Triggering a secondary or slave photographic flash is really not that hard as one might think. Although the duration of primary flash is often shorter than 1/10000 of a second, for modern day electronics it is not hard to synchronise a secondary unit to the primary one with the corresponding precision. There are several circuits posted on the net which can do the job but, as we could observe, most if not all of them are designed to operate only in the dark - they are not really able to adjust themself to strong ambient light conditions. Sadly, this renders fill-in flash daylight photography very tricky.

We designed simple but very reliable auto-biasing photo preamplifier which enables reliable triggering of the secondary flash unit even in the brightest noon daylight from the distance of at least 15 meters. A very nice feature is that the complete device literally costs pennies...