Connect the sensor following the steps in the Getting Started section of this user manual.
Internal Gate Mode and Laser Gate Mode
The Vernier Photogate operates in two modes. A shutter over the internal gate detector determines the operating mode. The shutter is on the inside of the thinner gate arm. Open the shutter to use the internal gate, and close the shutter to use the external laser gate. A red LED is on when the gate is blocked in either mode.
To use the internal gate mode, open the shutter and position the Photogate. When the gate is blocked the red LED will be illuminated.
To use the external laser gate mode, close the shutter for the internal gate. The laser port is on the outside edge of the gate adjacent to the captive bolt. Align your laser so the beam enters the port and turns off the LED. Blocking the laser beam at any point in its path will then turn the LED back on. The path of the laser need not be a straight line. You may want to use mirrors to create a complex path that is crossed by the moving object multiple times.
Laser Safety Note: Do not align the external laser gate by sighting by eye. Follow all safety precautions indicated by the laser manufacturer.
The Vernier Photogate can be connected in a daisy-chain mode. Connect one gate to the interface, and then connect the next gate to the first Photogate using the white BTD socket on the arm of the Photogate. Up to four Vernier Photogates can be connected to an interface at one time. The daisy-chain mode requires the cable from the VPG‑BTD model. If you have purchased the VPG-DG version for compatibility with your interface, you will need to purchase an additional cable to make each photogate-to-photogate connection (order code PG‑BTD).
In the daisy-chain mode, the analyzing software has no way to determine which gate has been blocked, so be sure that this information is not needed. One common setup is to use gate timing, so that the software reports the time a gate is blocked. If you know the order in which gates are blocked from the geometry of the experiment, then daisy-chain mode will work.
Note that in a typical collision experiment with two objects passing through the two gates, the gates may have overlapping block intervals. In this case, you must connect the gates to two separate channels on the interface. Motion timing mode can be used if the daisy-chained gates are equally spaced. Enter the distance between gates in your software to determine position, velocity, and acceleration of a single object passing through a series of photogates.
This sensor is equipped with circuitry that supports auto-ID. When used with LabQuest 2, LabQuest, LabQuest Mini, LabPro, SensorDAQ, TI-Nspire Lab Cradle, or CBL 2, the data-collection software identifies the sensor and uses pre-defined parameters to configure an experiment appropriate to the recognized sensor.
More Information on Geometric Aspects of Photogate Timing
Photogates have geometric complications that result in the effective length of an object passing through the gate being slightly less than the actual length.
The Motion Timing mode uses a photogate or 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.
In this mode, a measurement from when a photogate gets blocked to when it gets blocked again will be recorded.
For this mode, timing begins when the photogate is first blocked. The timing continues 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 mode uses a photogate connected to an interface. The timing begins when the photogate is first interrupted. The timing continues until the photogate is interrupted twice more, so that you get the time for a complete swing of a pendulum or other oscillating object.
In Photogate Timing mode, only a time and Gate State column are displayed. You may add other calculated columns as desired.
For a good discussion of these issues, see “Photogates: An instrument evaluation,” Eugene P. Mosca and John P. Ertel, Am. J. Phys. 57 (9), 840–844 (1989).
A more comprehensive tutorial can be found in the following documents:
- Logger Pro Introduction to the Vernier Photogate
- LabQuest Introduction to the Vernier Photogate
Experiments Using the Vernier Photogate
The Vernier Photogate is used in several experiments in the book Physics with Vernier. See this book for detailed experiments. Here are some brief examples of things you can do with a photogate.
- If you know the diameter of a ball rolling through a photogate, you can determine the speed of the ball from the ratio of the diameter to the time the gate is blocked by the ball. This requires only one gate, but the gate has to be positioned carefully so the light beam intersects the middle of the ball.
- Using two photogates positioned at a known separation, you can determine the speed of an object from the time interval between the breaking of the first beam to the breaking of the second. This mode is known as pulse timing.
- Set up a pendulum so that the bob swings through the photogate. The time interval from one block to the third block yields the pendulum period.
- Use the laser gate at floor level to measure the “hang time” of a jumper. The jumper’s shoes will block the beam while on the floor. The time interval of interest is then the unblock to block time.
- Use an Ultra Pulley Attachment to construct an Atwood’s machine, consisting of two masses connected by a flexible string. The string passes over the pulley, causing it to rotate as the masses move. Use motion timing to measure the position, velocity, and acceleration as a function of time.
- Measure the free fall acceleration of a picket fence using either the internal gate or the laser gate. Motion timing will give you the position, velocity, and acceleration as a function of time. Do the two modes give different results?