This circuit is a JK flip-flop. It only changes when the clock transitions from high to low. The inputs (labelled J and K) are shown on the left. When J = K = 0, it holds its present state. When J = 1, K = 0, the output is set to high. When J = 0, K = 1, the output is set to low. When J = K = 1, the output is toggled from high to low (or low to high).
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