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

Activity 4.  Triggering the flash unit with sound

Equipment needed:

• Vivitar 283 flash unit
• 4 AA batteries or SB-4 AC adapter
• Flash-to-trigger cord
• Piezoelectric sound trigger (requires 9-V battery)
• Balloons and pin
• Jeweler's screwdriver (flathead)

Background: You’ve already seen how to trigger a flash discharge by shorting across the flash terminals. In this activity, you'll use a sound trigger, a circuit that shorts the terminals electronically in response to sound.

The three parts of a sound trigger are a microphone, an amplifier, and a silicon-controlled rectifier (SCR). The diagram to the right shows how the components are connected. The microphone picks up the sound of the event to be observed. The amplifier boosts the current from the microphone signal in order to actuate the SCR.  The SCR serves as a switch to short the flash terminals, which are connected to the cathode (C) and anode (A) of the SCR. The amplifier output is connected to the gate (G) and cathode of the SCR. 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. Besides acting as a switch, the SCR isolates the amplifier from the voltage across the flash terminals.

The sound trigger for this activity uses a piezoelectric film as the microphone.¹ The film is pressure-sensitive, producing a voltage spike in response to the sudden deformation caused by a loud, sharp sound such as a hand clap or balloon burst. A diagram of the internal components of the trigger is shown below. The 5-kW potentiometer serves as a variable resistor to provide sensitivity control. A 9-V source is connected to pins 1 (+) and 3 (-), and the flash terminals to pins 2 (+) and 3.

1. Assemble the sound trigger, flash-to-trigger cord, and flash unit as shown in the diagram to the right.²  Turn the flash unit on (if using batteries) or plug in the SB-4 adapter.  The sound trigger will automatically be powered on when the 9-V battery is connected.
2. In order to test the trigger, clap your hands or snap your fingers to discharge the flash unit. You can test the sensitivity of the trigger by seeing how far you can stand from it before a clap no longer actuates it. You can also try adjusting the 5-kW potentiometer to see how it affects the sensitivity. Turn the dial clockwise to increase the sensitivity. (You'll need a flathead jeweler's screwdriver for this.) If you have trouble getting the sound trigger to function properly, make sure that all connections are secure and that the sensitivity is not too high or too low. If the sensitivity is turned too high, the flash unit will discharge spontaneously and then will not discharge again. If this happens, turn the sensitivity down just enough to allow the flash unit to be discharged.
3. Set the flash unit in the yellow automatic mode and point it at a nearby object so that the duration will be minimum. Then set off the flash unit repeatedly with successive claps or snaps. This works because each flash uses only a small portion of the stored energy, and it takes a very short time to replenish that energy.
4. Now try observing a balloon burst. Place the microphone of the trigger near the balloon and point the flash unit at the balloon. Turn the room lights down. Pop the balloon with a needle, watching it as you do.

Notes

1. If you wish to build your own sound trigger from scratch, go here.  If you prefer to work from a kit, go here.
2. Note that the PC cord has been modified by replacing the PC socket with an audio plug. This provides a convenient way of connecting the flash unit to a trigger.  Audio plugs (also known as RCA plugs) are available from neighborhood electronic stores.   PC plugs are difficult to come by.