Electronic flash can be used to take sharp photographs of events occurring at high speed. Most flash units designed for amateur photographers can provide flashes of light as short as 30 millionths of a second. This is sufficient to "stop" such events as the impact of a tennis ball with a racket, a foot with a football, and the burst of a balloon or a popcorn kernel. The main problem to solve in taking photographs of these events--or simply in using electronic flash to view them directly--is to trigger the flash discharge when the event is occurring.
Three types of triggers and an electronic delay circuit are described in this section. The simplest trigger, a metallic switch, closes automatically when the event occurs. One form of this triggered is described below. Two other types, sound and photogate, are described on the pages that follow. All of these triggers have the function of closing the flash terminals electronically.
Notes about the electronics: Our use of circuit diagrams assumes that the reader is familiar with basic electronics. Parts for the circuits are inexpensive and are available from local electronics stores and catalog outlets. For parts listings or kits for the various circuits, see Make your own.
A contact trigger is simply a metallic switch whose leads may be connected directly to a flash unit. (They may also be connected to a delay circuit for timing synchronization.) When the switch is closed by, say, dropping a rubber ball on it, a flash unit will be discharged after the preset delay. By varying the delay, the collision can be observed in various stages.
A large area contact trigger that works well for dropped objects uses 2 20-cm squares of stiff cardboard, 2 20-cm squares of aluminum foil, and 1 20-cm square of black construction paper or poster board. Do the following to construct it. A diagram is shown below.
1) Smooth any wrinkles out of a 20-cm square of Al foil. Then glue the foil to the same size cardboard. Tape the bare end of a 1-m wire lead to one corner of the foil.
2) Smooth out another 20-cm square of foil and glue it to black construction paper of the same size. Tape another 1-m wire lead to a corner of this foil.
3) Cut out a 15-cm square hole from the second square of cardboard, leaving a square frame of 2.5-cm width. Glue this frame onto the foil prepared in step 2.
4) Arrange the pieces as shown in the diagram. The cardboard frame acts as a spacer to separate the pieces of foil. When an object is dropped onto the upper piece, it is forced into contact with the lower.
Something interesting to try with the contact trigger is to drop a soft rubber ball or water balloon onto the trigger. If using a water balloon, protect the trigger with plastic. Vary the delay to capture the collision in various stages.