Issue
113 December 1999
Being
Cool is Easy
A
Temperature-Sensing Control Device
PUTTING
IT ALL TOGETHER
I
now had to decide what to do with an X-10 interface.
I certainly could make use of some sort of motion-sensing
device. After some thought, I decided that a temperature
sensing device would be a logical extension to my first
project.
I
also had a real need for such a device. My daughter’s
bedroom on the second floor tends to get hot during
sunny summer days. I had been using the Homebase controller
to turn on a fan on summer afternoons.
Most
of the time this solution was adequate. On occasion,
however, when we had a cool, rainy day (not uncommon
in central New York), the bedroom would get too cool.
The
obvious solution was to turn the fan on in the afternoon
only if the temperature was above a certain level. However,
I’d also like to be able to manually override the
fan control when I’m at home.
The
more I thought about the requirements for this fan controller,
it became obvious that an autonomous device didn’t
make sense. What I needed was a "dumb" sensor
device, which provided input to the Homebase controller.
My
first thought was to implement a remote temperature-sensing
device that would send an X-10 command to the Homebase
controller whenever the temperature transitioned through
a predefined threshold.
There
were two problems with this approach. First, the threshold
would have to be set in the remote device. Second, I
already had enough X-10 transmission devices and adding
one (or more) additional device(s) that transmitted
on their own would only increase the probability of
a collision.
What
I finally decided on was a device that would speak only
when spoken to. The Homebase controller sends a query
to the remote temperature sensor. This query specifies
the temperature threshold. The remote temperature sensor
then, and only then, responds (transmits) to indicate
if the current temperature is below, at, or above the
specified threshold.
The
sensor is programmed for a single house code and responds
to queries from the Homebase controller. These queries
consist of two consecutive X-10 unit On commands.
The
sensor decodes these On commands to determine the temperature
threshold value to use for the comparison.
The
decoded value is compared to the actual sensed temperature.
The sensor responds with a unit On command if the temperature
is greater than or equal to the queried temperature
value, or with an Off if not. The homebase controller
then takes the appropriate action based on the response
received from the sensor.
An
example of the protocol is illustrated in Table 1. The
controller sends a 75°F query and the temperature sensor
responds with a unit On command to indicate that the
current temperature is equal to or greater than 75°F.
Controller
Transmit 75°F query:
Unit A, Key Code 7
Unit A, ON Command
Unit A, Key Code 5
Unit A, ON Command
Temperature sensor
Compare current
temperature to 75°F
Transmit response:
Unit A, Key Code 16
Unit A, ON Command
Controller
Take appropriate
action for temperature at or above 75°F.
|
| Table
1— In this command protocol example, the
X-10 controller sends a 75° query to the temperature
sensor. The sensor responds to indicate that the
temperature is at or above this value. |
Note
that an On command consists of two X-10 transmissions.
Each transmission contains a House Code and a Key or
Function Code. The Key Code of the first transmission
identifies the unit (1 through 16) and the Function
Code of the second transmission identifies the Command
Code (On in this case).
