Originally Posted by resonableman

whih sensor do u suggest i should use??
thx everyone

That shouldn't be particularly critical; personally I would just use pairs of IR LEDs and photodiodes.

Why sould two people entering the room at once be a deliberate attempt to fool the system? People move in ways that are most condusive to their own needs. The possibility of multiple people entering/leaving the room is always there. There is no system that can reliably account for that. Secure buildings require each person entering or leaving to use a magnetic badge before passing through. But even that isn't 100% reliable, as one person can 'taligate' on another. Only by controlling how people pass through can you be sure you have an accurate accounting.

Originally Posted by resonableman

thx brownout.but the door does not have that

You'll have to construct your own. It can be primative and work in two directions ( entering/leaving ) Use a simple rotary switch or encoder for the signal.

I'm just an idea guy.

Originally Posted by BrownOut

Why sould two people entering the room at once be a deliberate attempt to fool the system? People move in ways that are most condusive to their own needs.

That's true, but I'm imagining a single door here, which would not (certainly not comfortably) fit two people in the doorway. Obviously this may not apply to a larger doorway, but in that situation two people passing through the doorway at the same time is unlikely to occur.

Electronics devil, I would just place IR LEDs on one side of the doorway (controlling them with PWM would probably be the best idea), and photodiodes on the other side. A microcontroller could be used to detect when people pass the doorway and in which direction, and then the rest is mainly software.

Originally Posted by giftiger_wunsch

That's true, but I'm imagining a single door here, which would not (certainly not comfortably) fit two people in the doorway. Obviously this may not apply to a larger doorway, but in that situation two people passing through the doorway at the same time is unlikely to occur.

You're also assuming light traffic. Not always a good assumption. I belive people can accomplish extraordianry things when they are pressed to. People don't have to pass thru at the same time, they only have to be close enough that there is no "make" of the beam between them.

Well unless given more information, a single door and light traffic is a reasonable assumption. To be close enough that the beam being broken twice is perceived as a single stimulus means being pretty damn close, depending on the recovery time of the photodiodes. And with three separate beams, that'd make it especially unlikely.

The most likely way that it would be inaccurate is if two people passed through the doorway side-by-side, thus producing a single uninterrupted err... interruption () to the beams. But if it is a single door, the only way two people are going to fit through a door at once is turning sideways to intentionally achieve this. If it's a double door, then clearly this method may not be accurate.

As you said BrownOut, the only way to make sure it's accurate is to restrict people's movement; turnstiles are ideal, but a narrow doorway should filter out most of the erroneous stimuli.

Anyway, it sounds like this is a homework project or something so maybe we're taking the limitations to heart too much

Standard entry door is 3 feet. Pleanty of room or two people to pass. It you use this at your house, then it'll probably be sufficient. In a public place, conference room for example, I wouldn't trust it. That's just my fastidious nature.

I agree, but given his requirements it seems a compromise is necessary. 3ft is barely enough room for two people to pass, generally if two people need to pass through one would have the courtesy to wait for the other to pass. Generally

I was considering making a much simpler version of this a while ago. By much simpler, I mean a single beam in front of my bedroom which will trigger a loud shout of "GET THE HELL OUT" if someone tries to enter my room while I'm not there or a remote alarm, whichever.

Originally Posted by giftiger_wunsch

I agree, but given his requirements it seems a compromise is necessary. 3ft is barely enough room for two people to pass, generally if two people need to pass through one would have the courtesy to wait for the other to pass. Generally

In GB, maybe. In the US, probably not. In Germany, NEVER!

Heh, well unless you have a suggestion which doesn't involve magnetic swipes and turnstiles (and apparently it can't even involve a microcontroller ) I think it'll have to be a compromise.

A trunstile would be easy to make. 3 broomsticks, rotary switch, a long bolt and a pedistal made of PVC and you're done. I was actually kidding about the trunstile, but there just isn't a reliable way to do this without them. Why do you think every sporting event and disney uses them?