LZ: I2C Temperature Sensor


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LZ: I2C Temperature Sensor



I2C Temperature Sensor
By:  Ed Plese, Jr.  (eplese@lnd.com)


Thanks to Per Finander for the basic layout of the schematic and for
his I2C ASM routines.


The parts needed for the Temperature Sensor are a capacitor, two
diodes, the IC and half of a link cord.  The capacitor can be just
about any value above 1uF.  I have tried 2.2uF, 10uF and 100uF
capacitors and they have all worked equally well.  I would recommend a
smaller one (around 2.2uF) because I think the larger ones have more
of an internal leak so they would draw more power from the link port. 
I wouldn't go much above 100uF because then the capacitor will draw a
lot of current through the port and possibly damage the link port. 
Also, the smaller you go, the easier it is to buy a surface mount one.
 I plan to make the whole thing a surface mount module and maybe it
will be small enough to fit inside the calculator.  You also can use
just about any kind of small switching diodes for the two in the
schematic.   I haven't tried any other than the 1N914 type that I used
in my prototype.  The IC is the LM75CIM-5 by National Semiconductor. 
They can be ordered from their web page at www.national.com.  For the
link cord, I used the half of my link that remained from when I made
my calculater to computer link.  In order to make this, you must etch
your own PCB or find a carrier board for it.  For my prototype I just
made it so each lead of the IC went to a wire.  This way I could
connect the wires to a breadboard for testing.  You could either do
that or make a PCB and solder everything to that.  Please note that in
the schematics, the chip is upside down with pin 1 being in the upper
right as opposed to the lower left.


The connections to the chip are as follows:
1.  SDA (bi-directional data line)
2.  SCL (serial clock)
3.  O.S. (no connection in the circuit but has other uses)
4.  GND
5.  A2
6.  A1
7.  A0
8.  +Vs


A0-A2 are address bits used for the chips address.  In the schematic
all of these inputs go to GND and are therefore a logical 0.  If you
want, you can change these because the software will allow you to
change what one it looks for.  This also allows you to have multiple
temperature sensors on the same bus if they are all different.  For
more information about the LM75CIM-5, there is an Acrobat Reader file
on their web site that tells ALL about it.


The schematics are in the included .GIF file.


If you cannot view this attachment, email me and I'll send you a
different format.


This should work better.  I'm now using Pegasus Mail instead 
Microsoft Exchange.