The flash
Found an old simple flash on the flee market, Vivitar 125. Paid
10e and it works just fine.
As most flashes, it is activated by shorting the two contacts in the
connector.
Sensor circuit and logic
Used this circuit. It allows defining a number of red-eye reduction pre-flashes to be ignored
before activating the slave flash.
I was unable to find the photodiode from the schematics, but used a
common phototransistor, PT331. The small-signal transistors (PN100 and
PN200), I also substituted with BC547 and BC557, respectively.
The 10k resistor from base to ground on Q1, I had to change to 47k,
probably due to different characteristics of the transistor or the
phototransistor.
Construction
Circuit assembled on experiment board.
Works surprisingly well on both short and long distances, and does not
react to other light sources than camera flashes.
Building final circuit on pre-drilled prototype board turned out to be
less work than etching a printed circuit board.
Finished circuit in action, 1/60s shutter
Finished
Board mounted in a box, and flash attached with the original flash
shoe.
The DIP switch is used to set (binary) the number of pre-flashes.
A maximum of 32 flashes can be defined.
Picture examples
Shutter 1/60s, aperture f/2.8, Canon A85
As you can see, these were taken in a large, dark storage house with
high ceiling where the built-in flash doesn't have much effect.
The pictures are resized only.
Without
With slave flash
Without
With slave flash