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B DUIOLDGA DETECTOR
BY MICHAEL SIMPSON
We have several pets in our home, including two dogs. One of the
problems we have is the dogs like to sneak into the kitchen and eat
the cat’s food. My wife asked me if there was any way we could create
some sort of detector that would chirp each time a dog entered the
kitchen. Of course, I said yes.
The key to this project is to detect only
the dog and not a person or cat
entering the kitchen. This can be done with
two sensors at various heights and a bit of
logic to decode the sensors.
I started doing some research and
started looking into some of the various
sensors available.
not well suited for this type of application. First, they are
not very directional, and even by placing them in an
enclosure they still suffer. Also, they tend to be sensitive
to sudden changes in IR like a shadow moving across the
sensor. After building and testing the Dog Detector
based on these sensors, I knew I would have to look into
another sensor.
Lasers and Photo
Detectors
Active IR Sensors
These would probably be the simplest
to interface, but require two separate power
sources and mounts: one for the lasers and
the other for the photo detectors. This way
also makes it difficult to relocate the sensor
and adjust the heights of the sensors. There
are also alignment issues. The small photo
detector surface means you can only have a
1/8” variance. For these reasons, I decided
these types of sensors are much too finicky
for this application.
The concept behind active IR sensors is much like that
of the PIR sensor. The main difference is that this sensor
actually sends out IR energy and measures the amount of
that energy reflected back. Many of these types of sensors
modulate the IR energy to give them a certain amount of
immunity from outside IR interference. Some of these
sensors can actually return range information. The sensor
shown in Figure 3 outputs a voltage based on the amount
of IR energy that is reflected back. While these sensors are
more immune to outside influences, they do have trouble
when incandescent lamps are in use. They also have
problems with various surfaces, as some do not reflect
■ FIGURE 1 Passive IR
Sensors
These are also know as PIR sensors.
This type of sensor monitors the amount
of IR energy hitting the internal sensor and
changes the state of the output lead when
it detects a sudden change in this energy.
The sensor shown in Figure 2 can be
purchased from RadioShack for about
$10. I actually built the Dog Detector
using these sensors but found that they are
40 December 2008
■ FIGURE 2
■ FIGURE 3
