Posted on Mar 2, 2013

This circuit is used for remote photographic flash units that flash at the~ame time as the flash attached to the camera. This circuit is designed to the trigger cord or hot shoe connection of a commercial portable flash unit and triggers the unit from the light produced by the light of the flash unit attached to the camera. This provides remoteoperation without the need for wires or cables between the various units. The flash trigger unit should be connected to the slave flash before turning the flash on to prevent a d VIdt triggered flash on connection.

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The L14C1 phototransistor has a wide, almost cosine viewing angle, so aligmnent is not critical. If a very sensitive, more directional remote trigger unit is desired, the circuit can be modified using an L14G2 lensed phototransistor as the sensor. The lens on this transistor provides a viewing angle of approximately 10° and gives over a 10 to 1 improvement in light sensitivity (3 to 1 range improvement). Note that the phototransistor is connected in a self-biasing circuit which is relatively insensitive to slow-changing ambient light, and yet discharges the 0.01-I"F capacitor into the Cl06D gate when illuminated by a photo flash. For a physically smaller size, the C106D can be replaced by a C205D, if the duty cycle is reduced appropriately.

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