Q1 is a phototransistor that is normally illuminated by a beam light. When the beam is intercoupled, pin 1 of U1A goes high, and forces pin 4 U2B low. Then, C2 discharges through R2 and R3. After a certain time delay, pin 10 U1C goes high, triggers SCR1, and sets off the flash. R4/C1 charges, causes U1D output to go low after about 1/2 second, and resets U1A and U1B to the initial state. This delay prevents accidental double flash exposure.