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Electronic Guidebook for High-Speed Flash Photography

revised 12-00

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Activity 12. Plucked cords and rubber bands


Equipment needed:

  • Vivitar 283 flash unit
  • 4 AA batteries or SB-4 AC adapter
  • Cord to connect flash unit to delay circuit
  • Flash clamp
  • Delay circuit
  • Two 0.01-µf capacitors
  • Bracket photogate and trigger circuit (transistor type)
  • Cord from photogate trigger to delay circuit
  • Elastic cord
  • Two supports for the cord
  • Rubber bands

Background:  In Activity 9, you used an interrupter photogate to observe waveforms on a plucked, elastic cord. You achieved different delays by moving the interrupter to various positions along the cord. In this activity, you’ll use a bracket-type photogate in a fixed position in order to obtain a wider range of delays. The bracket, shown to the right, holds the infrared LED and photodetector farther apart than the interrupter, thus allowing larger objects to pass through.

  1. In order to observe waveforms on an elastic cord, connect the output of the photogate to the input of the delay circuit. Use a 0.01-µf capacitor with the delay unit, and connect the flash unit to output 2. Adjust the photogate to maximum sensitivity as described in Activity 9. Set up the arrangement shown below. Tape the photogate to the tabletop and stretch the cord between two fixed supports. Pull the cord down into the bracket and release it in order to discharge the flash unit. Adjust the delay in order to show the cord at different times after release. If you need greater delay times, use a larger capacitor. Adding a second 0.01-µf capacitor in parallel with the first will double the maximum delay time.

  1. Try shooting a rubber band through the photogate. Adjust the delay to show the band at various times after it leaves the gun. In order to get reproducible results, you'll need to shoot the rubber band in a similar way each time. You may want to make a gun using a board and clothespin.



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