Author: F. Roesky
Current designs of slave flash units are, in the opinion of the writer, too complicated and may be simplified without any problems and without losing any of their usefulness as may be seen from the accompanying circuit diagram. This proposed circuit offers a number of advantages:
• no need for an additional power source, since power is derived via the sync contacts of the main flash unit;
• automatic reset (no need for a button);
• operates with new as well as older main flash units (contact voltage >100 V);
In spite of these proven practical properties, the design is simplicity itself. It is controlled by a low-current CMOS decimal counter IC1, a Type 74HC4017, which enables the entire circuit to be powered directly via the sync contacts of the main flash unit via resistor R1 and voltage limiter D1.
The control circuit based on transistor T1 also operates with minimal quiescent current. When a flash is detected, T1 provides a clock pulse to IC1. Depending on the position of switch S1, the low-power thyristor fires on the second or third flash pulse, so that it does not react to the pre-liminary flash that prevents the red-eye effect. After about 0.4 second, when the main flash unit is discharged, IC1 is reset via R4, C3 and T3. At that instant, the current drain of IC1 increases briefly and the voltage across C1 collapses. This is of no consequence, however, since after at most one second C1 is recharged to a level at which the circuit is operational again. If the circuit is to be polarity-sensitive, connect a small bridge rectifier rated at 400 V between the contacts of the main flash unit and terminals JP1 and JP2.
(Elektor Electronics Magazine – 07/2006)
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