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

Activity 14.  Using a light slave

Equipment needed:

A light slave is a trigger that sets off one flash unit in response to the flash of light from another. These are often used by photographers to illuminate a subject with more than one flash unit without the need of long cords. The master flash unit may be used on the camera hot shoe and the slave unit positioned to the side.

The delay circuit can serve as a light slave simply by connecting an NPN phototransistor to the input. Connect the emitter to the positive side of the input and the collector to the negative. In this configuration, a flash of light on the transistor will have the effect of dropping the voltage across it, thus triggering the delay circuit.

Using the delay circuit as a light slave makes it possible to interject a time delay between the discharge of the master and slave flash units. Place one flash unit directly on your camera so that it discharges upon pressing the shutter. Place the second flash unit, connected to output 2 of the delay box, somewhere away from the camera. Of course, you’ll have to select a shutter speed long enough so that the slave unit discharges before the shutter closes. And you’ll have to pick relatively slow-moving objects since you have to trigger the shutter manually.

If you have a camera with an electronic shutter release, you may want to try connecting a trigger (contact, sound, or photogate) to the input of the shutter release. In that way, the event that you are photographing will trip the camera. This would make it possible to work in a room with subdued lighting, since the shutter would not be open for long. Keep in mind, though, that for most cameras there is a significant delay (hundredths of a second) between the trigger signal and the opening of the shutter.

Note: By using the delay box as a light slave and connecting a single flash unit to output 2, you can cause the flash to trigger repetitively on its own flashes of light. There is, however, a minimum delay which is influenced by the recycling time of the flash unit.