A sound trigger discharges a flash unit in response to a sound or vibration created by either the high-speed event itself or some other action that initiates the high-speed event. The three parts of a sound trigger are the microphone, the amplifier, and a silicon-controlled rectifier (SCR). The diagram above shows how the components are connected. The microphone picks up the sound of the event. The signal from the microphone is then amplified in order to gate the SCR, which acts as a switch. The amplifier output is connected to the gate and cathode of the SCR, and the flash terminals are connected to the anode and cathode. When a sound is picked up by the microphone, the amplified electrical current flows in the gate-cathode circuit of the SCR. That in turn allows current to flow in the anode-cathode circuit, thus discharging the flash unit.

The SCR has a second function. It serves to isolate the amplifier from the voltage across the terminals of the flash unit. This can be as high as 330 V for older flash units. Newer units, however, may have low-voltage trigger circuits. In any case, an SCR rated for 400 V is sufficient.

When using a sound trigger, several factors influence the time delay between the high-speed event and the discharge of the flash. These include the intensity of the sound, the sensitivity of the trigger, and the position of the trigger in relation to the event. Changing the latter is a good way to fine tune the time delay, since one can use the fact that sound travels about 345 m/s in air at room temperature. (A convenient way to remember this is as a third of a meter per millisecond.)

The initial placement of the sound trigger is a matter of guesswork. If it is placed close to the subject and the flash discharges sooner than desired, the trigger can simply be moved farther away. Photographs need not be taken as these adjustments are made, because naked-eye observations are possible.

The fact that the delay between the production of the sound and the discharge of the flash depends directly on the distance between the source of the sound and the microphone can be exploited to photograph the event at different times. For example, with the sound trigger next to a balloon, one can catch a rip in the balloon just as it is beginning. As the trigger is moved farther away, the rip will also have moved farther. With two flash units, independently triggered at different times during the event, double-exposure photographs can be made to show the progression of the event.

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Piezoelectric sound trigger

Sound triggers are convenient to use in many situations. If the event to be photographed produces a loud, sharp sound, piezoelectric triggers work fine. The pressure of the sound pulse on a piece of piezoelectric film distorts the film, which produces a voltage pulse in response. A transistor switch serves to gate a sensitive SCR, that is, one with a low gate current threshold.

While one can spend a hundred dollars or more for a professional model sound trigger, there are inexpensive alternatives that work just as well in many situations. The circuit shown below can be assembled for less than $10. It consists of a piezoelectric microphone, a general-purpose transistor, a 5-kW variable resistor for sensitivity control, a 400-V SCR, and a 9-V source. The SCR must be sensitive to 0.2 ma of gate current or less.

Note:  The piezoelectric trigger doesn't work well for sounds like thuds (for example, tennis ball or racquetball collisions).  A trigger sensitive to a wide range of frequencies is needed. One possibility is the Freeze Frame kit available from PAIA electronics. An article about this trigger kit was published in the November, '92 issue of Electronics Now.

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Tape recorder sound trigger

An audio amplifier with a dynamic or condenser microphone may be used to provide a sensitive sound trigger with a wide frequency response. The output current is great enough to gate a less sensitive SCR, such as the 400-V SCR available from Radio Shack (part no. 276-1020). An amplifier intended for sound reproduction will also work as a sound trigger. A microphone is used for the input, and a speaker or earphone output is used to gate an SCR to which the flash terminals are connected.

A tape recorder with a built-in microphone is particularly convenient to use. The recorder must be one that gives an output signal while recording. The only part that must be specially constructed is the cord that connects the tape recorder to the flash unit. Two leads from the earphone jack are soldered to the gate and cathode of an SCR. The two leads from the flash terminals are connected to the cathode and anode. A method of making connections without having to cut the PC cord is described next.

1) Cut a 1-cm section from the top of a plastic ball-point pen cartridge. Insert this inside the collar of the connector at the end of the PC cord. If the plastic doesn't fit, shave or file off a little. The fit has to be snug to prevent the plastic from falling out. It serves as the insulation between the inner positive pin and the outer negative collar. A cross-sectional view of the assembly is shown to the right.

2) Clip the earphone from the earphone cable of the tape recorder. The leads that come from the recorder are the ones that will be used. They should be several feet long to allow the recorder to be positioned far from the flash. If they’re too short, splice extra wire onto them. Strip a quarter of an inch of insulation from the ends of the leads. Then wrap one lead (either one) around the gate of the SCR and the other around the cathode. Solder or tape them in place.

3) Carefully bend the cathode of the SCR closer to the anode. Bend the gate farther from the anode. Then push the anode into the plastic sleeve inside the collar of the PC terminal. Be sure that the anode is pushed in far enough to make contact with the pin. Also check that the cathode is making contact on the outside of the collar. You may need to use tape to hold the cathode firmly in place. A photograph of the completed assembly is shown to the right.

When using the tape recorder sound trigger, insert a blank tape and turn on the record function. You'll probably find that the trigger is so sensitive that the click of the camera shutter may trigger the flash discharge prematurely, giving unwanted film exposures. In order to avoid this, hold your hand over the camera lens while opening the shutter.

One thing to try with this sound trigger is capturing the collision between a tennis ball and the racket. Hit the ball into a hanging sheet to prevent the ball from rebounding from walls.