Connecting a Camera Hot Shoe to a homemade strobe
simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. A very simple opto-isolator with normally-off output.

How it works:


D1 allows C1 to charge up from the 6 V supply. See note on current limiting below.
When the strobe contact closes D1 is reverse biased and C1 discharges through R2, D2 and SW1. With R2 = 390 \$\Omega\$ the current will be limited to 10 mA.
While the LED is on Q1 will 'close its contacts'.


The beauty of this apart from its simplicity is that the LED is normally off, saving power.

Current limiting

We don't know exactly what is protecting the 6 V supply. I suggest you connect a 1k resistor across the jack and measure the voltage across it. From that you can calculate the voltage drop internally and figure out the effective series resistance. If this is less than 1k (indicated by > 3 V across the test resistor) then I'd be inclined to add a 1k resistor between D1 and the top of C1 to limit the surge current on connection.