temperature sensing using 2 wires

Discussion in 'General Electronics Chat' started by Dollarday, Nov 29, 2012.

  1. Dollarday

    Thread Starter Member

    Jan 25, 2012
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    Hi everyone :cool:

    I would like to know, do you know of any temperature sensors which you can use to measure multiple points, but only requires 2 wires to function? (In other words, you only require 2 core cable, but can measure temperature at mutliple points)

    One such IC is the Maxim DS18S20... Are there any other IC's that can accomplish this? (Which are preferably much cheaper?)

    Regards,
    Dollarday
     
  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    There are none. A temperature sensor can only sense the temperature at the place where it is, excepting of infra red sensors that sense the light emitted from a distant object and infer the temperature.

    There are some 1-wire (that also need a ground), many voltage devices (2 or 3 wires), and also several I2C devices (4 wires); I2C has the advantage of those same 4 wires may service multiple sensors.
     
  3. spinnaker

    AAC Fanatic!

    Oct 29, 2009
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    I'm confused. You say there are none then you say there are some. One wire devices like Dallas DS18B20 can be used with 2 wires. One ground one sense / parasitic power.

    But I am also confused because the op is already aware of those sensors.
     
  4. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    When I said there are none I am referring to the OP's request for "temperature sensors which you can use to measure multiple points"

    There are many that measure a single point given a simple interface.
     
  5. spinnaker

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    Oct 29, 2009
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    The one wire sensors from Dallas can measure multiple points. Each device has a 64 bit unique serial code. Not sure of the limitation (if any) but several can be multidropped on the same pair of wires.

    The code to parse the serial numbers is a bit hard to understand but not too awful bad. I have a library in C that I need to drag out of the closet for an upcoming project.
     
  6. MrChips

    Moderator

    Oct 2, 2009
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    As spinnaker says, there are 1-wire sensors that actually use two wires, signal and ground.
    I have built such devices using simple microcontrollers all networked across one wire.
     
  7. Dollarday

    Thread Starter Member

    Jan 25, 2012
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    Ah, I understand where the confusion is. Let me rephrase: Are there any temperature sensors in which a number of these sensors can be placed on multiple points along a cable which you can use to measure multiple points, but only requires 2 wires to function?

    In other words, you only require 2 core cable, but can measure temperature at mutliple points by connecting multiple sensors to the cable at each point where you would like to measure the temperature.
     
  8. MrChips

    Moderator

    Oct 2, 2009
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    Google 1-wire.
     
  9. Jaguarjoe

    Active Member

    Apr 7, 2010
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    If you are measuring the average temp, you can string some LM335Z's in series. If you want minimum temp, put them in parallel. These are 2 wire devices. GND and signal. You'll need to supply a ~6.2k current limiting resistor.
     
  10. spinnaker

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    Oct 29, 2009
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    I answered you long ago. It is called One wire and Dallas is one of a number of companies that manufactures such devices. See above.
     
  11. Dollarday

    Thread Starter Member

    Jan 25, 2012
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    Hi Spinnaker, I refer to my original question:
    "One such IC is the Maxim DS18S20... Are there any other IC's that can accomplish this? (Which are preferably much cheaper?)"

    This is in fact the same one wire temperature sensor from Dallas, which is a subsidiary of Maxim-IC.

    The DS1820 is a one wire sensor from Dallas, I'm looking for a similar IC which is cheaper.
     
  12. spinnaker

    AAC Fanatic!

    Oct 29, 2009
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    No idea. You will have to search.

    You can get 5 DS18S20s on ebay for less than $9.


    http://www.ebay.com/itm/5pcs-DS18S20-18S20-1-Wire-Digital-
    Thermometer-Dallas-/260920130130?pt=LH_DefaultDomain_0&hash=item3cc00d3a52

    That is really mot all that expensive.

    That is a lot of technology packed into a small chip. It would be surprising if you find anything much cheaper.
     
    Dollarday likes this.
  13. John P

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    Oct 14, 2008
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    It seems as if the DS18S20 is a unique product. You might just possibly be able to duplicate it with a PIC or similar processor plus an external temperature sensor, but you'd need to add a few other components, and you'd need a circuit board to put it on and by the time you were done, you'd have more bulk and most likely, more cost.
     
  14. PeterSt

    New Member

    Dec 15, 2012
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    As has been said "you won't probably get such a device for anything cheaper". Well, you won't. And yes it is unique because it is digital (and the reason why "1-wire" for signal is sufficient).

    Personally I think the mere point is that you hardly need anything else, assumed you take the TO-92 package. Take something like UTP cable, make a ring of 100 meters and connect as many of these sensors as you need. Cut two of the (twisted) wires, solder them again and solder the two legs on that. I have three of these rings (with that length) in my house and a couple of 100 of these 1820's.
    Notice that Dallas provides an serial interface which takes an Ethernet (UTP) connector and which at the other end goes into you serial port (but which may need an addtional interface card for the motherboards from today). That's all it needs on the hardware side.

    Do notice that 1-Wire consists of a whole host of sensor stuff. And all as cheap.

    There's standard software which can read out the sensors but this may not suit you. What I did was using "OPC" to have it really nice and next program with VB.Net all you like. This could be about their accuracy of 0.5C which only tells that one to the other may be off by that amount. But calibrate them (measure them relatively) and correct that in software - done. And next you have a temp sensor with a resolution of 0.001C !

    Apart from the programming you may not like, there's nothing much easier to accomplish - not even for one sensor. But when there are several or many, you will be nowehere near the super low price this costs. Remember, no supplies needed anywhere.
     
  15. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    May I ask how many actual DS sensors you have on each 2-wire "ring"?

    What about the limit on the number of devices that can be ghost powered (without needing a thrid power wire)?

    And I don't want to sound rude but why on earth do you need hundreds of temperature sensors in your house? That seems very unusual and quite interesting! :)
     
  16. spinnaker

    AAC Fanatic!

    Oct 29, 2009
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    Good catch. Maybe the poster has a HUGE home with hundreds of rooms? :)

    Gee this project would cost $200-$400+ US. Then again if you can aggord a house with hundreds of room.............:)
     
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