From: firstname.lastname@example.org (N. Thornton)
Subject: Re: Alternative design to this?
Date: 23 Sep 2002 10:00:37 -0700
NNTP-Posting-Date: 23 Sep 2002 17:00:37 GMT
email@example.com (Peet Grobler) wrote in message news:<firstname.lastname@example.org>...
> I'd like to start off with home automation (my own version thereof).
> First off, I'd like to have a computer monitoring the outbuildings (so
> my wife doesn't kill me if something goes wrong).
> I have 33 inputs, including all doors, windows, lights, etc. I want to
> have a PC monitoring all these, and logging changes in them. Later on
> I'll add temperature monitors.
Not very with it today, so this may be off the mark... you're sending
8 bit data decribing 1 bit situations, like lights on or off. So it
appears you could combine many pieces of info into one single byte of
8 bit data. That would reduce your 33 dramatically.
Point 2 is you only _need_ one wire, not a pair, assuming you've got
an earth wire already. It is possible that very occasionally you could
get a data byte messed up, but not often enough to have any real
effect in real world terms. If you never ever want that to happen,
just tell the PC to average the result of a few successive readings,
or to re-question the device if the reported value changes, or to only
believe a result when 3 successive readings are the same.
There are more old fashioned ways to do it, using very cheap parts.
One example would be that each detector interface has a voltage window
detector. When the PC end questions it, PC end sends the relevant V
corresponding to the target device, and target device responds by
either doing nothing or shorting the line until the i is removed. Thus
you have a lot of 1 bit data senders, and very simple ones too. One
very IC could do that.
The PC end questionsing voltage is developed by putting Rs on a PC
port, as a simple D-A. The PC end listening device is a simple opamp,
giving hi or lo output.
** Much better **
Then there are even simpler methods. Each detector connects to the
line via an R: the set of 8 detectors on one wire gives you an 8 bit
voltage value (or current). Very very simple. For 33 devices you'll
need 3 signal lines using 8 bit, or more if you use a lower bit code.
That sounds to me like a real winner. Ultra simple, ultra cheap, very
expandable. When you add temp senmsors later, you can decide what the
important temp break points are, then you'd only need say 4 bits per
temp.. or go the whole 8bit level per temp sensor.