|Slave-Flash controller suitable for digital camera (Questa pagina in Italiano)|
This circuit is able to count the pre-flash from a digital camera and trigger a slave-flash only after a pre-set up number of pre-flashes.
Before using it you need to know the number of pre-flashes of your camera, also you need to test it because in some cameras some pre-flashes are very fast and could be detected as one.
A couple of photo-diodes BPW34 in reversed polarization detect the flash light. The light detected is used by a mono-stable circuit that give the pulses to a binary counter. The counter’s outputs are connected to the selection jumper (P5) in this way the user can chose how many flashes must be ignored by the circuit.
The cathode of the series must be connected to the 1st pin of connector P8.
To the 1st pin of connector P1 must be connected the positive pin of the driven flash unit.
For some flash unit such as the sub-strobes that share the hot shoe contact with the battery, recharges leads, could happen that the unit stuck after the 1st flash. To avoid this problem the circuit provides an impedance adaptation through the connector P4: jumper 2-3 direct connection, jumper 1-2 impedance adaptation.
The connector P6 is the SCR:
pin 1: Gate
pin 2: Anode
pin 3: Cathode
The best suitable component is TIC106M (or equivalent) any gate sensitive TTL compatible can work.
WARNING touching the high voltage flash’s pin could be harmful!!!
About the integrate circuit the suggested ones are:
SN74LS191N Texas Instruments
SN74LS123N Texas Instruments
You need to avoid MM74HC123AN from Fairchaild because it does not perform the self-reset when you turn it on.
Following the link it is possible to download the PCB and the schematic in CIRCAD98 format.