Description The heart of my flash trigger is an LM555 timer and a DM7473 flip-flop. The 555 provides an adjustable delay between an event being triggered and the flash being fired — for example, we want to wait for a falling drop to fall a few more inches before the flash goes off. The flip-flop triggers at the end of the delay and fires our flash. A circuit is required that would give a change in voltage when a light beam is interrupted. A phototransistor allows current to flow only when light is present. A large resistor between the phototransistor and ground causes 5V to be output when light is present, but pulls the output to ground when the beam is interrupted. Unfortunately, there’s bound to be ambient light hitting the phototransistor even when the LED beam is interrupted. The phototransistor may only reduce the voltage at its output by 1V when the beam is interrupted — we normally output 5V, and the 555 timer won’t trigger until the voltage dips below 1.67V, so the change in v