[TriEmbed] Raspberry Pi & remote temp sensor

[Scott Hall]

On 05/02/2014 07:13 PM, jonathan hunsberger wrote:
> Not sure about the 1-wire.  I have some DHT22 / AM2302 (one signal wire, but
> not "1-wire" protocol) running about 16 meters on wire that was once used for
> security system sensors.  Hooked to an Arduino right now, but they worked
> similarly on a Raspberry Pi before I used that for another project.
>
>
> On Fri, May 2, 2014 at 4:06 PM, Grawburg <grawburg at myglnc.com
> <mailto:grawburg at myglnc.com>> wrote:
>
>     I have an RPi set up with two DS18B20 temperature sensors - one is fixed
>     on a breadboard and one is the waterproof unit with about a meter
>     extension (got from AdaFruit).  Works fine as is. I do not need a
>     waterproof sensor but would like to put the sensor 10 to 15 feet away from
>     the RPi: we are measuring the temperature at different points in a room.
>      What's the reasonable limit for extending the sensor away from the Pi?
>

I was reading over the specs
(http://www.maximintegrated.com/datasheet/index.mvp/id/2815) and see that since
this a "parasitic" one-wire device, it derives its power from the signal line. 
So therefore, as long as its getting sufficient power and its signal can be
distinguished beyond noise and delay artifacts, 1-wire buses can be built quite
long.  According to this article from Maxim,
http://www.maximintegrated.com/app-notes/index.mvp/id/148, pulling up the bus to
5V and using 5V driving signals and using cat-5 twisted pair wiring, the usual
configuration is a long bus up to 450-meters with less than 50-meters between
stubs and less than 3-meter stubs to the devices should be fine:


You can build a bus with long stubs if you shorten the main bus:

however, due to reflections and such, you should add a resister at the point
where the stub connects to the main bus:


For these long buses, you should probably drive it with a transistor to reduce
noise:


To isolate the bus from the R-Pi's data pins directly, for small to medium sized
network up to 100-meters you can use a bus driver, like a DS2480B or DS9097U and
a simple R-C impedance matching circuit:



1-Wire dataspecs and implementation:
http://www.maximintegrated.com/app-notes/index.mvp/id/5134

More info here: https://en.wikipedia.org/wiki/1-Wire

-- 
Scott G. Hall
Raleigh, NC, USA
ScottGHall1 at GMail.Com

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