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'Home Automation with PICs [OT] (was: Internet'
2000\02\18@222345 by Patrick J

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ok, but what if the room has 2 doors ?



> Well, I suppose this would work in most situtations :)
>
> John Mullan

> Why not place another PIR sensor at the exit/entrance to the room (call this
> sensor 'E' .. call the one in the room 'R').  Then if sensor 'E' is tripped
> before sensor 'R' then someone entered the room.  If 'E' is tripped last
> then someone left the room.
> Ken

2000\02\18@232737 by Ken Webster

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>ok, but what if the room has 2 doors ?

The general case would be to assume that the subject(s) is/are in the
vicinity of the most recently tripped sensor or in any area lacking coverage
that lies beyond that sensor.

Thus if
  R = sensor in room
  D1 = sensor near (just outside) door #1
  D2 = sensor near (just outside) door #2

Then the following conclusions may be drawn from the following sequences of
sensor interactions:

D1 first then R --> subject entered room via door #1
D2 first then R --> subject entered room via door #2
R first then D1 --> subject exited room via door #1
R first then D2 --> subject exited room via door #2

Of course, a system this simple could be fooled by someone else simply
walking past the door when someone is alredy in the room and sitting still.
If you used more sensors you could track each individual and conclude that
it was merely another individual walking past the door.  Of course, this too
could be fooled if two individuals entered the room together, one sat down,
and the other left.  But at least it is a starting point.

If you wanted a perfect system you may have to make people wear PIC-based ID
transcievers that identify each individual to each sensor they pass (via a
radio signal).  You could even implant the chips under the skin like they do
to track wildlife.  Maybe the government will like this idea too and require
that each individual be implanted with such a chip at birth so that thier
location can be unambiguously resolved at all times :o)

A less invasive but more expensive approach would be to place cameras at
various locations around the house and have a computer vision system pick
out the human subjects and track them from room to room.  I don't think it
would be practical to attempt this using a PIC though :o(  ... maybe if the
human subjects had a big bar-code tatoo on their foreheads :o)

Cheers,

Ken

2000\02\19@041108 by Byron A Jeff

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>
> ok, but what if the room has 2 doors ?

The presumption is that you put an 'E' sensor at each entrance/exit.

The reason that this won't solve the problem is that PIR's aren't
sensitive enough to distinguish between one person and two people
entering the same entrance at the same time. So you get the situation
where two folks enter at the same time, then one leaves. The sensors think
that one person entered then exited. So it turns the lights off leaving
the second person in the dark.

This person sensing problem has no simple solutions. In the end it'll require
sensor fusing at least two or three different technologies (PIR, beams,
RFID, capacitance) to accurately detect people.

BAJ
{Quote hidden}

2000\02\19@204324 by John Mullan

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Okay, okay.

I'm back to my original idea of monitoring the signal difference from the
transmitting wire on (or in) one wall to the receiver wire on the other.
Something like they do for traffic lights.  Signal changes when a body
occupies the room.

Seems to be even more practical now if you start setting up all this IR
detectors all over the place.

John Mullan


{Original Message removed}

2000\02\19@214920 by andy howard

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> Okay, okay.

> I'm back to my original idea of monitoring the signal difference from
the
> transmitting wire on (or in) one wall to the receiver wire on the
other.
> Something like they do for traffic lights.  Signal changes when a body
> occupies the room.

I think noise would be your enemy with this, A big lump of steel a few
inches above a sensor is much easier to detect with inductive sensors
than a few pounds of water several feet away. You'd need to carry some
resonant device.

Other possibilites?


You could have an IR transponder consisting of an IR receiver, some
small intelligence - or even something hardwired - to give a return
code, and an IR diode. Each room has a similar IR transceiver that polls
the room. If you're in there your transponder signals back. It could
easily be made into a badge, pen-type device etc, minimum of processing
required, individual IDs available. Unambiguous indication. Best of all,
pretty cheap to implement.



Hmmm, or how about hacking a serial output from a casio GPS watch
http://www.casio.com/gps/ ) and sending that to your system via IR, RF,
induction loop or whatever?
Neat if it could be done. If e.g. the receiver is separate from the
display driver and not all under one big epoxy blob.
Not cheap, possibly not accurate enough in all cases, but cool as
anything...



> Seems to be even more practical now if you start setting up all this
IR
> detectors all over the place.

Then there's I-Buttons.

Or you could have a coded electromagnetic badge or hang-tag like some
building security systems use.

Or barcoded ID Cards, or wristbands. There are surplus supermarket POS
barcode readers
for sale cheaply online, I'm sure I've seen some recently.

You'd only need one pickup per room for a simple toggle system, and a
bi-colour led to confirm if it thinks you're in or out.

With a wrist or belt worn tag you could have an in sensor and an out
sensor on either
side of the doorway if you wanted positive log-in/log out switching.

Hmmm, this is getting silly. I'd go for the IR transponder, cheap,
cheerful, small, simple to install and relatively unambiguous.













.

2000\02\19@220157 by Ken Webster

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>Okay, okay.
>
>I'm back to my original idea of monitoring the signal difference from the
>transmitting wire on (or in) one wall to the receiver wire on the other.
>Something like they do for traffic lights.  Signal changes when a body
>occupies the room.
>
>Seems to be even more practical now if you start setting up all this IR
>detectors all over the place.
>
>John Mullan

This sounds interesting ... I would love to hear about your results if you
get it to work.  I'm not sure how one would do this though.  Inductive
sensors for traffic lights are simple because cars are large metallic
objects.  The electromagnetic properties of humans aren't nearly so
convenient.

If the subject comes close enough to the antenna you could sense the change
in capacitance like a theremin.  But to cover an average-sized living room
may be difficult.  Perhaps you could make a big grid (approximating a plate)
under the carpet?  Seems like a lot of work.

PIR sensors may do the trick with a little modification -- how about using a
diffuse lense instead of the typical Fresnel lense and have a PIC directly
sample a high-resolution A/D reading from the sensor.  Any reasonably rapid
increase in reading could be interpreted as a human entering the room and a
rapid decrease could be interpreted as a human leaving the room (under the
right circumstances ... the software would have to reject changes due to
lights being switched on and off, etc., but, if it is part of a home control
system then it should know this information anyway).

Ken

2000\02\19@223136 by John Mullan

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The principal behind this is more like changing the ground plane of the
transmitter therefore changing the signal that is picked up by the receiving
antenna (remember, there is a transmitting wire along one wall and receiving
wire along an opposing wall).

There is actually a device that does this already.  They pump a signal
through the one wire and compare it to what is received at the receiving
wire.  They provide another connection if you need to put a metallic/foil
sheet under your flooring to increase the ground plane effect.

I have seen their page on the internet and will try to find it.

I don't want to just buy this though, I want to build something like it.

John Mullan


{Original Message removed}

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