Flash Detector

As the flash is very bright, in this circuit we just use a 1.5K resistor, which pull the emitter low when Q1
is off and raise its high (to 5V-Vce(sat)) when there is a flash. The signal is then pass through a Schmitt
trigger to assure a clean signal into the E128.

PT4 is then hi at default and low when there is a flash. In the program we will wait for a falling edge.