get page for DS1820. Search on DS18B20, and you get different results.
DS1820 seems to be obsolete.
DS1822 has flags, but I don't want them, but lower accuracy.
DS18S20 seems to be the chip for me....
Many 1-Wire chips can be connected using just two wires. (The "One Wire to rule them all", plus the wire for "ground", or "zero volts".)
The eponymous "1-Wire" carries both the power to run the 1-Wire chips on the MicroLan, and the data going to and from those wires. This is pretty cool... but actually, not terribly complicated. The idea sort of presages the "power over ethernet" extension for what we usually call a LAN. But major elements of it were anticipated many, many years earlier by the system used "in the old days" for street corner fire alarm call boxes.
I have written a separate page covering the issue of Parasitic Power or Other. This a question that applies to many (not all) 1-Wire chips. (Some cannot be powered parasitically, and need a local Vcc).
There is a big advantage, of course, if you have long cable runs.
Be careful... not all of the DS18xx temperature sensors send the temperature encoded exactly the same way. They all work in generally the same way... but if you are working on a project and getting consistent... but wrong... readings from your sensor, pull up the relevant data sheet and look closedly at exactly how the temperature was encoded for transmission over the MicroLan. Your temperature reading subroutine may be guilty of "good data (not garbage) in, garbage out"!
Do you have an "always on" internet connection, e.g. DSL or broadband? (Even a basic home user account is fine. You don't need a fixed IP address.) Want to set something up that lets you check all is well at your home or business.... from any internet terminal in the world? (The terminal needs nothing more than a standard browser.) You don't have to spend anything on software, everything you need is free. You might want to dedicate an old Win98 "box" to doing the work, but you don't have to. You'll probably want to spend a little money to attach one or more 1-Wire temperature sensors to the system, but there are ways around even that expense.
Sound interesting? See the pages about my FarWatch system.
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