Although the output of a Photogate is quite simple (blocked or unblocked), timing is determined by the photogate timing mode in the data-collection software. There are different modes for different types of experiments. The mode you use depends on what quantity you want to measure.
Photogate Default Setting
The default timing mode is Motion Timing, which is perfect for measuring g with the Picket Fence (PF). For more information, see How can I use a Vernier Photogate to measure g?
Performing a Vernier Experiment?
If you are doing an experiment from one of the Vernier lab books (e.g., Physics with Vernier), instructions for how to set up the Photogates with either Logger Pro or LabQuest are in the student versions of the experiment.
Writing Your Own Lab?
If you are writing your own lab, you will need to set the timing mode and enter the correct distance/length to get meaningful data. Below is an outline for some common data-collection setups.
Photogate Set Up using Logger Pro
- Using the table below, determine which Logger Pro sensor file to open and which distance/length to measure.
a. The Logger Pro sensor files for use with Photogates can be found in the Experiments>Probes & Sensors>Photogates folder.
b. Take special care in measuring the appropriate distance/length. Inaccurate distance measurements will lead to bad velocity and acceleration data.
c. Enter the distance/length into the appropriate parameter box, when applicable. If no parameter box is on the page, the distance/length can be entered by selecting “Set Up Sensors” from the Experiment menu, then selecting your interface. Click the Photogate and select “Set Distance or Length.” In the dialog box, enter the appropriate distance/length and the corresponding units.
- Modify the sensor file, as necessary. In many cases, the sensor file will already provide the information you need. If not, you may need to create your own user parameters and calculated columns to generate the data you want.
- Save your modified sensor file under a new name in a location accessible to your students.
|Desired Measurement||Sensor File||Distance/Length to Measure||Details|
|Instantaneous Speed||One Gate Timer.cmbl||Object length
Note: If the length entered differs from the actual length of the object that passes through the beam, the speed measurement will be off. For example, if you rolled a marble through the gate, you would need to know the diameter or secant of the ball that went through the beam.
|Measure the length of the object that will pass through photogate. Set up the photogate so that the object moves through it. When the object passes through the photogate, it measures how long the beam is blocked. The object’s speed (at the position of the photogate) is determined by dividing the length of the object by the blocked time.|
|Average Speed||Pulse Timer – Two Gates.cmbl||Distance from first photogate to second||Set up two photogates so that the object will pass through both of them. Measure the distance between the gates. Timing starts when the object breaks the beam of the first photogate and ends when it breaks the beam of the second photogate. The average speed of the object is determined by dividing the distance between the photogates by the measured time.|
|Speed and Acceleration||Gate and Pulse Timer.cmbl||Object length||Set up two photogates so that the object will pass through both of them. Measure the length of the object. Three times are measured: the blocked time of the first photogate, the blocked time of the second photogate, and the elapsed time between the photogates. The length of the object and the blocked times are used to calculate the object’s speed through each photogate. The object’s average acceleration is determined by dividing the change in speed from the first photogate to the second by the time elapsed between the two.|
|Speed and Acceleration||Motion Timer Picket Fence.cmbl, Pulley.cmbl, Cart picket fence.cmbl||If you are using a Vernier Picket Fence, Cart Picket Fence, or Ultra Pulley, no distance measurement is necessary. (Sensor file uses the dark band spacing as the default distance.)
If you are using another or homemade picket fence, measure the distance from the leading edge of one dark band to the next.
|Attach a picket fence to the object and set up a photogate so that the object will pass through it. As the picket fence moves through the photogate and the beam is alternately broken and unbroken, each event is given a distance, based on the known spacing of the picket fence. From that distance data, velocity and acceleration can be calculated.
This same timing mode is used with an Ultra Pulley (Pulley.cmbl) or Bar Tape (Motion Timer Picket Fence.cmbl)
|Period of a Pendulum||Pendulum Timer.cmbl||N/A||Place the photogate at the bottom of the pendulum swing. Photogate measures the time from the first bob pass to the third, i.e., when the bob swings back through the photogate in the original direction.|
|Projectile Motion||Varies||Varies||Photogate set up for projectile motion is discussed here (How do I set up Vernier Photogates (VPG-BTD) for data collection in a projectile motion experiment?).|
- The following diagram summarizes the various timing modes and how they differ in timing measurements. The diagram shows a horizontal line representing time. The vertical position represents the gate state. When the line is high, the gate is unblocked. When the line is low, the gate is blocked. Time T1 is the time that software records as the gate being blocked. In the case of Gate Timing, the diagram shows that the gate is initially unblocked, then blocked, and then unblocked again and the interface records only the time it is blocked.
- There are also other, less common, timing modes useful for specific applications. Here is a complete list of the terms used in Logger Pro sensor files:
- Bounce.cmbl: Useful for studying the bounce of a ball using a Photogate mounted one ball diameter above the floor.
- Flash.cmbl: Simply measures how long a Photogate is unblocked. It can be used to measure how long a camera shutter is open.
- Strobe.cmbl: Measures the time between unblocking events of the Photogate. It can be used to check the timing of a strobe light, if the light replaces the IR source of the photogate.
- Collision.cmbl: This file is like gate timing, but assumes two Photogates are used to measure speeds before and after a collision for two different objects. The moving objects are assumed to have “flags” of known length on them that go through the Photogates.
- Jump.cmbl: This file is set up to use with a laser beam along the floor pointed into one of our newer Photogates which allows you to use the laser beam as the Photogate beam. It measures the hang time of someone jumping.
- Daisy-chained Photogates.cmbl: Up to four Photogates can be daisy-chained together and spaced at equal intervals to mimic the functionality of Motion Timing. This file reads the blocking/unblocking events of the daisy-chained gates and uses the known spacing to calculate speed and acceleration.
- A comprehensive tutorial can be found at
How can I use Vernier Photogates (VPG-BTD) to time the Science Olympiad Scrambler event?
Why does my photogate record a really high velocity at the beginning of a trial with a picket fence?
How can I use a Vernier Photogate to measure g?
Why is my picket fence blue and black instead of clear and black?
How can I use a photogate to count events?
Note: For data collection with TI devices, see https://www.vernier.com/products/texas-instruments/data-collection/