To explore how different motion detectors operate.
To successfully build and test an ambient-light-ignoring
active IR motion detector.
There are a wide variety of motion detectors available
currently. To allow a better understanding of motion
detectors, the following section provides a detailed
description of a few different types.
Ultrasonic transducers can be used to detect motion in
an area where there are not supposed to be any moving
objects. This type of motion detector is most commonly
used in burglar alarm systems since they are very
effective in this application.
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Figure 1 shows the operation of an ultrasonic motion
detector. There are two transducers: one emits an
ultrasonic wave and the other picks up reflections from
the different objects in the area. The reflected waves
arrive at the receiver in constant phase if none of the
objects in the area are moving. If something moves, the
received signal is shifted in phase. A phase comparator
detects the shifted phase and sends a triggering pulse
to the alarm.
Ultrasonic motion detectors have certain advantages and
disadvantages when compared with other types of motion
detectors. The main advantages is that they are very
sensitive and extremely fast acting. However, the
largest problem with this type of motion detector is
that it sometimes responds to normal environmental
vibration that can be caused by a passing car or a plane
overhead. Some types of motion detectors use infrared
sensors to avoid this problem, but even these detectors
have some problems.
In passive infrared motion detectors, a sensor
containing an infrared-sensitive phototransistor is
placed in the area to be protected. Circuitry within the
sensor detects the infrared radiation emitted by the
intruder's body and triggers the alarm. The problem with
using this type of detector is that it can be falsely
triggered by warm air movement or other disturbances
that can alter the infrared radiation levels in an area.
In order to prevent this problem, newer systems use two
infrared sensors which monitor different zones within a
protected area. Logic within system triggers the alarm
only when the two zones are activated in sequence, as
would occur if a person walked through the protected
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Figure 2 shows the operation of an active infrared
motion detector. In the active system each sensor
consists of two housings. One housing contains an
infrared-emitting diode and an infrared-sensitive
phototransistor. The other housing contains an infrared
When positioned in front of an entrance to a protected
area, the two housings establish an invisible beam. A
person entering the area interrupts the beam causing an
alarm to be triggered. An active motion detector is much
more reliable than a passive one, but it requires
careful alignment when it is installed. The detector can
be falsely triggered if one of the housings moves
slightly and causes a discontinuous beam.
For our project, we decided to construct an active
infrared motion detector. Originally, we wanted to build
both an IR and an ultrasonic detector, but we decided
that an ultrasonic detector would require too much time
for a three week project.
However, we also decided that just building an IR motion
detector would probably be a trivial exercise. So, we
decided to expand on the concept by building an ambient
light ignoring motion detector.
This type of motion detector uses the same basic concept
as the active infrared motion detector. An interruption
in a 5 kHz modulated pulsating beam that is transmitted
by an infrared diode and received by an infrared
transistor sets off the alarm. A schematic of this
motion detector is given in Figure 3.
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Figure 3. Ambient-Light-Ignoring Active Motion Detector
The circuit on the left is the transmitter circuit that
establishes a 5 kHz modulated infrared beam. As you can
see from the schematic of the receiver circuit, a
resonance-tuned narrowband amplifier reduces the
detector's sensitivity to stray light. C1 and L1 in
IC2A's feedback loop cause the op amp to pass only those
frequencies at or near the LED's 5 kHz modulation rate.
IC2B's output increases when the received signal is
sufficient to drop the negative voltage across C2 below
the reference set by R2. The output of this circuit is
then attached to some load resistance, which can be an
alarm or, for demonstration purposes, an LED.