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A sound trigger is an electronic circuit that fires a flash unit in response to a sound or noise. This allows to capture high speed events like an exploding balloon, the breaking of glass or splashing objects.

To capture high speed events we need very fast responsive equipment.

A camera (compact or DSLR) is too slow in these situations: the delay between the shoot command (pressing the button or via any electronic way) and when the photo is taken is very high (with the fastest cameras the delay is around 10-20 msec).

A flash unit, instead, is very fast, and lights up almost immediately when it's triggered. An other benefit of the flash is the ability (with the modern flashes) to set up the power of the light. Reducing the power means that the light is emitted for a shorter time than at the full power, so, in this way, we can “freeze” the fastest movements. We could take a photo with an equivalent shutter speed of 1/41000 sec (circa) setting up the flash at 1/128 of the standard power.

 

Using the sound trigger

To take advantage of capturing images using the flash light, we need to operate in a completely dark room. The camera must be set in bulb mode and manual focus. A tripod and a remote control are required. Prepare the scene and focus on the desired area, turn off the light and then, with the flash ready to fire, press and hold the shutter button of the camera. Wait for the sound that fires the flash, then release the shutter button.

 

Known limits

This circuit, like all of the cheaper sound triggers suffer of the same annoying problems:

• If the camera is not far enough from the microphone, when the mirror of the camera is moved up (pressing the shutter button) the noise is sufficient to fire the flash, and the final result is compromised.
• If the object produces the sound twice or more (ie a bouncing object), the flash will be triggered more than once, causing unwanted results.
• If the sound produced is very short the flash will be triggered for a very short time, and light power will be less than the desired power.
• Preparing the scene with the flash switched on, will cause continuous annoying flash triggered at each little noise or when people speak.

Using a microcontroller, which supervises the circuit, it’s possible to bypass these problems. If you are interested, take a look a the advanced sound trigger.

 

Time delay

Several factors influence the time delay between the moment of the event and the flash trigger. 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 position is a good way to fine tune the time delay, since the sound travels about 345 m/s in air. Moving the circuit 35cm far away from the object a time delay of 1 msec will occur (70 cm = 2 msec etc).

 

Technical details

• Built-in microphone
• Extremely high sensitivity
• Sensitivity control using a multi-turn trimmer potentiometer
• Monitoring led (useful to check if a very slight sound is enough to activate the trigger and if the whole circuit is powered and working)
• Output : compatible with low and high voltage flash (max 400V)
• Powered by a 9V battery or by any 6-12 Vdc power supply
• Power consumption : 0.8 mA
• Dimensions : 22 x 92 mm approx. Thickness : 25 mm