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Re: [nslu2-linux] Temperature sensors and A-D converters

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  • Mike Westerhof
    ... Indeed. Apparently somebody thought that it might be possible to use the ground in a building, for example, as the return conductor. I think that s
    Message 1 of 16 , Apr 2, 2010
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      clerew5 wrote:
      > --- In nslu2-linux@yahoogroups.com, Philip Tait <pjtait@...> wrote:
      >
      >> 1-wire sensors: http://en.wikipedia.org/wiki/1-Wire
      >>
      >
      > Ah, but that actually requires 2-wires :-( .
      >

      Indeed. Apparently somebody thought that it might be possible to use
      the ground in a building, for example, as the return conductor. I think
      that's marketing talk, as the devices really work better with a proper
      2-wire system.

      > And it would seem that the idea is to put the A-D converter at the far end of some wire alongside the sensor.
      >

      Yes. The wire (or pair of conductors, if you prefer) is actually a
      digital network connection. Many of the devices do A-D conversion, and
      send the results digitally back to the host. However, some devices are
      completely digital. And some do the reverse -- I have a relay board
      that I intend to use to control the heating/cooling system once I finish
      this project, although right now it just blinks LEDs and looks cool -
      everyone likes blinking lights ;-).

      > And how does one interface that to a Slug? Can it be done from an i2c bus, or is it necessary to bring out the UART connection from the Slug? And can somebody recommend a suitable sensor/converter for use with that system (I am looking for a temperature resolution of 0.1°C, and an absolute accuracy not too much worse than that).
      >

      I use a pre-packaged USB device:
      http://www.hobby-boards.com/catalog/product_info.php?cPath=23&products_id=1503

      > I would also like some advice on the path I have been following so far:
      >
      > I intended to attach an i2c bus (that seems a well-established hack to the Slug). On that bus there will be assorted LEDs and pushbuttons, an LCD, an EEPROM, and a 4-channel 12-bit A-D converter. I had intended to use LM35DZ temperatures sensors (as manufactured by both MS and Texas). And I had identified an i2c A-D chip from the Farnell website (AD7991) that appeared to have all the required characteristics.
      >
      > But I did not check enough. It turned out to be a SOT23 style chip which was about the size of a baby's fingernail :-( . Not nice for building on a breadboard. I looked for a suitable breakout board, but the only available one for an 8-pin SOT23 was in California, and I am in the UK. The only breakout boards readily available in the UK are for 6-pin SOTs (well, there is a possible one from Microchip, but at a ridiculous price).
      >
      > So back to the Farnell website for a chip with a larger footprint. But they all seem of be surface mount and with TSSOP or MSOP format, where the pin spacing is still 0.025in (the same as SOT23).
      >
      > So can anyone suggest any better chip? I might get away with SOIC format (0.05in spacing). The only chips I can find with good ol' DIL fomat are from Microchip, and they use Yet Another Nonstandard Interface (Microport).
      >

      Check out the kits and parts at the supplier I listed above. That will
      give you some idea about size and nature of the various one-wire chips
      and devices, and from that you can decide if the one-wire parts are more
      practical.

      In terms of the temperature sensors, I needed a network of them, and
      found that the pre-packaged or kitted parts were all too expensive -- I
      was paying more for connectors than the parts. So I purchased a dozen
      of the temperature sensors from a taiwan-based supplier (via eBay).
      Each part is in a small transistor package, and were very easy to solder
      to short (8 inches approx) twisted pair.

      Each "pigtail" was tested, and the network address recorded. I attached
      a label with the network address to the pigtail as well -- otherwise you
      have no idea which sensor is which (they are, of course, wired up to
      the same twisted pair).

      I wired the building up with ordinary telephone twisted pair, running a
      length through the first-floor rooms, then dragging it through the
      crawlspace beneath the building to get a sensor down where the plumbing
      is at most risk of freezing.

      I estimate the total cost for that part of the system to be about USD 3
      per sensor, by the time I was done with the sensors, solder, and
      telephone wire. Hard to beat that for price.

      -Mike (mwester)

      >> On Thu, 2010-04-01 at 13:47 +0000, clerew5 wrote:
      >>
      >
      >
      >>> --- In nslu2-linux@yahoogroups.com, "Mike Westerhof (mwester)"
      >>> <mwester@> wrote:
      >>>
      >>>
      >>>> I have a slug installed at an old house I have been working to
      >>>> refurbish. It uses a network of one-wire sensors to monitor
      >>>>
      >>> temperature...
      >>>
      >>> Eh? What sort of sensor uses one wire?
      >>>
      >
      >
      >
      >
      > ------------------------------------
      >
      > Yahoo! Groups Links
      >
      >
      >
      >
      >
    • clerew5
      ... Yes, there seems to be some neat stuff there. But I am already well down the i2c route. Actually, I have now reached the conclusion that the easiest way to
      Message 2 of 16 , Apr 5, 2010
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        --- In nslu2-linux@yahoogroups.com, Mike Westerhof <mwester@...> wrote:

        > > But I did not check enough. It turned out to be a SOT23 style chip which was about the size of a baby's fingernail :-( . Not nice for building on a breadboard. I looked for a suitable breakout board, but the only available one for an 8-pin SOT23 was in California, and I am in the UK. The only breakout boards readily available in the UK are for 6-pin SOTs (well, there is a possible one from Microchip, but at a ridiculous price).
        > >
        > > So back to the Farnell website for a chip with a larger footprint. But they all seem of be surface mount and with TSSOP or MSOP format, where the pin spacing is still 0.025in (the same as SOT23).
        > >
        > > So can anyone suggest any better chip? I might get away with SOIC format (0.05in spacing). The only chips I can find with good ol' DIL fomat are from Microchip, and they use Yet Another Nonstandard Interface (Microport).
        > >
        >
        > Check out the kits and parts at the supplier I listed above. That will
        > give you some idea about size and nature of the various one-wire chips
        > and devices, and from that you can decide if the one-wire parts are more
        > practical.

        Yes, there seems to be some neat stuff there. But I am already well down the i2c route.

        Actually, I have now reached the conclusion that the easiest way to embed surface mounted chips (even the very small ones) is to manufacture a one-off PCB. This turns out to be much easier that one might have thought. Google turns up lots of advice. All you need is a laser printer, some suitable transfer paper (old glossy magazines are recommended), and a willingness to handle ferric chloride etch (which is pretty messy stuff by all accounts).
      • Thomas Cooper
        It s much easier to buy what you need ready to go from Spark Fun Electronics, or some place similar... They cost about a buck or so. A bottle of ferric cloride
        Message 3 of 16 , Apr 5, 2010
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          It's much easier to buy what you need ready to go from Spark Fun Electronics, or some place similar... They cost about a buck or so. A bottle of ferric cloride will cost you more than that.

          On Mon, Apr 5, 2010 at 4:09 AM, clerew5 <clerew5@...> wrote:
           


          --- In nslu2-linux@yahoogroups.com, Mike Westerhof <mwester@...> wrote:

          > > But I did not check enough. It turned out to be a SOT23 style chip which was about the size of a baby's fingernail :-( . Not nice for building on a breadboard. I looked for a suitable breakout board, but the only available one for an 8-pin SOT23 was in California, and I am in the UK. The only breakout boards readily available in the UK are for 6-pin SOTs (well, there is a possible one from Microchip, but at a ridiculous price).
          > >
          > > So back to the Farnell website for a chip with a larger footprint. But they all seem of be surface mount and with TSSOP or MSOP format, where the pin spacing is still 0.025in (the same as SOT23).
          > >
          > > So can anyone suggest any better chip? I might get away with SOIC format (0.05in spacing). The only chips I can find with good ol' DIL fomat are from Microchip, and they use Yet Another Nonstandard Interface (Microport).
          > >
          >
          > Check out the kits and parts at the supplier I listed above. That will
          > give you some idea about size and nature of the various one-wire chips
          > and devices, and from that you can decide if the one-wire parts are more
          > practical.

          Yes, there seems to be some neat stuff there. But I am already well down the i2c route.

          Actually, I have now reached the conclusion that the easiest way to embed surface mounted chips (even the very small ones) is to manufacture a one-off PCB. This turns out to be much easier that one might have thought. Google turns up lots of advice. All you need is a laser printer, some suitable transfer paper (old glossy magazines are recommended), and a willingness to handle ferric chloride etch (which is pretty messy stuff by all accounts).


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