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This counter can be used to know the number of visitors present in the room at any given time. It is useful for places such as movie halls, buldings and offices. To keep the cost low it uses simple calculator instead of a counter-and display circuit.

The calculator can be used as a normal calculator is plugging it off from the circuit. All the components are readily available in market and the circuit is easy to build.

Circuit description

Two transmitter-receiver pairs are used at the passage:

One pair comprising light source A (transmitter) and light -dependent resistor LDR1 (receiver) is installed at the entry side of the passage, while the other pair comprising light source B (transmitter) and LDR2 (receiver) is installed exit side of the passage.

Light from. two light sources (torches) should continuously fall on the respective light dependent resistors (LDRs), so proper orientation of light beams and LDRs is essential.