counter circuit, HELP!!!

i want a circuit that will switch on when a pendilum goes past a photodiode, count from when the pendilum first goes past the device and comes back down again and hold the count on 3 7-seg displays (sec, 10'th of sec, 100'th of sec).
in other words, start count on first impule, stop count on second impulse, and freeze screen until button is pressed.
i know i will need a fairly fast clk, and some sort of latching circuit.

can any one help? :?

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