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Activity
9. Using an interrupter photogate to observe a
plucked cord
Equipment needed:
Vivitar 283 flash unit
4 AA batteries or SB-4 AC adapter
Cord to connect flash unit to photogate
Flash clamp
Interrupter photogate trigger
Elastic cord (approx. 1/8" diameter and 0.5 m long)
Two supports such as ring stands for the cord
Background: A photogate consists of an emitter
and detector of light aimed at each other. The emitter
may produce a beam of visible or infrared light. The interruption of
the beam serves as the triggering event. The emitter may be a light-emitting
diode (LED), a laser, or a beam of white light from, say, a
penlight. The detector is a phototransistor, serving
as a variable resistor whose resistance depends on the intensity of
detected light.
A transistor-switched photogate trigger will be used in this
activity. The circuit is described in the Tools
section.
This trigger has a very rapid response that will be needed for
capturing a fast-moving cord.  The
photogate itself is an interrupter, shown to the right and being
held by a pair of needle nose pliers. One post of the interrupter
posts houses an infrared LED and the other post houses the detector.
The two parts face inward across the narrow gap between the posts.
When the cord passes between the gap, a flash unit will discharge.
If you did Activity 8, you used a
sensitive sound trigger to observe the shape of a plucked, elastic
cord. In this activity, the interrupter photogate will be used for
observing the shape.
- You’ll first need to adjust the photogate trigger for
maximum sensitivity. Turn the sensitivity knob (or variable
resistance) one way or the other until the flash discharges
spontaneously. Then back off slightly to the point where the
flash will discharge when something is passed through the gap of
the interrupter. What you have done is found the threshold for
spontaneous triggering. As long as the resistor is just below
the threshold, the trigger will be at its most sensitive. Above
the threshold, the trigger will not function.
- Stretch the elastic cord horizontally between two fixed
supports as shown in the diagram above. Pull the cord upward at
the center and hold it steady. Bring the gap of the interrupter
down over the cord on one of the diagonal sides so that when the
cord is released, it will pass out of the slot and discharge the
flash unit.
- Try placing the interrupter at different positions. This will
change the time delay between the release of the cord and the
discharge of the flash unit. The closer the interrupter is
placed to one of the fixed ends, the longer the time delay will
be, because the wave will have to travel farther before reaching
the interrupter.
- Try releasing the cord from different positions. For example,
pluck the cord one-quarter of the way from one end to the other.
- If you wish to take photographs, review the techniques of Activity
5.
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