Below is a link to a useful table for anyone thinking of using non-digital flash units on their expensive camera. I will be importing a couple of old Honeywell Strobonar Press Flashes this week to use in cave photography along with my Minolta units. Though I hadn't planned to use them on my digital camera I was surprised to find they could be. The table lists the trigger voltages on a number of flashes including to my surprise one model of the old Honeywell Strobonar Press Flash. The old 1950s Honeywell is actually safe to use. Too high a trigger voltage will eventually fry your camera. Another option is a Wein Safe Synch if you are in doubt.

http://www.botzilla.com/photo/strobeVolts.html

Thanks for that info.

I had never given it a thought.

Just closing a contact, and the voltage level would not be critical......

With my 40D I use a Canon Speedlite to take full advantage of the electronic control that Canon offers.