ucontrollers (AVR/ATMega) and radio transceiver modules

[Tim Watts]

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(!)

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.


Any pointers as to any technogolgies or modules that would be worth
looking at would be most gratefully received. I've looked - wood and
trees syndrome - don't know how to filter the choices down...

Ta muchly

Tim

 

[Tim Watts]

How bothered are you about power consumption? Can you use mains
signalling or just lay cable?

Hi John,

Thanks for the reply.

Cabling: Well, I will have a wired DC SELV supply to avoid falling foul
of the IEE Wiring Regulations if I want to deploy any of these in
"Special Locations" like bathrooms. So current consumption will not an
issue there, but obviously, this rules out mains signalling.

I had considered dropping cables round and using a bastardised CAN bus or
RS485 drops or something, but radio would be cool for a number of
reasons, including the education factor. If it's cheap enough it may be
cost comparable with a cabled method (bearing in mind cables,
terminations, junction boxes etc). Also means I can extend the system to
outside use.

If you decide to go for basic modules without complex modulation methods
and software stacks, then there are three main types to consider:

1) On-off (amplitude) keying

2) Frequency shift keying - wideband

3) Frequency shift keying - narrowband

The range and cost increase in the order 1 -> 3.

I've never been any good with analogue stuff, but I do understand those
terms.

For the same transmitter power, receiver sensitivity and receiver
bandwidth lower frequencies will generally give longer range than
higher, mostly because the lower frequency receiver antenna (of
equivalent directivity) will capture more of the incident power .
However, higher frequencies will make it easier to use directional
antennae as they will be smaller. High frequencies diffract round
corners better, while low frequencies go through walls better.
OK.

I have successfully used the GT1 25mW transmitter and GR1 superhet
receiver modules available from several UK suppliers over a range of
about 2km in open air with a well-matched dipole antenna at each end. I
picked a frequency of 434.225 MHz because it is relatively quiet in my
area.

That seems very impressive for the power (thinking how many milliwatts my
WIFI runs at)

Another approach to getting long range is to take advantage of the UK
0.5W band at around 868 MHz, but the modules are more expensive than the
lower power ones.

Ah - thanks - I didn't know powers like 0.5W were permissable.

There are some very easy-to-use single-chip Manchester coder/decoders
available for use with FSK modules - RF600 and RF800.

Very interesting - just scanned the datasheet.

Have a look at Radiometrix, Low Power Radio Solutions, MK Consultants
web sites.

Just for starters, I really like the look of this one:

http://www.lprs.co.uk/product_info.php?cPath=35&products_id=56

(easy-Radio 433-4MHz FM transceiver)

Probably about the level I'm looking for.

And of course the Ofcom website for frequency allocations
and other rules such as permitted power levels and duty cycle limits. In
particular, look for frequencies which are allowed in the UK but not in
the rest of Europe. These will generally be much quieter than those
which can be used in any European country.

Good idea. Most manufacturers will be going for common frequencies in a
common market - makes sense to try for a quieter band.

If you do decide to go for Zigbee, Farnell sell reasonably priced Zigbee
modules, but the development tools are quite expensive.

I'll have a look there next.

Many thanks John, lots of excellent pointers

Cheers

Tim

 

[Tim Watts]

Just for starters, I really like the look of this one:

http://www.lprs.co.uk/product_info.php?cPath=35&products_id=56

(easy-Radio 433-4MHz FM transceiver)

Incidently, please excuse my ignorance: what would be a sufficient
antenna for one of these? Bit of straight open ended wire, 1/4 wavelength
(so 17cm or so for 433MHZ)? Or is it better to go for 1/2 wavelength if
possible, or some sort of loop? Does it need to be precise for resonance
purposes?

 

[Rich Webb]

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.

 

[Tim Watts]

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]

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

 

[D Yuniskis]

Hi Tim,

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).

 

[Tim Watts]

Hi Tim,

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"... :-/

This is true.

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.)

Good point. I was going to use the "repeated-call-for-demand" strategy -
ie rather than the thermostat sending one request for heat (even if it is
ACK'd) then staying silent for 6 hours, it will send a call for heat
(say) every 10 minutes when heat is required. That way a missed
transmission (or a power cycle where state may be forgotton) doesn't
break the system for long.

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

I think it will be a small packet (+ return ACK) every 10 minutes or so,
unless remote programming or data dump operations are in progress (from a
PC), in which case the channel will get very bursty for short periods.

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'm not too worried about encryption, but I might consider a very simple
challenge-response mechanism to authenticate the end points (never ceases
to amaze me that one can set off a neighbour's radio doorbell so easily
in this day and age).

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).

And the expense. Ideally, I'd slap ethernet+TCP/IP on everything and
shove it all through a seperate VLAN to my main network. But ethernet+TCP/
IP is rather heavy (doable but heavy) on a poor AVR and it seems for the
same or maybe less money, I can dispense with the wires.

Those Xbee modules look very cute - been reading the data sheets some
more. Basic collision avoidence and retransmit, packet framing and simple
addressing if my skim reading is correct. Just the ticket. Can soon pop a
simple protocol on top of that.

As it's advantageous to keep the user touchable parts on SELV DC (no
mains or earth references), power isn't a problem. One advantage is that
certain actuators are cheaper in the mains powered version (eg motorised
water valves for radiators) so it would now be possible to stick a direct
from mains powered Xbee next to a water valve with nothing more than a
suitably fused loop in loop out 240V supply, but keep the user interfaces
on DC.

I envisage lots of paired timer/thermostats (LCD display, few buttons,
one per room) paired with a valve + a second relay for boiler/pump
demand. The system would work without any PC and failure of any part (bar
the boiler demand interface) would knock out a room, not the house. The
flexibility comes in then being able to remote manage the lot as a whole.

I've been looking at Heatmiser timer/stats. Very nice, RS485 data link
with documented protocol but not radio link. But around 50 pounds. I
winder if I could do something for a fraction less, with more education
value and possibly (after the nth firmware upgrade!) more features

Honeywell CM-Zone stuff is also pretty nice and radio based but pricey
and without a documented protocol no fun at all.

Thanks for all your comments!

 

[D Yuniskis]

Hi Tim,

This is true.


Good point. I was going to use the "repeated-call-for-demand" strategy -
ie rather than the thermostat sending one request for heat (even if it is
ACK'd) then staying silent for 6 hours, it will send a call for heat
(say) every 10 minutes when heat is required. That way a missed
transmission (or a power cycle where state may be forgotton) doesn't
break the system for long.

But, if it goes silent, you don't know if the device has "died"
(in which case, you don't know whether the *room* wants heat/cooling
or not) or if it just "doesn't need 'anything'".

What I have done is download small "fail safe" applications into
each node. These allow local intelligence to keep things "safe"
in the event communication with the "server" fails for some
reason. I.e., the "thermostat" knows to maintain temperature
between some absolute limits regardless of whether or not it
has been commanded externally to do so. Likewise, the
irrigation system knows a default watering schedule that it
will enforce if it loses contact with the server.

(you have to consider what might happen if you are away from
the house for an extended period -- weeks? -- when the failure
occurs... no possibility of outside intervention to *fix*
things in your absence)

I think it will be a small packet (+ return ACK) every 10 minutes or so,
unless remote programming or data dump operations are in progress (from a
PC), in which case the channel will get very bursty for short periods.

ZigBee is good for very low bandwidth operation using very little
power (sleeping most of the time). E.g., it was designed to be
retrofittable in places like hotels, hospitals, etc. where you
may not already have the (network + power) infrastructure in place
to support things like this. So, you want to operate on batteries
yet not have to be replacing 500 batteries every month! :>

I'm not too worried about encryption, but I might consider a very simple
challenge-response mechanism to authenticate the end points (never ceases
to amaze me that one can set off a neighbour's radio doorbell so easily
in this day and age).

Note that this will leave you vulnerable to "session" attacks.
I.e., once you legitimately "open" a connection, someone else
can inject their own "commands" -- until you "close" the
connection. If the attacker can obfuscate your eventual
"close" message, then he can leave the channel open indefinitely
(unless your protocol has built in timeouts)

I only mention this as you need to gauge your own level of
paranoia. Nowadays, it is just too easy for folks to tinker
with stuff like this: "Hey, wanna watch me make Tim's
garage door go up and down nonstop for the next hour??"

And the expense. Ideally, I'd slap ethernet+TCP/IP on everything and
shove it all through a seperate VLAN to my main network. But ethernet+TCP/
IP is rather heavy (doable but heavy) on a poor AVR and it seems for the
same or maybe less money, I can dispense with the wires.

You needn't go Ethernet to be "wired". I opted for it simply
because I want one network fabric that I can apply to *all*
of my needs (automation, security, telephony, entertainment, etc.)
instead of having to run Cable Type X to these N places and
cable type Y to these other places.

E.g., I can put a "display" anywhere to control any*thing*.
(instead of having the display for the thermostat *at* the
thermostat, etc.)

Those Xbee modules look very cute - been reading the data sheets some
more. Basic collision avoidence and retransmit, packet framing and simple
addressing if my skim reading is correct. Just the ticket. Can soon pop a
simple protocol on top of that.

As it's advantageous to keep the user touchable parts on SELV DC (no
mains or earth references), power isn't a problem. One advantage is that
certain actuators are cheaper in the mains powered version (eg motorised
water valves for radiators) so it would now be possible to stick a direct
from mains powered Xbee next to a water valve with nothing more than a
suitably fused loop in loop out 240V supply, but keep the user interfaces
on DC.

I have been moving everything to PoE. Electric Code here (US)
allows this as "low voltage, power limited" wiring.

I envisage lots of paired timer/thermostats (LCD display, few buttons,
one per room) paired with a valve + a second relay for boiler/pump

I've opted to do everything "headless". Just black boxes with
suitable field wiring. The user interface being something
decoupled from the "controllers", "sensors" and "actuators".

demand. The system would work without any PC and failure of any part (bar
the boiler demand interface) would knock out a room, not the house. The
flexibility comes in then being able to remote manage the lot as a whole.

I've been looking at Heatmiser timer/stats. Very nice, RS485 data link
with documented protocol but not radio link. But around 50 pounds. I
winder if I could do something for a fraction less, with more education
value and possibly (after the nth firmware upgrade!) more features

Pounds = Dollars (regardless of the current exchange rate :> )
So, you should be able to put an EIA485 transceiver on a tiny
AVR and get the same sort of performance. But, then you have
to build the boards yourself...

Honeywell CM-Zone stuff is also pretty nice and radio based but pricey
and without a documented protocol no fun at all.

Good Luck!
--don

 

[Tim Watts]

Hi Tim,

Tim Watts wrote:

But, if it goes silent, you don't know if the device has "died"
(in which case, you don't know whether the *room* wants heat/cooling
or not) or if it just "doesn't need 'anything'".

True - perhaps a better strategy would be positive on and off signals, but
repeated every 10 mins. Continued lack of reception would signal an error -
though there's not much that can be done of the water valve controller has
died, other than manually override it[1] I suppose if the thermostat died,
the water valve could default to the last timed pattern it saw.

[1] Zone valves are the cheapest motorised valves I can find, but I'm
considering butchering a standard thermostatic radiator valve head and
adding an heating resistor to control it by bias. I know of someone who has
successfully done that (around 5W heating required IIRC) and the parts are
cheaper. Also allows for full manual override (turn the knob).

What I have done is download small "fail safe" applications into
each node. These allow local intelligence to keep things "safe"
in the event communication with the "server" fails for some
reason. I.e., the "thermostat" knows to maintain temperature
between some absolute limits regardless of whether or not it
has been commanded externally to do so. Likewise, the
irrigation system knows a default watering schedule that it
will enforce if it loses contact with the server.

There won't actually be a server as such - the room stats will be paired
with a valve controller (likely to be remote where it's convenient to mount
a valve, prolly in the attic space where the pipes are). If a room stat
fails, there's not a lot you can do, but I'm allowing mechanical manual
override as I don't trust computers (!).

(you have to consider what might happen if you are away from
the house for an extended period -- weeks? -- when the failure
occurs... no possibility of outside intervention to *fix*
things in your absence)

True. With the paired strategy though, at most one room is likely to fail
(might have to double up on the boiler receiver to guarantee that) so it
won;t really affect frost protection.

Note that this will leave you vulnerable to "session" attacks.
I.e., once you legitimately "open" a connection, someone else
can inject their own "commands" -- until you "close" the
connection. If the attacker can obfuscate your eventual
"close" message, then he can leave the channel open indefinitely
(unless your protocol has built in timeouts)

I only mention this as you need to gauge your own level of
paranoia. Nowadays, it is just too easy for folks to tinker
with stuff like this: "Hey, wanna watch me make Tim's
garage door go up and down nonstop for the next hour??"

No one round here will even be close to hacking at that level and the one's
who are up to it won't. But I take your point. It would be good to engineer
a modicum of protection in. Has to be lightweight - don't fancy doing
polynomials on an AVR... Must do some reading...

E.g., I can put a "display" anywhere to control any*thing*.
(instead of having the display for the thermostat *at* the
thermostat, etc.)

I like your approach

I have been moving everything to PoE. Electric Code here (US)
allows this as "low voltage, power limited" wiring.

The UK IEE regs define SELV (separate extra low voltage) as less than 60V
DC, so that does allow PoE too, in dry areas. But there is still a special
case to do with bathrooms where the limit is 30V DC depending on area and IP
rating.

I've addressed that problem by siting the backbox for the device out in the
hall and that one will have a remote sensor in the bathroom ceiling, which
is considered outside the vulnerable zones provided it's high enough.

I've opted to do everything "headless". Just black boxes with
suitable field wiring. The user interface being something
decoupled from the "controllers", "sensors" and "actuators".

That's a nice idea. If I opt for cheap enough RF transceivers and given AVRs
are dirt cheap, that would be quite sensible.

Good Luck!

Cheers! Have to fix the rest of the house first In the middle of a DIY
renovation - doing the mains wiring at the mo.

 

[D Yuniskis]

Hi Tim,

But, if it goes silent, you don't know if the device has "died"
(in which case, you don't know whether the *room* wants heat/cooling
or not) or if it just "doesn't need 'anything'".

True - perhaps a better strategy would be positive on and off signals, but
repeated every 10 mins. Continued lack of reception would signal an error -
though there's not much that can be done of the water valve controller has
died, other than manually override it[1] I suppose if the thermostat died,
the water valve could default to the last timed pattern it saw.

If you have local intelligence, you can continue to operate the
valve even in the absence of temperature (sense) data. E.g.,
remember the *pattern* that the valve was following (something
akin to duty cycle) and just have the valve continue that
*pattern* of operation in the absence of any "command input".
No, it won't be perfect. But, I suspect it will be better than
"full on" or "full off" :>

[1] Zone valves are the cheapest motorised valves I can find, but I'm
considering butchering a standard thermostatic radiator valve head and
adding an heating resistor to control it by bias. I know of someone who has

Ah, that's clever. Some "setback" thermostats sold here
ages ago worked on a similar principle -- generate some
heat *under* your regular thermostat to trick them into
thinking the house didn't need heat.

successfully done that (around 5W heating required IIRC) and the parts are
cheaper. Also allows for full manual override (turn the knob).


There won't actually be a server as such - the room stats will be paired
with a valve controller (likely to be remote where it's convenient to mount
a valve, prolly in the attic space where the pipes are). If a room stat
fails, there's not a lot you can do, but I'm allowing mechanical manual
override as I don't trust computers (!).


True. With the paired strategy though, at most one room is likely to fail
(might have to double up on the boiler receiver to guarantee that) so it
won;t really affect frost protection.

Understood. It's been so long since I've used steam heat that
I forgot how easy it is to have "multiple zones" :<

No one round here will even be close to hacking at that level and the one's
who are up to it won't. But I take your point. It would be good to engineer
a modicum of protection in. Has to be lightweight - don't fancy doing
polynomials on an AVR... Must do some reading...

Even if you aren't at risk for hacks, consider the effects
"bad data" can have on operation. I.e., what happens if
you erroneously turn the valve on? off? Think about
how each type of error can affect you. (e.g., timeouts
are often used).

Once you've done *that* thinking, consider how *that*
scheme can fail. E.g., what happens if your valve turns
on, then times out and turns off, then (erroneously) turns
on again, etc. I.e., it may be better off if it "fails"
one way or the other -- I'm not saying this is the case
for a steam valve but, rather, consider this when you
think of other automation applications (garage door
set to automatically close after 5 minutes to safeguard
against a faulty "open" command; then opens shortly
after having closed; closes again 5 minutes later and
again reopens, etc.)

I like your approach


The UK IEE regs define SELV (separate extra low voltage) as less than 60V
DC, so that does allow PoE too, in dry areas. But there is still a special
case to do with bathrooms where the limit is 30V DC depending on area and IP
rating.

Yes. I believe a consequence of the difference between
wet skin "breakdown voltage" vs. dry.

I've addressed that problem by siting the backbox for the device out in the
hall and that one will have a remote sensor in the bathroom ceiling, which
is considered outside the vulnerable zones provided it's high enough.

I think 7 feet, here. (or maybe it's 8?)

That's a nice idea. If I opt for cheap enough RF transceivers and given AVRs
are dirt cheap, that would be quite sensible.

The point was to allow the sensors and actuators to be simple,
inexpensive and "pot-able". Once you remove the human user
from the equation, you can more readily achieve these goals.

At the same time, it lets the user interface become much
*richer* than it would otherwise be for any of these
individual devices.

Cheers! Have to fix the rest of the house first In the middle of a DIY
renovation - doing the mains wiring at the mo.

<grin> Been there, done that, have the T-shirt to prove it.
Tedious though rewarding experience!

--don

 

[Charlie E.]

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,
When you say power, are you talking 24VAC for powering the thermostat,
or full 120VAC. If the latter, it is a big code no-no to put ANYTHING
low voltage in there!

Even if it is the 24VAC, you are going to couple to it with any
antenna you put in there, and get very 'interesting' results!

Charlie

 

[D Yuniskis]

Hi Tim,

Agreed. The only thing the valve doesn't have is the current room
temperature, so it would have to replay the operation timings - or an
approximation of them (like x% duty cycle from 1pm to 2pm etc). But yes,

I wouldn't even bother trying to track time of day, etc.
Pick some period -- "the last 24 hours" (?) -- and have it track
(and continuously update) the average duty cycle that it was
*commanded* to use over that period. Then, in the absence
of "commands" (for some "significant period"), just have it
reproduce that duty cycle in some <mumblemumble> period
(I don't know the time constants involved in steam heat so
you'd be better at guessing that!)

You aren't trying to work perfectly. Rather, you are
trying to come up with something better than "all on",
"all off" or "whatever I was commanded to do most
recently". If things break while you are "available",
you will *fix* it! (you've heard of the Master-Slave
principle? Well, in this case, *it* is the Master and
*you* are its Slave! :> ) You just want to cover your *ss
in case you *aren't* available.

that would probably give a sensible performance for constant weather
conditions and at least means it goes off at night and on in the day.


I think all the mechanical stats I've ever seen here have had that -
provided the person remembered to take a neutral there(!).

I went through loads of sources of random valves. The motorised "standard"
valves are fairly ubiquitous, but not so cheap and are usually 240V
operation. 24V exists, but being aimed at the fancy boat market cost far
more than they're worth.

Yup. With steam, you're kinda stuck on your choices.
E.g., if it was hot water heat, you might have a wider
variety to chose from.

I found one neat little valve that was pulse operated at 12V (bistable,
needed no power to maintain either state) but it both severely restricted
flow and had some bloody awful *plastic* 1/2" BSP threads on it. Having had
experience of some plastic threads on a small water heater where varying
amounts of PTFE tape (tried water grade and the thicker gas grade) failed to
seal it and had to resort to a product that's basically a thick Loctite for
plumbing[1], I never want to see one again!

Plastic and metal don't usually mix well. E.g., always plastic
*into* metal and not the other way around. Even that is no guarantee.

[1] Rocol Threadseal XS - suitable for hot, cold and drinking plumbing and
methane-gas plumbing, on any taper thread joint - don't know if you have it
or something like it, but it is superb stuff.

Understood. It's been so long since I've used steam heat that
I forgot how easy it is to have "multiple zones" :<

We think it's the state of the art here ;->

Yeah, well... we also have INDOOR PLUMBING on this side of the
pond! Fantastic invention! Makes going to the loo on a cold
winter day much more comfortable!

Seriously - last house I lived in had one timer and stat in the hall for the
whole house and that was only 15 years old. Building regulations now require
I think 2 zones and thermostatic rad valves. hmm. advanced...

Here, I would wager most homes have single zone HVAC. Things
like forced air and hot water are expensive to route into
multiple zones. I don't think I have seen steam heat used
in many modern homes.

I was planning on lobbing a CRC in at least And framing the data. SLIP is
quite a nice way, simple and reasonably bomb proof - used that a lot at my
last job for binary data transmission over radio (well, the electronics
geniuses did, I just had to understand it when it fell out of a TCP stream
and process it on a linux box...)


Indeed. I'm rather used to programming larger computers (PC vs embedded),
but I find a lot of wisdom in higher level protocols can be borrowed,
simplified and put to good use...

Yup. The problems most folks face when moving into an embedded
world are lack of resources (memory, MIPS, etc.), 24/7 operation
(rebooting is not an option) and "Gee, that CAN'T HAPPEN!?"

Agreed. Even the boiler control can be fail safe - cycle it at a reasonable
periodicity and duty cycle. If all the valves are closed, boiler's internal
stat will cut out (safely) and the water will just get pumped around the
mandatory bypass loop so nothing lost.

But, *you* have to figure out what those times and duty
cycles are. Can't turn to a chart on page 47 for the answer.

Simple and modular is good. It's likely that the human bit will have a
240x128 or similar LCD dotmatrix display (cheap and usable) and a few
buttons, or a touch panel. Running that *and* doing comms *and* doing
control logic is probably inviting bugs. Of course it's doable, but making
everything basically one IO blob on one side and a comms link on the other
is probably more rational.

I'm looking into repurposing some "digital photo frames" for
wall mounted displays (they tend to use far less power than
regular LCD monitors -- also much more appropriate sizes!).

Even there, I plan them to just be simple "display devices".
I.e., all the smarts reside in the server and this is just
a "terminal" -- much the same way that the sensor nodes are
just "input devices" and the actuator nodes are "output devices".

This helps keep the displays inexpensive, lets me power them
down when not being used, makes support for multiple "user
interfaces" an inherent part of the design, etc.

The one *must have* would be the ability to reload the firmware over the
comms link, be it radio or wire. If the code is small enough that I can have
2 code bases in flash and an invariant boot section that should be possible
and fairly bullet proof.

That's reasonably easy to do with today's parts.

Fun when something works and/or doesn't blow up/fall off ;->

Yeah, as Buckaroo Bonzai would say: "Don't tug on that -- you
don't know what it's connected to!"

 

[Tim Watts]

Charlie E. <[email protected]>
wibbled on Wednesday 27 January 2010 02:23

Tim,
When you say power, are you talking 24VAC for powering the thermostat,
or full 120VAC. If the latter, it is a big code no-no to put ANYTHING
low voltage in there!

240V mate, in Blighty here...

I know (unless the ELV (<60V DC) has insulation to the standard of the
adjacent LV (240V) then it's permissable from a safety standpoint, but not
desireable from an electronics POV.

But no, the room stats and human interface bits and probably the valve
controllers will be ELV drops, either:

24V if I needed to run motorised actuators anywhere
5V if I don't. Or possibly both, depending on cost of dual PSU vs DC-DC
converters in every module... The PSU will be beefy, fully isolated from
mains and earth and will probably supply a few DC points for phone charging
etc, too (I have way too many 5W wall warts).

Single SELV isolated from everything supply back somewhere.

Even if it is the 24VAC, you are going to couple to it with any
antenna you put in there, and get very 'interesting' results!

Accepted. Most of my backbox drops (this is plaster on solid brick walls, no
sheetrock except upstairs) have 2 x 16mm uPVC oval conduit, so at least the
antenna can go up the other one and be > 1" away.

One or two ended up with 1x 20mm round uPVC tube for logistical reasons (too
much random wood in the ceiling area over the box) - just have to suck that
and see...

If I got desperate enough, I could always score a fine channel in the
plaster and bury a couple of antenna wires in the wall surface next to the
unit - not sure how well that would work...

 

[Tim Watts]

Hi Tim,



I wouldn't even bother trying to track time of day, etc.
Pick some period -- "the last 24 hours" (?) -- and have it track
(and continuously update) the average duty cycle that it was
*commanded* to use over that period. Then, in the absence
of "commands" (for some "significant period"), just have it
reproduce that duty cycle in some <mumblemumble> period
(I don't know the time constants involved in steam heat so
you'd be better at guessing that!)

You aren't trying to work perfectly. Rather, you are
trying to come up with something better than "all on",
"all off" or "whatever I was commanded to do most
recently". If things break while you are "available",
you will *fix* it! (you've heard of the Master-Slave
principle? Well, in this case, *it* is the Master and
*you* are its Slave! :> ) You just want to cover your *ss
in case you *aren't* available.

Wise words - I like that principle.

Yup. With steam, you're kinda stuck on your choices.
E.g., if it was hot water heat, you might have a wider
variety to chose from.

There is some confusion. I never said steam - someone else did ;->

No-one uses steam here expect as a primary distribution round a large site
(eg Hospital, University) and even then it's converted down to HW by local
calorifiers.

Nope - just plain old water at around 160F.

My Building Inspector just about accepted I would be testing my own
electrical work with the correct instrument and filling in the correct forms
for him; he had a mild stroke when he saw I'd declared gas work on the
Building (works) Notice (he agreed it *was* legal for me to do my own (but
illegal for me to do it for others for pay - need to be registered for
that), but they'd never had anyone ever bother to tell them before). I'd
imagine he'd have a full on coronary if I mentioned steam heating ;->

Hope you saw that as tongue in cheek ;->

Yeah, well... we also have INDOOR PLUMBING on this side of the
pond! Fantastic invention! Makes going to the loo on a cold
winter day much more comfortable!

;->

My great aunt had an outside bog. It was grim in winter...

Here, I would wager most homes have single zone HVAC. Things
like forced air and hot water are expensive to route into
multiple zones. I don't think I have seen steam heat used
in many modern homes.

Some use air here. I'd have a problem - so much wood under the floor
upstairs due to a previous roof room conversion, it's very hard for me to
even route one 4" extract duct for the bathroom...

I'm looking into repurposing some "digital photo frames" for
wall mounted displays (they tend to use far less power than
regular LCD monitors -- also much more appropriate sizes!).

Even there, I plan them to just be simple "display devices".
I.e., all the smarts reside in the server and this is just
a "terminal" -- much the same way that the sensor nodes are
just "input devices" and the actuator nodes are "output devices".

This helps keep the displays inexpensive, lets me power them
down when not being used, makes support for multiple "user
interfaces" an inherent part of the design, etc.

That's a very cool idea - I like that.

That's reasonably easy to do with today's parts.


Yeah, as Buckaroo Bonzai would say: "Don't tug on that -- you
don't know what it's connected to!"

And when you have 5 identical copper pipes coming up from the boiler, be
sure you know which one is gas before wielding the pipe cutter ;->

 

[D Yuniskis]

Hi Tim,

There is some confusion. I never said steam - someone else did ;->

Ah, my apologies!

No-one uses steam here expect as a primary distribution round a large site
(eg Hospital, University) and even then it's converted down to HW by local
calorifiers.

Nope - just plain old water at around 160F.

Wow! And you have individual control over each (baseboard) radiator?
Someone spent a wee bit extra on all that bypass pipe! Usually,
all radiators are in a single loop (you have a "loop per room"?)

My Building Inspector just about accepted I would be testing my own
electrical work with the correct instrument and filling in the correct forms
for him; he had a mild stroke when he saw I'd declared gas work on the
Building (works) Notice (he agreed it *was* legal for me to do my own (but
illegal for me to do it for others for pay - need to be registered for
that), but they'd never had anyone ever bother to tell them before). I'd
imagine he'd have a full on coronary if I mentioned steam heating ;->


Hope you saw that as tongue in cheek ;->


;->

My great aunt had an outside bog. It was grim in winter...

Yeah, that sort of thing is *almost* fine when you're a kid
"away at camp". I can recall experiences on a *frozen* lake!
But, I much prefer to be 25C when I crap! :>

Some use air here. I'd have a problem - so much wood under the floor
upstairs due to a previous roof room conversion, it's very hard for me to
even route one 4" extract duct for the bathroom...

Many houses here don't employ return ducts. Just supply.
(wonky system).

That's a very cool idea - I like that.

I like X Terminals. The idea grew out of that. The software
lets me draw on displays (from the server) wherever they may be.

And when you have 5 identical copper pipes coming up from the boiler, be
sure you know which one is gas before wielding the pipe cutter ;->

Yikes! Gas in copper? We use "black pipe" (similar to galvanized
pipe) for gas. Copper *prefered* for potable water (though I have
see PVC and other wonky things used -- poorly!

 

[Tim Watts]

Hi Tim,



Ah, my apologies!


Wow! And you have individual control over each (baseboard) radiator?
Someone spent a wee bit extra on all that bypass pipe! Usually,
all radiators are in a single loop (you have a "loop per room"?)

No - we do parallel feeds for each rad (flow+return). Usually in 8/10/15mm
dropped off a main run of 22mm copper or plastic. So yes, each rad will have
a valve at one end (old=flow control, new=thermostatic) and a flow balancing
valve at the other end.

I'm likely to dispense with the valve at the rad and stick it where the flow
for that rad tees off the main 22mm circuit.

Yeah, that sort of thing is *almost* fine when you're a kid
"away at camp". I can recall experiences on a *frozen* lake!
But, I much prefer to be 25C when I crap! :>

Indeed. I used some proper "dunnies" (wooden hut and box) outside in Latvia
when I toured there. Not bad in summer. Wouldn't fancy it at 0F!

I like X Terminals. The idea grew out of that. The software
lets me draw on displays (from the server) wherever they may be.
Ditto.

Yikes! Gas in copper? We use "black pipe" (similar to galvanized
pipe) for gas. Copper *prefered* for potable water (though I have
see PVC and other wonky things used -- poorly!

Yep. It's fine but not *that* mechanically strong. Fine inside, wouldn't put
it in front of a doorstep (though I met a registered gas fitter who wanted
to).

You can use iron, but no one can be arsed with that now, except the gas
companies for the last leg into the house, especially where it's likely to
get a good kicking, eg in the garage.

Can't use plastic for gas, except in the road (that's standard now for the
gas company).

There is a new thing called TracPipe which is some sort of light corrugated
steel inside a plastic tube which can be hand bend minimising the number of
joints needed.

For water, most people use plastic. For what it is, it's actually very good.
Works at 10 bar (less for hot), push fit demountable fittings that are
compatible with copper and it's incredibly fast to install so all the
plumbers default to it. I've used it for temporary lash ups. Mice can eat it
though. So for proper work, I still prefer soldered or compression fittings
on copper. Little longer to do, should last decades. Again, no one uses iron
any more except industrial, though there's plenty still in use, as well as
lead.

 

[D Yuniskis]

Hi Tim,

No - we do parallel feeds for each rad (flow+return). Usually in 8/10/15mm

OK. So, twice the pipe that would otherwise (e.g., series connected
radiators) be needed. Besides the obvious capability of "local
control" at each radiator, you also don't have the problem of
the "room at the end of the line" getting the least heat! :<

dropped off a main run of 22mm copper or plastic. So yes, each rad will have
a valve at one end (old=flow control, new=thermostatic) and a flow balancing
valve at the other end.

I'm likely to dispense with the valve at the rad and stick it where the flow
for that rad tees off the main 22mm circuit.

But wouldn't that be "buried" (in a wall, below a floor, etc.)?

Indeed. I used some proper "dunnies" (wooden hut and box) outside in Latvia
when I toured there. Not bad in summer. Wouldn't fancy it at 0F!

Note that "not bad" is not the same as "good"! ;-)

Yep. It's fine but not *that* mechanically strong. Fine inside, wouldn't put
it in front of a doorstep (though I met a registered gas fitter who wanted
to).

You can use iron, but no one can be arsed with that now, except the gas
companies for the last leg into the house, especially where it's likely to
get a good kicking, eg in the garage.

Our gas feed was just replaced (I guess they do it periodically
as I have seen trucks methodically digging up people's yards
all around town). The feed *at* the meter is metallic. But,
below grade, some kind of plastic/nylon.

Amusing to watch them "pull" this new gasline *through* the
old one. Have a gizmo that looks like a conical knife.
Attached to a 3/8" (?) diameter steel cable. New gas line
follows the broad end of the cone.

Cable is threaded through the existing gas line (from either the
street end or the meter end, depends on logistics). Then, the
free end is attached to the bucket on a back hoe. Back hoe operator
repeatedly lifts and lowers the bucket (taking up the slack
each time) to *pull* the assembly through the old gas line.
Knife edges on the exterior of the cone slice the old gas line
to shreds as it makes way for the new gas line to follow it.
Incredible labor saver. However, if old gas line is pinched
anywhere (e.g., tree root growing around it) I guess the forces
exerted can snap the cable (connection to the "mule")

Always interesting to see how people solve problems.

Can't use plastic for gas, except in the road (that's standard now for the
gas company).

There is a new thing called TracPipe which is some sort of light corrugated
steel inside a plastic tube which can be hand bend minimising the number of
joints needed.

For water, most people use plastic. For what it is, it's actually very good.
Works at 10 bar (less for hot), push fit demountable fittings that are
compatible with copper and it's incredibly fast to install so all the
plumbers default to it. I've used it for temporary lash ups. Mice can eat it
though. So for proper work, I still prefer soldered or compression fittings
on copper. Little longer to do, should last decades. Again, no one uses iron
any more except industrial, though there's plenty still in use, as well as
lead.

I use 1" galvanize outside for feed lines to irrigation.
Once below grade, I transition to PVC (we have essentially
two grades of PVC here: thin wall and thick wall). Indoors,
potable water is all copper.

 

[Tim Watts]

D Yuniskis <[email protected]>
wibbled on Wednesday 27 January 2010 18:21

OK. So, twice the pipe that would otherwise (e.g., series connected
radiators) be needed. Besides the obvious capability of "local
control" at each radiator, you also don't have the problem of
the "room at the end of the line" getting the least heat! :<

Yes - it leads to the ritual of "balancing the system" where you run round
all the rads (usually several times) and measure the temperature
differential between flow and return then fiddle with the flow restrictor
valve on that rad (that needs a spanner, and is usually capped afterwards).
The tedium of this process is seconded only by the "let's get all the
trapped air out everywhere". Thankfully you only need to do the former once.

There is a variation, where every rad has a pair of 8mm or some other snall
size back to a central manifold (so called "microbore") which solves the
balancing problem more or less. More liable to sludge up, though if the
right chemicals are added, it's not usually a problem.

But wouldn't that be "buried" (in a wall, below a floor, etc.)?

Normally yes, but in my house, it will be in the side storage cupboards off
the attic room so fairly accessible.

Note that "not bad" is not the same as "good"! ;-)

I was surprised. I expected a "certain smell", but what I got was a face
full of ammonia. Quite logical but unexpected!

Always interesting to see how people solve problems.

Yeah - those dudes and the water co's are incredibly inventive when it comes
to not having to dig miles of trench.

I use 1" galvanize outside for feed lines to irrigation.
Once below grade, I transition to PVC (we have essentially
two grades of PVC here: thin wall and thick wall). Indoors,
potable water is all copper.

Presumably you work to 1/2" and 3/4" copper inside? It's always fun working
in an old house here because the copper could be 1/2" or 3/4" or 1" or the
modern 15mm/22mm/28mm that looks absolutely identical. Until you try to make
a compression fitting, then it all goes pear shaped. Personally we should
have stayed imperial for plumbing - all the threaded stuff including taps is
still BSP anyway...

Going to get told off soon for going off-topic ;->