Slave flash trigger with counting logic

Text and images copyright (c) 2005 Jan-Erik Skata. All rights reserved.

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.


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


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.


With slave flash


With slave flash