6v6gt:
Why has a thyristor (BT149) been specified ? Is that the best choice in this application ? Simply that I have not seen these in Arduino applications before.
I have come across many hand held power timing lights that use the SCR input method, and it is very good for noise immunity and a nice sharp negative pulse to trigger the LM555 and keep timing delays to a minimum.
Handheld power T/L usually have an inverter in them to get 400Vdc for the flash tube, this can create a "bit"of noise.
It looks weird,but it works.
Tom... 