Photogates

Photogates are digital sensors that connect to the digital inputs of your interface (DIG 1 or 2 on LabQuest, DIG/SONIC 1 or 2 on LabPro).
 

Connecting Photogates

Note: Older Photogates do not have the auto-ID feature and must be set up manually.

Once the Photogate is set up in the Sensor Setup window, the gate status will be displayed. Block the Photogate with your hand; the status should report as Blocked. Once you remove your hand, it should change to Unblocked. The status of the gate is also displayed under the Toolbar.

The default mode is motion timing. This mode is appropriate for a picket fence experiment, a smart pulley, or for a string of four or more daisy-chained photogates. (Only auto-ID photogates may be daisy-chained.) Click on the Photogate Channel Box in the Sensor Setup window to change the settings.

Set Distance or Length:

Current Settings

The default Timing Setting for the Photogate is Motion Timing (blocked or unblocked) vs. Time. In the data table, blocked status is indicated as a "1" and unblocked indicated by a "0."


Motion Timing

The Motion Timing mode uses a photogate or Smart Pulley connected to the digital input. During operation, times are recorded as leading opaque edges of  a "picket fence," bar tape, or a pulley spoke pass through the photogate beam. These times are displayed in a data table. More importantly, if you enter the distance between the leading edges of the opaque bands in the Length of Object field, the program can analyze the times, and calculate velocities, displacements, and accelerations.

When a picket fence or bar tape is used, the width of each of the bands (b in the figure) should be at least 0.5 cm, The distance between the leading edges (d in the figure) should be at least 3 cm or larger if the picket fence is to be moving rapidly (for example in the free fall experiment). A closer spacing can be used if the object will be moving slowly; for example, on an air track glider. Motion timing uses the block to block timing interval for most calculations.


Pulse Timing

Pulse Timing refers to the measurement from when a photogate gets blocked to when it gets blocked again.


Gate Timing

For this mode, timing will begin when the photogate is first blocked. The timing will continue until the gate is unblocked. The duration of the interruption is thus timed. If the length of the object is entered in the Length of Object field, the velocity is calculated.


Pendulum Timing

The Pendulum Timing mode uses a photogate attached to an interface. The timing will begin when the photogate is first interrupted. The timing will continue until the photogate is interrupted twice more, so that you get the time for a complete swing of a pendulum or other oscillating object.


Photogate Timing

In Photogate Timing mode, only a time and Gate State column are displayed. You may add any other calculated columns as desired.


Calculated Columns for the Photogate

 Logger Pro contains functions you can use to create calculated columns based on photogate data.
1. Select New Calculated Column from the Data Menu. 
2. Click the Functions tab and select photogate > mode
3. Choose from the Variables pull-down menu the variable to be inserted in the equation. The variable "GateState" refers to the blocked/unblocked status of the photogate.
4. Enter a name and a short name for you new column and click Done.

The new column will be listed in the Data Table.

 
Photogate Functions
Blocked To Blocked ("Time", "Gate1", "Gate2")
"Time":    Optional. A column of real numbers (the times of events)
"Gate1":   A column of photogate states (1's and 0's)
"Gate2":   Optional. A column of photogate states (1's and 0's)
Returns a column of the times between successive blocked events in gate 1 and blocked events in gate 2.  If you don't enter a "Time" column, the program will find one.  If you don't enter "Gate2", "Gate1" will be used.
Blocked to Unblocked ("Time", "Gate1", "Gate2")
"Time":    Optional. A column of real numbers (the times of events)
"Gate1":   A column of photogate states (1's and 0's)
"Gate2":   Optional. A column of photogate states (1's and 0's)
Returns a column of the times between successive blocked events in gate 1 and unblocked events in gate 2.  If you don't enter a "Time" column, the program will find one.  If you don't enter "Gate2", "Gate1" will be used.
Unblocked to Blocked ("Time", "Gate1", "Gate2")
"Time":    Optional. A column of real numbers (the times of events)
"Gate1":   A column of photogate states (1's and 0's)
"Gate2":   Optional. A column of photogate states (1's and 0's)
Returns a column of the times between successive unblocked events in gate 1 and blocked events in gate 2.  If you don't enter a "Time" column, the program will find one.  If you don't enter "Gate2", "Gate1" will be used.
Unblocked to Unblocked ("Time", "Gate1", "Gate2")
"Time":    Optional. A column of real numbers (the times of events)
"Gate1":   A column of photogate states (1's and 0's)
"Gate2":   Optional. A column of photogate states (1's and 0's)
Returns a column of the times between successive unblocked events in gate 1 and unblocked events in gate 2.  If you don't enter a "Time" column, the program will find one.  If you don't enter "Gate2", "Gate1" will be used.
Blocked Mid Times ("Time", "Gate1", "Gate2")
"Time":    Optional. A column of real numbers (the times of events)
"Gate1":   A column of photogate states (1's and 0's)
"Gate2":   Optional. A column of photogate states (1's and 0's)
Calculate the average times between blocked events from Gate 1 to Gate 2.  If you don't enter a "Time" column, the program will find one.  If you don't enter "Gate2", "Gate1" will be used.
Blocked to Unblocked MidTimes Blocked to Unblocked MidTimes
"Time": Optional. A column of real numbers (the times of events)
"Gate1": A column of photogate states (1's and 0's)
"Gate2": Optional. A column of photogate states (1's and 0's)
Calculate the average time between successive blocked events in gate 1 and unblocked events in gate 2.  If you don't enter a "Time" column, the program will find one.  If you don't enter "Gate2", "Gate1" will be used.
Unblocked to Blocked MidTimes Unblocked to Blocked MidTimes
"Time": Optional. A column of real numbers (the times of events)
"Gate1": A column of photogate states (1's and 0's)
"Gate2": Optional. A column of photogate states (1's and 0's)
Calculate the average time between successive unblocked events in gate 1 and blocked events in gate 2.  If you don't enter a "Time" column, the program will find one.  If you don't enter "Gate2", "Gate1" will be used.
Unblocked Mid Times ("Time", "Gate1", "Gate2")
"Time":    Optional. A column of real numbers (the times of events)
"Gate1":   A column of photogate states (1's and 0's)
"Gate2":   Optional. A column of photogate states (1's and 0's)
Calculate the average times between blocked events from Gate 1 to Gate 2.  If you don't enter a "Time" column, the program will find one.  If you don't enter "Gate2", "Gate1" will be used.
Pendulum Period ("Time", "Gate1")
"Time":    Optional. A column of real numbers (the times of events)
"Gate1":   A column of photogate states (1's and 0's)
Calculate the time between every other blocked event on Gate 1.  If you don't enter a "Time" column, the program will find one.
derivativeTimeShift DerivativeTimeShift ("Y", "X"): returns the derivative of "Y" with respect to "X".  This function is specifically designed to be used with photogate and picket fence data. The derivatives returned are adjusted to estimate values at the start of the timing interval, instead of the midpoint. For details see The Physics Teacher, Vol 35, April 1997, p.220. Average velocity during the time interval is equal to the instantaneous velocity at midpoint of the time interval.

Where
secondDerivativeTimeShift Numerical time-shifted second derivative("Y", "X")
"Y": A column of real numbers
"X": Optional. A column of real numbers
Numerical time-shifted second derivative. Calculates the second numerical derivative of "Y" with respect to "X".  The values are shifted so that the derivatives are calculated at the midpoints between each two values.  If you don't supply an "X" column, the program will find one.

collapse collapse("X"): Returns a column with all the non-numerical cells (blanks and text) removed.
collapseIndirect collapseIndirect(X, Y) Returns a column of only the rows in "X" corresponding to rows in "Y" that have valid numerical cells.

See Also:

How is the use of photogates different in Logger Pro 3 from Logger Pro 2?