In the world of photography, the exposure triangle is immutable. To get a properly exposed photo (not too bright or too dark), you need a balance of aperture size (how much light gets in), shutter speed (how long the light gets in), and ISO (sensitivity to light at the expense of noise). But the shooting situation often limits how the photographer can adjust each parameter. To freeze action, for example, you need a very fast shutter — reducing the light you let in and therefore exposure. To compensate, you might need to use a flash and this DIY device can help with the timing.

There is a reason that photography flash units only come on for a split second (about 1/10,000th of second is normal): they’re incredibly bright and would burn out if left on for any length of time. To freeze action, such as a balloon popping, you need a fast shutter speed. Too slow and the photo will be all blurry. Exact numbers vary, but 1/8,000th of a second isn’t unusual for the mechanical shutter on a modern mirrorless camera. To get proper exposure, you need to time the shutter to open at the exact same time that the flash is illuminating your subject and that is something you could never achieve through manual control.

That’s hardly a new problem and so cameras are capable of releasing the shutter at the proper time in relation to the flash, but how do you sync those two events with whatever action you want to freeze? If that action happens to make a noise, this device is the solution.

This device, based on an Arduino UNO Rev3 board, uses a microphone to listen for loud noises. If a noise exceeds a set threshold, the Arduino triggers the flash. An isolation circuit made with a Reed switch protects the Arduino from the high voltage of the flash. Reed switches are relatively slow, but they’re affordable. For better performance, an opto-isolator could be used instead.

To demonstrate this, students at Rochester Institute of Technology froze the action on some balloon pops and the results look great.

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