Roff--
Sorry, I missed your last comment...the question was I really that I needed to figure some of it out myself. I now understand the input voltage divider and the zener diode limiter.
What is the "2.5V reference IC"? Does it make the voltage differential exactly 2.5 volts regardless of the input? What chip might this be?
At the local Radio Shack, I was only able to get a LM339 which is a Quad Comparator similar to the LM393. I made the first circuit powered first by 3 AA cells, then 4 AA cells (5.6 volts), with the same results in each case:
Findings:
1) To fire the flash- short across the flash terminals (input pin 3 to ground). This will only occur if the positive leg of the flash is attached to the circuit ground and the negative leg of the flash goes to pin 3 which is marked as the positive input. In the circuit for the LM311, the flash input is marked as the negative, while it is marked as the positive in the LM393 circuit. Does this polarity matter?
2) Relay is immediately energized upon connecting the power supply, even if flash trigger voltage goes to zero, or flash is taken out of circuit.
Components of circuit and wiring have been checked and rechecked...
I tried wiring without the transistor as in the second schemetic, but the LM339 didn't provide enough output to drive the relay at all.
The relay should not energize upon adding the 6 volt power supply when the flash is not connected, and I am not sure why it seems that the flash is connected in reverse.
I do know I don't know enough to figure out why it is not working...
Do you have additional suggestions for me to test?
I will try and secure both a LM393 and the LM311 to try things further.
Thanks