Notes from Sand Hill
XTension Tech Notes
ID: TN.proximity |
Subject: Sensing 'presence' rather than 'motion'
Stop waving at your motion sensors !
The common 'motion sensors' that are available from X-10 and others, are based on the premise that a warm body moving in the field can be sensed by a 'passive infrared' detector.
Much of the time, these PIR's are great, but what if we sit in a chair to read a book or watch TV, and remain motionless for a period ? Just as we're getting comfortable, the motion sensor says 'the room is empty', and turns off the lights !
Of course all we have to do is get out of the chair, or wave our arms, and the PIR will turn the lights on again...but what we really need is a 'presence sensor' rather than a motion sensor.
There is now a device available which actually senses when a person moves into its 'field', and can determine when all persons have moved out of the area.
This last part is specially nice. For example when there are multiple people in a room, the lights can be kept on until everyone has left.
How it do that ?
The device emits a very low power radio signal which sets up a 'field' between its transmitting and receiving 'antennas'. When we set it up, we 'tune' it while the field is empty. So that the signal strength between the antennas is just less than what it takes to engage a very small relay.
When someone enters the field, their body will actually increase the signal strength between the antennas, and will cause the relay to engage.
If we take the output of that relay to the inputs of a Powerflash module, we get a X-10 ON signal when the field goes from 'empty' to 'not empty', and an OFF when the field is completely empty again.
The most important use for home automation, is probably the one above, where it is important to know when a room is really empty or not. In this case, the device can cover an area about 12 feet wide.
Another use is similar: that of determining that a chair, sofa or bed is occupied. In this case, the device is tuned so that it only senses a body which is a few inches away from the antennas.
There are of course many other uses for this sensor. It can be used to detect someone passing through a doorway, or approaching a kiosk, etc. But for this technote, we will only look at the first two modes.
How's this go together ?
Now this looks like a big bother, but it really isn't. Here's a picture of just the Bik sensor board :
Chad Gard assembled two of these and was kind enough to take pictures of the process. He now has a website for home automation. Thanks Chad !
And here's most of the parts that you have to buy :
When putting these together, you must be MOST careful of the polarity of the power supply connections.
Now we make the Antennas
The Antennas are really just a pair of single wires which are connected to the center conductors of the Audio cables ( coax -- pronounced 'CO-AX')
There are two parts of each Antenna. The coax cables run from the Bik board to the location you want to monitor:
The center conductor is really the carrier of the signal, and the outer braided conductor 'shields' this signal, keeping it inside until it gets to the end where the braid is stripped back exposing the center wire.
The business end of the antennas
Any metallic wire or screen that we connect to the center conductor of the coax, will 'radiate' a signal from the Transmitter end, or detect the radiated signal on the Receiver end (as in radio).
You can use anything from metal plates or screen, to simple wires or even aluminum foil (hard to solder).
Shield the center conductor all the way to the area of interest
The idea is that wherever the outer shield of the coax is exposed, there begins the point where the signals will be radiated or detected. Any metallic object that you connect that to will increase the 'sensor' area or breadth.
So you don't want to expose too much, but you want to have enough for the area you want to monitor.
One inch of exposed antenna would be fine to guard a wrist watch etc, but would not be sufficient for a doorway or a bed...
In the case of a whole room, you must consider that the transmitter antenna and the receiver antennas are up to 12 feet apart, and both must connect to the Bik. So one run of Coax may be longer than the other.
The Bik document says that whenever the cables must be of unequal length, the shorter of the two should be the Receiver.
Install everything before trying to 'tune' the Bik
This means that you really want to think about where you want it to go before you go off half cocked and cut everything to the wrong length...
You may want to install the antennas under the carpet, or horizontally in opposite walls, or under the sofa cushions, but you must plan out the final installation carefully. If you do not, you will be making your own 'tar baby'.
This doesn't mean that you can't cut and re-solder etc, but after the third time, it gets to be a gumption trap (please take my word for it).
Tuning the circuit for the final location
This may be easy, or it may be something that makes you lose your religion.
According to the documentation from Bik, there are two 'modes'. They are called the "Transmitter" mode and the "Shunt" mode.
For your needs, these differ only in the way that we set up the Bik controls.
In the first case, we set up the Bik to detect a diminished signal, and in the other, we make it look for an increase in signal strength.
The difficult part of all of this is the construction of your house. How much metal is there? Is the floor cement slab or raised?
The problem is one of signal propagation. Whether signals can easily be passed to earth ground or not. A cement slab with metal 'rebar' is a good conductor, and will 'grab' these signals. A raised floor (like the second story), will likely not be a good 'sink' for the signals
Regardless of which way we have to set it up, we want it to be reliable.
I think that this is one thing that will only be answered when you finally have everything physically settled.
This is where I have to quit for now as the next section could drive you crazy if it's not clear and deterministic.
In short, in one mode we tune the Bik so that it will close the contacts of the relay when a body enters the field, and in the other, we set it to open the contacts.
The documentation that you get with the Bik is correct for each mode, but it is not clear that you have to 'discover' which mode is right for the application.
The 'tar baby' part is that you might just luck onto a good setting the first time you set one up, but the next time you move something around, you might find that the same procedure won't work and you will try and try....
As soon as I can get a deterministic sequence for setting up both modes, I can't recommend that you buy it unless you have a BSEE or a lot of patience.
UPDATE : September 1, 2007
I have been using this for 4 months now and it has proven to be much less bother than I anticipated.
I have not had to re-tune the circuit board at all !
I have re-aligned the antennas each time I changed the sheets of the bed,
but even when they get messed up, they still seem to work well.
I still don't have a good 'story' about how to tune the circuit board for your particular application, but I really do like this product.
OH ! you might want to know where to buy this thing:
try : the BIK site
(still in work) mf
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