counter circuit

I want to make a project in which I have to use an electronic circuit which will count the no of people passing at a particular entrance in a given time interval and this number or equivalent signal (propotional voltage) can be used as a input for subsequent part of my project circuit.Suggest any possible circuit.

Light source, detector, counter IC. what do you plan to do with the number when you get it? You have to give more info if you want useful feedback.