The topic is SAFE FLASH SYNC for third party guns. Like many others I was concerned about frying my 300D circuitry by using third party flash guns with relatively high trigger voltages. The option were:

(1) Try it & see what happens
(2) Buy Canon flash gear.
(3) Don’t use flash
(4) Buy a Wein isolator unit

I did not fancy 1, 2 or 3 and 4 is a tad expensive, so I decided to design and build my own isolator, details as follows.

Total cost of electronic components £3.20 (2004 prices), plus sync lead, battery and hot shoe adaptor.

Design is for maximum flashgun trigger level of 200V, and should work OK down to about 6 volts. For higher voltages you need to increase the value of R2.

Isolation level camera to flashgun is 7,500 volts.

Just one point if you make one to this design, you do so at your own risk and I cannot accept any responsibility for damage to cameras or people etc.

Mine works fine and triggers reliably down to about 5 volts:

NOTE! (November 2005)

At least one person who has tried this circuit has reported triggering problems due to varying operating tolerances on the active components (opto isolator/thyristor) used. If you try it and encounter similar problems, you will need to play around with the resistor values to compensate for any differences and provide the correct operating voltages/currents for your components.

Have fun.