Re: My latest cool DIY automation idea

["Robert Green" <ROBERT_GREEN1963@xxxxxxxxx>]

> I would mount one of these http://www.maxbotix.com/ opposite the door
> and range find the object as it moves through the doorway.

A pretty neat solution and certainly something to consider in addition to
all the other technologies.

The problems of occupancy detection are in some ways similar to aircraft
radar detection (sky occupancy, if you will).  Single targets are easy to
identify.  It's when two targets merge and then separate (flight KAL 007 for
example) that trouble begins.  I don't have any figures to back me up, but I
feel that a two IR beam detector at each doorway would be less confused by a
room full of people than a rangefinding device.

There's no doubt, however, that even two IR beams could be confused by two
people walking through the doorway together.  That's where pressure mats on
either side of the door would provide the definitive answer.  Even if the IR
sensed two people in the doorway, the mats would tell the tale of which way
they went.  No weight detection on one side meant they both went the other
way.   I do see rangefinder technology as a further way to test the
assumptions of occupancy.  The units you pointed to are nice, small, have
very low power consumption and are fairly cheap (25 to 29 bucks) and I would
probably use them where my ultra-cheap sensors were problematic.  The look
especially useful for robotic designs.

Gotta love the builder's motivation: "my daughters robot was stuck on a
wall."

Any idea what happens if the units are placed where they can "hear" each
other?

(Answered, actually, by the FAQ:

http://www.maxbotix.com/MaxSonar-EZ1__FAQ.php

at their very simple but very well done website)

The designer details several ways of using multiple sensors without them
interfering with each other.  He also points out the problems facing
ultrasonic detectors and "soft acoustical targets" like human beings.

That discussion leads me to believe that double beam IR doorway sensors are
still the most reliable ways to detect exit and entry and therefore
occupancy.  The only hard part is designing a circuit that keeps the two
LEDs from interfering with each other.  That will probably require a design
similar to theirs that reads each sensor "round robin" style.

I thought of doing it cheaply by have a low sensor and a high one, but I
really wanted both sensors at the same height so that I could reliably sense
motion.  At the same height, one would always be "eclipsed" for a very short
time before the other.  A simple latching relay circuit could detect
direction based on which LED fired first.  With detectors at different
heights, you could mess up the direction if someone's leg crossed at the
exact time that their torso did.  At exactly the same height, that wouldn't
happen.

The only unanswered question remaining about the Maxbotics ultrasonic units:
Will they attract bats?  ;-)

--
Bobby G.


"rhamer" <rhamer.2b50gg@xxxxxxxxxxxxxxxxxxxxxxxxxx> wrote in message
news:rhamer.2b50gg@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
>
> I would mount one of these http://www.maxbotix.com/ opposite the door
> and range find the object as it moves through the doorway.
>
> Getting closer = entering, getting further away = exiting.
>
> Just an idea.
>
> Cheers
>
> Rohan
>
>
> --
> rhamer