This Project is a simpler version of a circuit that's in use every day around the world. If you've ever wondered how such places as stores or airports are able to count how many people enter their buildings each day, this Project will show you how it's done.
As you can see from the schematic, this Project uses a divide by four counter circuit and line decoder. Each time the light falling on the CdS Cell is interrupted, a clock signal is sent to the first J-K flip-flop. A different LED will light to represent the number of "interruptions" the circuit manages to count.
You'll find this Project works best in a dark room using a light (such as a flashlight) directly striking the CdS Cell. Wave your hand between the beam of light and the CdS Cell and you'll notice the LEDs go on and off to indicate the count.
You'll have to adjust the potentiometer to set how "sensitive" this Project is to change in light. You might also want to use different values for the 100K resistor to see what effect they will have on the operation of this Project.
You can see more complicated versions of this circuit located near doorways, stairs, elevators, etc., in many buildings. Each time a person crosses a certain point, a beam of light is interrupted and the counter circuit advances by one. Of course, a digital display of some sort is used instead of LEDs.
Very often these devices are located very close to the ground or floor — at foot or ankle high level. You can identify them by a light on one side and another opening on the other side.
How many of these devices can you spot in the buildings in your town?