Electronic Guidebook for High-Speed Flash Photography, revised 12-00

Activity 11.  Using an electronic delay with a contact trigger

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 with selection of capacitors (1-mf, 0.1 mf, 0.01 mf)
• Contact trigger with wire leads for connection to delay circuit
• Soft plastic ball
• Water balloon

Background: When using a photogate trigger, the flash discharges as soon as the beam is broken. For some situations, it may be desirable to have a time delay between the breaking of the beam and the discharge of the flash unit. For example, if a ball were dropped through the photogate, a delay would be necessary in order to capture the ball as it collided with a table or the floor. A delay circuit that can be used with the photogate trigger is described here. We'll refer to the symbols in that diagram in the instructions below.

The purpose of this activity will be to learn how to use the electronic delay with a contact trigger. A contact trigger is simply composed of two metal strips or plates that are normally held a small distance apart by an insulator. The two plates are connected with wires to the PC cord of a flash unit. When the plates are forced together by, say, a colliding object, the flash unit will discharge. A method of making a contact trigger such as the one that will be used in this activity is described here.

1. Connect a flash unit to output 2 of the delay circuit. Use a 1-µf capacitor for the capacitance, C. This capacitor and the setting of the 1-MW variable resistor determine the length of the delay. Turn the resistor to mid range. Then test the trigger by touching the input to ground. You should notice a short delay before the flash unit discharges. Try turning the resistor to different values to see how that affects the delay. What is the maximum value of the delay? the minimum value? You’ll probably find that when the resistor is turned too low, the flash will not discharge at all. Now replace the 1-µf capacitor with a 0.1-µf capacitor. Is the delay noticeable? Whenever using the delay circuit, you’ll need to decide what capacitor to use in order to get best results.¹
2. Place the contact trigger on a table. Connect the wire leads to the input of the delay box. Aim the flash unit so that it will illuminate a ball as it strikes the table. A soft plastic, hollow ball works best for short drops. Use the delay circuit, with an appropriate capacitor selection, to observe the collision of the ball with the trigger at different times after contact. Some trial and error may be necessary to determine which capacitor is best. The best selection should allow you to scan the entire time interval of the collision--but not much more than that--by turning the variable resistor through its entire range. One problem that you may notice in using the contact trigger is that the flash unit discharges on consecutive bounces of the ball. This problem can be eliminated by replacing the 1-kW resistor with a 100-kW resistor. This increases the amount of time that the delay circuit, once triggered, waits before resetting.
3. Try dropping a water balloon onto the trigger. First, put the contact trigger into a plastic bag with only the wires coming out. Seal the bag well to keep water from leaking into it. Set the trigger on a slightly raised platform in a tray to catch water in case the balloon breaks. You can probably use the same capacitor as for the plastic ball. The water balloon takes on an interesting variety of shapes as it is collapsing onto the trigger and also while it is rebounding. Be sure to scan the entire time interval of the collision and a little beyond.

Notes

1. The delay (in seconds) is approximately equal to the product of the capacitance (in µf) and the setting of the variable resistor (in MW). If the maximum value of the variable resistor is 1 MW, then the maximum time delay with a 1-µf capacitor is 1 second.
2. You may wish to calibrate the delay circuit so that you know the values of the delays for various resistor settings. This can be done by connecting flash units to both of the outputs and measuring the time interval between flash discharges with a high-speed clock. (See Activity 3.)