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Electronic Guidebook for High-Speed Flash Photography

revised 12-00

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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.

    1. 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.

    1. 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.
    2. 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.
    3. Try releasing the cord from different positions. For example, pluck the cord one-quarter of the way from one end to the other.
    4. If you wish to take photographs, review the techniques of Activity 5.

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