object, counting the number of rotation of a wheel or motor etc.

The LDR is the main component of the circuit that is used to detect the presence and absence of the laser beam. When any person or object come between the laser beam and the LDR then the laser beam will be cut and the resistance of the LDR will increase due to which the transistor will become actuated and activate the Photocoupler which switches the "=" key of the calculator. Now each time a person or object passes through the laser the calculator add 1 in the counting.

For using the circuit first you have to adjust the calculator for counting mode that can be easily done by simply pressing 0+1 on the calculator and then your calculator will show 1,2,3,4 counting. Make sure to enclose the circuit with the LDR in a suitable case and make a small hole for the LDR to receive only the laser light and not other lights or the circuit will not work properly. The operating voltage of the circuit 4.5 to 12V DC. If you want to operate the circuit with low voltage for example 4.5V to 6V then change the 470R resistor to 180R.
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The figure below shows a very simple and useful project of an object counter circuit. The circuit is using a laser light, a cheap 8-digit calculator and few other components. This circuit can be used variety of purposes like for counting persons through the entry gate, counting a large quantity of any
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Object Counter Circuit Diagram