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Im buildin a tachometer an...

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I am building a tachometer for a class. I will use it to demonstrate the measurement of RPM (revolutions per minute) for a fan. The basic setup is as follows: I will have a laser beam pointed at a photoresistor, which is apart of a circuit. The fan will be placed in between the beam and the photoresistor. Every time one of the fan blades crosses the beam path, the resistance of the circuit will decrease enough so that a "trip current" is flowing. This trip current will go through a not gate, which will output a logical one. This logical one will then enter a counting component of the system, which will display the number of times one of the blades has crossed the beam path (one for the first interruption). This process will continue for one minute, the display showing the number of interruptions. At one minute, the display will freeze, displaying the RPM for the fan. The counting component of the system will have an element that divides the total number of times the beam has been interrupted by the number of blades to provide this number. A switch will be available that will restart the process (such as in the case that I wanted to repeat the process just for testing precision or getting a bad measurement). I would appreciate any advice I could get as to what parts I can use for this project, particularly what parts I will need for the counting component.

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