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From: Rich Webb on 25 Jan 2010 08:42
On Mon, 25 Jan 2010 10:22:40 +0000 (UTC), Tim Watts <tw(a)dionic.net>
wrote:

>Hi,
>
>New to the group... But did google around the archives first, so please
>excuse if this has been done to death - I did try(!)

The guys over at comp.arch.embedded may also have some thoughts. I added
that group as a cross-post, with follow-ups directed to the original
group at s.e.d.

>I play with AVRs (Tiny/Mega) as a hobby (linux sysadmin, perl/C
>programmer by trade).
>
>I fancy having a play with some radio datalinks (with a view to home
>automation) - but there seem to be a million modules and a fair number of
>frequencies and standards. I'm just after some pointers based off some
>simple requirements:
>
>Important:
>
>a) Range - 20m through masonry (11" brick wall and 4" wall sort of thing,
>no rebar), 40m free air (could accept 10/20m)
>
>b) Usable data rate - 10's to 100-ish kbit/sec
>
>c) Cheap - 20 pounds sterling give or take, say 40 US dollars for a small
>easy to mount module (wire pads/header pins - no funky surface mount
>modules, my soldering isn't that advanced)

Have a look over at http://www.sparkfun.com/commerce/categories.php?c=16
and http://www.dontronics-shop.com/wireless.html for an idea of what's
available. For one-off, the simplicity of throwing on an XBee module is
hard to beat (and it meets your price point) but there are several other
choices.

Cost increases with data rate and complexity, so you'll have to make the
decision as to where your comfort zone is.

--
Rich Webb Norfolk, VA
From: John Walliker on 25 Jan 2010 08:49
On 25 Jan, 11:17, John Walliker <jrwalli...(a)gmail.com> wrote:
>  High frequencies diffract round corners better,

Correction - its the other way round. What I should have said is that
high frequencies are better at getting through small apertures, such
as the joints between sheets of aluminium foil coated plasterboard.

John

From: Tim Watts on 25 Jan 2010 09:13
On Mon, 25 Jan 2010 08:42:14 -0500, Rich Webb <bbew.ar(a)mapson.nozirev.ten>
wibbled:

> On Mon, 25 Jan 2010 10:22:40 +0000 (UTC), Tim Watts <tw(a)dionic.net>
> wrote:
>
>>Hi,
>>
>>New to the group... But did google around the archives first, so please
>>excuse if this has been done to death - I did try(!)
>
> The guys over at comp.arch.embedded may also have some thoughts. I added
> that group as a cross-post, with follow-ups directed to the original
> group at s.e.d.

Thanks - I'll check that group out too :)

> Have a look over at http://www.sparkfun.com/commerce/categories.php?c=16
> and http://www.dontronics-shop.com/wireless.html for an idea of what's
> available. For one-off, the simplicity of throwing on an XBee module is
> hard to beat (and it meets your price point) but there are several other
> choices.

Wow - some of those Xbee modules look exceedingly cool.

> Cost increases with data rate and complexity, so you'll have to make the
> decision as to where your comfort zone is.

Indeed. Data rate per device isn't likely to be high, but given many
devices, the net bandwidth and wasted bandwidth due to collisions is a
consideration...

Many thanks for your pointers Rich :)

Cheers

Tim

--
Tim Watts

Managers, politicians and environmentalists: Nature's carbon buffer.
From: Tim Watts on 25 Jan 2010 09:18
On Mon, 25 Jan 2010 05:49:09 -0800, John Walliker <jrwalliker(a)gmail.com>
wibbled:

> On 25 Jan, 11:17, John Walliker <jrwalli...(a)gmail.com> wrote:
>>  High frequencies diffract round corners better,
>
> Correction - its the other way round. What I should have said is that
> high frequencies are better at getting through small apertures, such as
> the joints between sheets of aluminium foil coated plasterboard.
>
> John


Ah yes. Fortunately I don't have any shielding issues until I get to the
roofline (1st floor as it's a bungalow) when the foil lined celotex will
be an issue.

Re the antennae: thanks for the info regarding monopoles and dipoles.
Many of these devices (thermostats) if I make them, will be in backboxes
in the wall. I've laid in lots of plastic conduit, so it has occurred to
me I could make a 1/2 wave dipole, coax centre fed and insert it up the
conduit, which gets it clear of the metalwork in the wall and the
electronics. Downside is there might be a power cable up there too
(mostly I have a pair of oval conduit drops per box, but a couple have a
single round 20mm tube). Suck it and see I guess :)


--
Tim Watts

Managers, politicians and environmentalists: Nature's carbon buffer.
From: D Yuniskis on 25 Jan 2010 18:47
Hi Tim,

Tim Watts wrote:
> I play with AVRs (Tiny/Mega) as a hobby (linux sysadmin, perl/C
> programmer by trade).
>
> I fancy having a play with some radio datalinks (with a view to home
> automation) - but there seem to be a million modules and a fair number of
> frequencies and standards. I'm just after some pointers based off some
> simple requirements:
>
> Important:
>
> a) Range - 20m through masonry (11" brick wall and 4" wall sort of thing,
> no rebar), 40m free air (could accept 10/20m)
>
> b) Usable data rate - 10's to 100-ish kbit/sec
>
> c) Cheap - 20 pounds sterling give or take, say 40 US dollars for a small
> easy to mount module (wire pads/header pins - no funky surface mount
> modules, my soldering isn't that advanced)
>
> Pie in the sky wish list
>
> d) Ideally simple framing built in - ie I clock a bunch of data in and
> hit "send" so to speak, and it transmits. Receive buffers frame and
> wiggles an interrupt. "Frame" could mean 8 bit word, or entire long
> packet (100's words).
>
> But ultimately I *could* live with wiggling some pins on a simple RF
> module where the 2 pins send different signals over the carrier to
> indicate 1 and 0.
>
> d) would be nice as it simplifies programming by miles, but not if it
> impacts on c)
>
> Not too bothered whether 433MHz, 868MHz, or 2.4GHz (this is the UK BTW),
> though I suspect 868MHz would be slightly better being less crowded.
>
> Zigbee looked interesting but I don't really need a whole protocol stack
> (I can do that) and it's not cheap.

I start looking at these things with the big criteria being:
- power consumption
- range
- data rate + duty cycle
Cost tends to fall out based on the above.

First, think about how you are going to power these devices.
The MCU's you reference are *relatively* low power. But,
the choice of radio can quickly aggravate your power budget.
E.g., if you intend to battery operate the devices, then
you have a very strict power budget. If you can tolerate
having a real "(mains) power supply", then power is less
of an issue (though this will affect where you site the
devices and can also be a cosmetic problem). However, you
have to ensure that you don't *need* power to be available
at all times. E.g., a mains powered HVAC controller can
make sense -- if the power is out, the controller is dead
but so is the HVAC plant, typically. OTOH, a mains powered
alarm system leaves you vulnerable to someone turning off the
power (or, a normal outage) -- sure, you may have a battery
backup on your alarm/siren, but if the sensors suddenly
go "off-line"... :-/

The same sorts of arguments apply to the reliability of
the data link. Can you tolerate short and/or long term
losses of connectivity? Will your HVAC system cease
to operate if it loses contact with the "temperature
sensor"? (etc.)

If you need to push lots of bits lots of time (e.g., continuous
duty), then this will conflict with a low power solution.

For things like home automation, most data channels can be *very*
low bandwidth -- watching for door/window closures, monitoring
temperature, etc. With these low data rates, ZigBee can be
a win *if* you can't afford a wall wart or other AC power
source nearby.

Of course, a wireless solution leaves you vulnerable to
interference from outside sources (unintentional as well
as deliberate). And, it "leaks" information to outside
eavesdroppers (unless you emply an encryption protocol
in the data stream and authentication).

I opted to deploy a wired system, here, to meet my needs.
This allows me to remove the unsightly wall warts/power
cords that would accompany each device (power comes down
the data cable). It also lets me decide which devices
need to be "backed up" during a power failure and lets
me centralize that backup power source (instead of having
to backup each individual node *at* the node). And, it
provides considerably more security against eavesdropping
and tampering than a wireless approach would have (imagine
someone accidentally/intentionally commanding your thermostat
to 30C in the Summer; or, turning on the heater for your
outdoor jacuzzi in the dead of winter)

The problem with any wired approach is, of course, the *wires*!
<grin> We had been remodeling at the time so installing the
extra cabling only carried the cost of the cable itself
(plus the terminations thereto).
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