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Slave Flash Trigger Unit
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Integrated flash is not always the optimal solution in photography. In many cases better results can be achieved, if many flash units operate in sync.

This article describes a device that can transform any flash unit into a wirelessly controlled device. The slave flash unit will be activated as soon as the master flash unit emits light.

The trigger unit is quite a simple yet effective device (see the schematic diagram on the picture below).

Attention! Some flash units have dangerously high voltage (up to 350 VDC) on their trigger contacts! To avoid electrical shock, be very cautious during your experiments with this device.

VD1 OS351 VS1 KU103V R1 36 K R2 1 K C1 1200 pF C2 0.47 µF GB1 1.5 - 3 VDC

The general idea behind this solution is quite well-known. I only added a separate battery GB1 to the traditional scheme with a thyristor and a differentiating circuit. (It is quite possible I am not the first to do that.)

The advantages of this solution are as follows:

1. The device can be connected to any flash units, including those that do not tolerate even a small current drain on the trigger contact.
2. False triggering is significantly eliminated.

Almost any photodiode or phototransistor can be used as VD1. Also feel free to experiment with the voltage of GB1. It seems to influence sensitivity of the circuit. Since the current drain is very small, GB1 can be practically of any type. Small tablet-like watch batteries will perfectly do.

Any suitable thyristor can operate in this circuit. When choosing VS1 keep in mind that it must operate under high voltages of at least 350 VDC.

One more thing to know: in order to operate properly, the voltage drop across R1 must not exceed 70 per cent of the battery voltage. Tipically you will not have to worry about it when shooting indoors.

This solution can also be used to reduce high voltage on trigger contacts of budget flash units. The details are discussed in the corresponding article.

Although every effort was made to make the described device both safe and functional, I decline any responsibility for any damage caused by such devices. If in doubt, please use the devices the manufacturer of your camera recommends.

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I would like to thank Pavel Lachayev for his constructive comments, which prompted a couple of additional remarks:

1. The device is not very sensitive. Its sensor should be oriented directly towards the master flash unit. Due to the low sensitivity, there is almost no false triggering. Typically, the slave flash unit will not snap into action by reflective light. I suggest the device be placed into a separate housing connected with the slave flash unit by a two-wire cable. In this case, you will be able to aim the flash unit and the trigger unit independently in any direction.

2. The trigger unit can operate under fluorescent lighting without any problems. Thus, you can use it in offices, colleges, etc.

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