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What is a Trigger Circuit?

By Paul Scott
Updated May 17, 2024
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A trigger circuit is an electronic device used in photography to activate or trigger external or supplementary flash units. When using such supplemental or fill flash units, it is critical that the camera flash and the external flash units fire at exactly the same moment. This is typically achieved by using a trigger circuit which fires all supplemental flashes simultaneously with the camera flash. This allows for clear, shadowless images to be shot in conditions where harsh primary lighting throws areas of the subject matter into deep shadow. There are two basic types of trigger circuits: the synchronized output circuit and the slave circuit.

It is often said that photography is the art of successfully capturing light. Unfortunately, this is not an easy task, as the camera is less sensitive as the human eye when it comes to describing subtle differences in light intensity. So while we may see a model in a studio or a wildlife subject clearly, a photo taken of the same subject may be ruined by deep, detail-robbing shadows. The best way to counter this shortcoming is to use fill lighting and flashes to illuminate the subject evenly. The use of natural or static studio lighting is easy to manage as the lights are on constantly but flash fills need to be synchronized with a trigger circuit to be effective.

A basic trigger circuit works on the principle of forming a connection or circuit between the external flash and a trigger source. When an initiation command is sent from the source, the external flash units fire. Synchronized circuits are connected to the external flash units and directly to a port or syncro connector on the camera itself. The camera's internal circuitry then becomes the trigger source, and when it fires the onboard flash, it will simultaneously fire the external flash units as well.

Slave trigger circuits have no direct connection to the camera and draw their trigger source from external sensors. The most common of these is a photo-sensitive trigger circuit which uses a photo sensor which reacts to the light from the main camera flash to initiate the external flashes. Another type of slave trigger circuit uses a sound sensor to activate the camera's shutter release and fire the flashes together and is commonly used in applications such as nature photography.

Many top end professional cameras allow for several settings to be applied to their syncro outputs which gives the photographer a lot of flexibility regarding lighting set up. Slave trigger circuits also range from basic, single-function units to sophisticated professional modules that allow greater control of the way the unit triggers the fill flashes. All types, however, are essential parts of a professional photographers equipment and allow for consistent, high-quality results in and outside of the studio.

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