Pyroelectric echolocation Detectors & Mechanical Reaction

May I suggest you read:
http://en.wikipedia.org/wiki/Pyroelectricity

What you describe with sound waves is ultrasonic detectors:
http://en.wikipedia.org/wiki/Ultrasonic_sensor

Check out the "References", "See also" and "External links" on those pages.

Positive or negative logic can be used in any way you like, depending on how you design your circuit, and the output can be designed to activate any kind of electro-mechanical transducer.

One caveat, if the sensor only generates a pulse upon detection, then "negative logic" can be a bit tricky. For instance. Someone walks in front of the detector and then stops. A pyroelectric detector reacts to differences in IR radiation, so it will pulse once or twice, but stop pulsing until that person moves again. if your mechanical action could injure that person, then you have a problem.

Now, an ultrasonic detector will continue to indicate the presence of the person, even if they stop. In THAT case, negative logic might work. BUT, if your mechanical action could injure anyone, I suggest you design in some redundancy. That could be, simply more than one ultrasonic sensor, perhaps from different angles.

Probably the most reliable sensor, for this application, is a beam that would be broken by a persons presence. This method carries the added safety feature of defaulting to the "person present" state should the source of the beam fail (i.e. emitter burns out, or such). If a large area needs to be covered, then multiple beams. Or an array of mirrors that reflect one beam across the area of detection multiple times, thus covering the whole area. The drawback there is alignment complications.

You could also use a scanning beam with a detector that looks for a particular modulation pattern in the light reflected off the person's body assuming what they are wearing will reflect whatever the beam is composed of. The beam could be amplitude modulated or could, even, be somehow frequency modulated (or a combination of several beams at different frequencies -- anything that is enough different from the ambient radiation pattern to make it stand out.

There are also video systems that monitor changes in the image. Such a system can cover a larger area than a single beam.

BTW: "pyro" is Greek for "fire" and used as a prefix means "fire" or "heat", thus pyroelectric means "electronics for detecting heat" -- in this case, body heat or the infrared given off by a warmblooded creature (we're all 'creatures', right? Or, are we 'critters'?!?).

Addendum: I tend to shy away from using ultrasonic sensors [US] to detect people, because of the noise they generate. I've yet to encounter a US that doesn't make a clicking sound -- in some cases a rather annoying clicking sound. Also, it could cause problems for guide bogs and such. And, there is even a possibility that focusing ultrasonic sound a some people my actually affect their nervous system or otherwise compromise their health or well-being.

jl

Safety scanner is a better option for human presence or any object, which are detected. They emit infrared ray within maximum of 180 degrees. We have applied them in our AGV. It scans area wise. The area is selected by giving input in digital. It has negative logic as you say and i hope such applications do have a negative logic widely.Go for Sick scanners

By the by please give us some information regarding pyroelectric detectors, since it functins by emitting sound waves. Seems to be interesting.

Pyroelectric detectors do not emit sound waves. In fact, they don't emit anything. They are passive detectors in that they detect changes in infrared [IR]. Typically a multifacited plastic lens focuses IR on to a pyroelectric sensor. Anything that emits IR (such as a human body) that passes in front of the sensor (i.e. into the field of view) causes a change and triggers the detector.

Another name for them is Passive IR Motion Sensor.