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Hi and welcome to the forum!
This really sounds a good idea.

By the way, how about having seperate doors for entry and exit. For each door you have a LASER (or some other???) beam, which is connected to your circuit. Every time a LASER beam is blocked your circuit gets a signal. If your circuit gets a signal from entry door, you increment a counter, if it gets a signal from exit door, you decreament a counter.

Also, this idea of 2 doors is just a suggestion. We can work on single door idea as well.

Let me know what you think !

Just a thought came from the above post.

Why not have two lasers at the door. Say 'A' and 'B' are two lasers. If laser 'A' breaks first then count it as person entering & if 'B' breaks first then count it as a person leaving.

just a thought. Hope it helps.

I will think on any other way out also.

Yep, ashuashi's suggestion would work. Though, if you are not using microcontrollers for programming the logic, just make use of logic gates and timers 😀

I'd recommend infrared rather than lasers, for safety reasons 😛

main problem is a person will break laser `A` and `B` entering or leaving the room through the only one door.a timer should be used.but how to implement?

The lasers are not on top of each other. What Ashuashi meant was that they have a distance horizontally apart (maybe around 10 cm). So, to determine whether the person entered or exited, the order in which the laser beams are broken will tell you. So, if you place beam A at the front end of the door frame, and beam B and the furthest end, a person entering will break beam A first, and then beam B. When exiting, it will be opposite.

I suggested a timer because it might help your use with logic gates (AND, OR). Have you used microcontrollers before? If so, then you can program your logic directly with code, which is much easier.

Exactly. They would be horizontally apart. Use simple gates logic to activate timer in forward and reverse increments.

Well.....this too is good option which will faciliate entry and exit through same door. Just one needs to take care of having door small enough so that at a time one person can pass through it. Otherwise it may create a problem to identify entry or exit. Am I right?

By the way, can one start designing a circuit diagram for this? It would be pretty simple I guss. 😀