Shadow Counter Circuit

Block Diagram of Shadow Counter

Shadow counter is a circuit which can count the number of shadows incident on it. Shadow counting is the main principle of many devices like currency note counting machines, visitor counters, many security systems , etc.
In this article i will teach you how to design a shadow counter circuit.

I have made this circuit based on Shadow Alarm by D.Mohan Kumar published in Electronics For You- January, 2006.


The block diagram of shadow counter is shown above. The intensity of light is sensed by the Light sensor. I have used a Light Dependent Resistor (LDR) as light sensor.

The comparator circuit is designed such a way that its output changes from HIGH to LOW when the shadow occurs. The output of the comparator triggers the monostable multivibrator which in-turn triggers the counter circuit. I have used µA741 Op-Amp as a comparator.

The counter circuit stores the number of shadows occurred in the past and increments the count by 1 unit when triggered. The output of the counter is given to a display using a display driver. I have used a common cathode seven segment display in my circuit.

The monostable multivibrator is used to avoid false triggering. For example, when a shadow is falling for a quite long time (a few seconds), then due to the partial shadows that may occur due to the same object, the comparator gets triggered multiple times for the same object. This increments the count many times for the shadow of only one object. Using a monostable multivibrator in between as I have done eliminates or at-least reduces that problem. I have used a 555 timer as a monostable multivibrator.

You can see the video of Shadow counter here:

The circuit is quite easy to implement. So, I encourage you to do it yourself. This circuit is basically aimed at hobbyists and is also suitable as a college project (mini project) for Electronic Engineering students.

However, I have also done the entire circuit on board along with complete documentation and presentation (PPT) of shadow counter. If you are interested in getting the circuit along with the documentation and presentation, you can contact me by clicking here.