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Build A Beam-Break Flash Trigger

Here's an easy-to-build accessory for the Time Delay Photoflash Trigger described in our February 2009 issue. It triggers the delay unit and your photoflash in response to an object interrupting an invisible beam of infrared (IR) light. Alternatively, it can be used on its own to directly trigger a photoflash.

By Jim Rowe

A Few months ago (in February 2009), we described a “Time Delay Photoflash Trigger”. This unit was triggered by a sudden sound picked up by an electret microphone. It then immediately opened the camera’s shutter and then fired the photoflash shortly after, depending on the delay period programmed into the unit.

Using sound pick-up in this manner is a popular and effective method of triggering a flash for “stop motion” and other kinds of special effects photography. However, in addition to the electret mic input, we also gave the delay unit a second “contact closure” input, so that it could be triggered using other techniques. Which was just as well, because as soon as the delay unit was published we started getting requests for a light beam trigger.

This simple “Beam Break Trigger Unit” is the result of those requests. It’s mainly intended as an alternative triggering front-end for the Time Delay Photoflash Trigger and is connected to the latter’s “contacts” input. However, it can also be used to trigger a photoflash unit directly if you don’t need the programmable time delay capabilities.

Note, however, that using the unit to directly trigger the flash has one important limitation. Unlike the Time Delay Photoflash Trigger, it doesn’t also trigger the shutter. This means that you have to open the shutter manually before the infrared beam is interrupted (eg, at night or in a darkened studio).

The new project is in two parts: (1)an IR Source unit which produces the IR beam and (2) a Detector unit which monitors the IR beam and closes its output trigger contacts briefly if the beam is interrupted. These two units are linked with an interconnecting cable which supplies the Source unit with power.

By the way, if you’re already wondering how you accurately line up the Source and Detector units when the IR light beam is invisible to the human eye, wonder no more. That problem has been solved by providing the detector unit with a visible green LED which lights when the IR beam is being received. This makes the lining-up process easy.

Both parts of the project run from a 9V battery fitted inside the Detector unit’s box. The total current drain is about 15mA which means that the battery should be either a set of six AA (1.5V) alkaline cells or a single high-energy 9V lithium battery. A standard 9V zinc-carbon or alkaline battery is not up to the job, as its life would be too short.

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