012-06379B
Photogate Timer
3
®
SPECIFICATIONS
Detector rise time:
200 ns max.
Fall Time:
200 ns max.
Parallax error:
For an object passing through the photo-
gate, within 1 cm of the detector, with a velocity of less
than 10 m/s, the difference between the true and effective
length of the object will be less than 1 millimeter.
Infrared source:
Peak output at 880 nm; 10,000 hour life.
Figure 3: Timing an Air Track Glider
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NOTE:
If additional photogate interruptions
occur after the second time is measured, and before
the MEMORY switch is flipped to READ, they too
will be measured by the timer and included in the
cumulative time.
TIMING SUGGESTION
Since the source and detector of the photogate have a
finite width, the true length of the object may not be
the same as the effective length seen by the photo-
gate. This parallax error may be minimized by hav-
ing the object pass as close to the detector side of the
photogate as possible, with the line of travel perpen-
dicular to the beam. To completely eliminate the
parallax error in experimental data, determine the
effective length of the object as follows:
c
With the Timer in Gate mode, push the object
through the photogate, along the path it will fol-
low in the experiment.
d
When the photogate is triggered (the LED on top
of the photogate comes ON), measure the position
of the object relative to an external reference
point.
e
Continue pushing the object through the photo-
gate. When the LED goes OFF, measure the posi-
tion of the object relative to the same external ref-
erence point.
f
The difference between the first and second meas-
urement is the effective length of the object.
When measuring the speed of the object, divide
this effective length by the time during which the
object blocked the photogate.
Memory Feature
When two measurements must be made in rapid succes-
sion, such as measuring the pre- and post-collision veloci-
ties of an air track glider, use the memory function. It can
be used in either the Gate or the Pulse mode.
To use the memory:
c
Turn the MEMORY switch to ON.
d
Press RESET.
e
Run the experiment.
When the first time (t
1
) is measured, it will be immedi-
ately displayed. The second time (t
2
) will be automati-
cally measured by the timer, but it will not be shown
on the display.
f
Record t
1
, then push the MEMORY switch to READ.
The display will now show the TOTAL time, t
1
+ t
2
.
Subtract t
1
from the displayed time to determine t
2
.
Figure 4: Photogate Timing a Pendulum
