From: firstname.lastname@example.org (Peet Grobler)
Subject: Re: Alternative design to this?
Date: Thu, 26 Sep 2002 10:12:15 GMT
References: <email@example.com> <firstname.lastname@example.org>
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On 25 Sep 2002 07:20:09 -0700, email@example.com (N. Thornton) wrote:
>firstname.lastname@example.org (Peet Grobler) wrote in message news:<3d915b78.9241288@
>> On 23 Sep 2002 10:00:37 -0700, email@example.com (N. Thornton) wrote:
>> >> 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.
>> >** 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.
>> Yes... I understand this, and would definately investigate it further.
>> If I understand correctly, you have 8 different R's on one line, each
>> giong to a sensor. Feed this line into an A/D, and get a value (in the
>Yup. Here's how I'd set it up:
>Each sensor is a simple switch, and has a resistor in series with it.
>At the PC end I'd put a power supply to the wire, via a small
>resistor. An opamp can measure the Vdrop across the R, and feed it to
>the PC or A to D it and feed that in.
Can you supply me with a simple schematic? I think you lost me
slightly again. Just 4 inputs, or so. Please?
>Now any combunation of open and closed sensors can be read. In an 8
>bit number each bit corresponds to one of the sensors. So all 8 can be
>read simulatneously on one line.
>> Use this value to determine which one of the 8 devices got
>> triggered. This would speed up the entire scanning process, since I'll
>> be doing 3 scans to read everything, instead of 33. Also, I can check
>> 8 devices at a time, if nothing changed, read on...
>> Hey, thanks. I'll experiment with this real soon. This is something I
>> haven't done before, that's why I'm so interested!
>> >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.
>> Nope, you lost me. I only need one wire? So let's say the sensor gets
>> activated. Then what?
>When the sensor is activeted it draws current down the 1 wire. That
>current is measured at the PC end. So the 1 wire is both supply and
>Presumably things like door open closed detectors will just be passive
>switches. If you have some active sensors online as well, your active
>sensors wil need to be the bits that draw the most current: just put a
>R on the sensor's output so that when the sensor goes on it draws a
>known amount of current.
Yes, all switches /detectors are passive switches. No active sensors
>This works with 2 wires, supply/data and 0v return, but if you have a
>common mains earth connection you can use that as the return wire, so
>you'd only need to install one wire for each 8 bits.
I do have a common mains earth, but would that work? Otherwise I'll
simply use two wires.
>If you do that put spike protection on the circuitry, as under mains
>faults the earth can momentarily rise upto mains voltage. So 2 wires
>might be easier actually.
Yes, I think so. I'd rather use the 2 wires.
>Good luck, NT
Thanks. Seems like I need it.