Suggestions for electronic "trip wire"

[lrmall01]

Since Halloweeen is coming up I have been thinking about setting up some "smart" decorations to scare folks getting candy.  Basically I'm wanting to detect when people walk up a few starts towards the front door so I can then set some actions in motion.

Would a Rangefinder typical of those used in robotics be suitable or are there other sensors that might work better?

Thanks for any suggestions! 

[mike_522]

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[Soeren]

Hi,

[..] Basically I'm wanting to detect when people walk up a few starts towards the front door so I can then set some actions in motion.

Would a Rangefinder typical of those used in robotics be suitable or are there other sensors that might work better?

A Sharp PSDs like the R316-GP2Y0A710YK could be used, but it's $20 and the beaam of detection is somewhat narrow.

An Ultra Sound echo device is another option, but may be hard to find in a ready-to-use version

A LASER fence is fairly cheap to make, but you need to mount it on something firm, to keep it in alignment and it will take some work to setup and fog, dust etc. (any air pollutant of a certain size really) will make it visible.

You could go advanced with capacitive sensing, RADAR or radio field disturbance, but they take some (a lot) experience in electronics, which you probably don't have as you ask.

The PIR (Passive InfraRed) device that the previous poster linked to (or any other PIR unit), will be the best solution for you, I think, as it works right out of the packet, perhaps with a little adjustment of the sensitivity control to suit your project and the field of view can be up to ~180°.
Plenty of other PIR units can be found, both as stand-alone units and built into lamps and alarms, which in some instances may be cheaper, but takes a little hacking into. Some drives a relay, which makes it fairly easy to connect up your own stuff.

How much tolerance on the distance of your "customers" position can you tolerate?

[lrmall01]

Soeren - the distance is fairly well defined.  The will have to pass up a set of stairs with a handrail on each side and I planned to detect them while moving up that flight of stairs.  At any rate, max distance would be 3 feet but likely to be 1.5 since most people walk up the middle of the stairs.

[waltr]

For a 1 meter distance a simple IR LED and detect across the path would work.

[Soeren]

Hi,

Soeren - the distance is fairly well defined.  The will have to pass up a set of stairs with a handrail on each side and I planned to detect them while moving up that flight of stairs.  At any rate, max distance would be 3 feet but likely to be 1.5 since most people walk up the middle of the stairs.

OK, it was more the tolerance (eg. 1m +/-20cm or whatever) that I wondered about.

As waltr stated, an IR gate might do (at least when it's dark outside), or an integrated IR receiver like what's in a TV set (for the remote) with a modulated transmitter LED - but you have to get the type used for industrial "light curtains" used to secure heavy machinery against human intervention as the TV types will reject a continuous signal as noise.
 
With any kind of light gate (beam break detector), you have a very precise triggering point (line), but a PIR mounted high and pointing more or less straight down will probably be the least complicated and PIR units are usually very reliable.

[Mastermime]

Laser Tripwire circuit / photoresistor tutorial