The flash that is built into digital cameras is designed for indoor photography. At subject distances of greater than five metres or so, the light is usually not powerful enough to take a satisfactory picture. Unfortunately most such cameras do not include a shoe for an external flashgun, and so a light-triggered slave flash is called for. The flash built into the camera produces a very rapid change in light intensity which is picked up by a phototransistor in the slave.
The pulse is transmitted via C1 to the transistor, which then briefly shorts the contacts on the slave gun together. The sensitivity of the device can be adjusted using P1. The circuit can be connected to the shoe contacts of the slave flashgun using a coaxial cable, or, with space permitting and a nimble bit of DIY, can be built inside it. The circuit is not suitable for use with flashguns which have a voltage of more than 20 V across the trigger contacts, or with cameras that emit a number of ‘preflashes’ before taking the picture proper.