Flip Flop Application - Event Detector

An event detector circuit is one that is capable of detecting and holding a signal until the event that changed the signal can be addressed. A flip flop can be used as an event detector as shown in the circuit below. When the flip flop receives light (or when the P switch is closed in the circuit), the output becomes 1. Further toggling of the P switch will have no effect on the output. It stays on 1 until it is cleared (when the C switch toggles off). If the C switch is off, then the output will never become 1, no matter how many times the P switch is toggled.