This isolation circuit requires no battery, it's powered by the flash trigger circuit itself.
Voltage above 5.6v is shunted to the negative rail, and the TRIAC carries the trigger load.
Zeners as low as 4V can be used. Select a TRIAC based on voltage and current of your flash.
Q1 is a 400+V TRIAC
ZD1 is a 5.6V Zener Diode