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433MHz + Picaxe = Magic!
You’ve no doubt heard of Murphy’s, Ohm’s and Moore’s
Laws . . . but how about Swan’s Law – “You can never have
too many thermometers”?
by Stan Swan*
H
ere’s a Picaxe-controlled
wireless version that should
suit many needs and YES! –
it’s legal, as the only Australian/NZ
433.92MHz LIPD ISM regulatory restriction is that the transmitter should
not exceed 25mW EIRP (Effective
Isotropic Radiated Power).
Since Picaxe-08M microcontrollers
work so well with 433MHz UHF data
modules (see last month), it’s tempting
to further link the pair to industrystandard DS18B20 Dallas Semiconductor (Maxim) digital temperature
ICs and make a simple Picaxe-08M
driven wireless thermometer.
Direct Celsius temperature data can
be then transmitted some 50 metres
and shown on a PC attached to the
433MHz receiver (with perhaps further treatment under Excel).
A simple antenna extends this range
to more like 300m, while a Picaxecontrolled data repeater can even push
ranges to perhaps 500m and may allow
coverage when obstacles otherwise
block weak signals.
What’s involved
As initially mentioned in the December 2005 SILICON CHIP article,
we’ve now migrated to the Mk.2
PICNIK box layout approach (see the
The first of Stan’s breadboard circuits for this month: both use the wireless techniques explained last month but now
they’ve taken on Picaxe control. Aaaaah – Stan’s two loves in one circuit? He’s in rapture . . .
98 Silicon Chip
siliconchip.com.au
The DS18B20 digital temperature IC
is, confusingly, a look-alike to cheap
BC547 transistors and mixups may
arise unless it’s boldly marked – here
whiteout and a red felt tip dot has
been used to avoid any possible circuit
confusion.
As outlined last month, the various
LIPD modules are usually pin-for-pin
compatible, so most of the common
433.92MHz transmitters can be used,
although the antenna position may
vary on some. One I found even had
its antenna pre-wound and bonded to
the module.
The receiver assembly may need
more consideration, since the Jaycar
version needs a nominal 5V supply
and may be less tolerant of three 1.5V
AAs (ie, 4.5V) for the supply unless the
cells are fresh. Consider perhaps four
NiCd/NiMHs (4 x 1.2 = 4.8V) instead,
or even 4 x 1.5V cells (thus 6V) and a
series silicon diode to drop that back
to around 5.4V (as outlined later).
The Mk.2 PICNIK box has room for
either a three or four AA-cell switched
battery box anyway
Once the Tx and Rx boards are assembled and powered up, simply port
over the correct code (www.picaxe.
orcon.net.nz/434tx.bas and www.
picaxe.orcon.net.nz/434rx.bas) from
the Picaxe Editor PC to the matching
setup.
Following the energy saving SLEEP
command (initially set to about one
minute – modify to suit), the DS18B20
Here’s the Picaxecontrolled wireless
thermometer – the
circuit at top and
the breadboard
layout at right.
There are subtle
differences
between this
layout and the
photo at left –
neither is “wrong”
but the one at right
is a little easier to
follow in printed
form.
www.picaxe.orcon.net.nz/picnik2.gif
“slide show”), which conveniently
has room enough for a Picaxe-08M
and the 434MHz Tx/Rx units. It’s again
strongly recommended that you first
lay out the circuit on such solderless
breadboards (as we’ve shown here), allowing things to be better understood
and tweaked. The final soldered versions should be the last stage in your
design– not the first!
However, for eager constructors
“more confident in their abilities”
and wanting to build just the final
version, it’s suggested that Dick Smith
Prototype Board (DSE H-5605) be
used. Its 0.1-inch-spaced solder pads
siliconchip.com.au
are laid out exactly the same as the
breadboards, allowing almost a “paint
by number” approach to board stuffing. Rather than soldering the Picaxe
and 433 units directly onto this board,
use 8-pin IC sockets. The same can be
said for the transmitter and receiver
modules – cut the IC sockets in half
lengthways.
The DS18B20 can of course be
extended away from the board with
three wires but ensure their solder
joints are waterproofed with epoxy or
neutral silicone sealant for measurements in damp areas. See a possible
approach at www.picaxe.orcon.net.
nz/pvdemo.jpg
The alternative construction methods:
our familiar breadboard and above
it, a DSE Prototype Board. It’s easy to
transfer circuits from one to another
because both use the grid system.
January 2006 99
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100 Silicon Chip
4007
Here’s the
matching receiver module
– again, there
are differences
between this and
the photos.
Virtually any of
the commonly
available (and
cheap!) 433MHz
wireless modules
can be used in this
circuit as most are
pin-for-pin interchangeable.
will power up to read the temperature, which is then transmitted as a
variable (b1) before shut down again.
A red LED winks to indicate outgoing data, which has been reduced in
speed to just 300 bps for reliability.
There’s little point in sending faster
when the unit will spend considerable
time idling between readings and the
Picaxes could even be under-clocked
to further slow data rates if superior
reception is needed – this may also
prolong battery life.
At the receiver (if in range) the unit
first has to be given a preamble to
ensure it’s listening carefully – experimentation showed that a good burst
of ASCII 85s (“U” being 01010101)
ensured it was suitably responsive.
A further “ABC” qualifier is then
added to the transmitted serial string,
with a similar sequence at the receiver,
to ensure that data will only be re-
sponded to if this preceding ABC is
present. Naturally, with numerous
wireless garage door openers, door
bells and the like now abounding, you
don’t want false triggering every time
the place next door has visitors – or
vice versa.
There’s a parallel here with WW2
coded BBC messages of course – only
if a pre-determined phrase such “My
hovercraft is full of eels” was broadcast would the listening partisan
group blow up the rail bridge, etc.
Being 2006 rather than 1945, instead
of bridges the alerted SERIN command
takes the b1 temperature variable and
directs it via the Picaxe programming
cable to the PC for editor “F8” 4800bps
terminal window display.
Other readouts, perhaps an LCD
module or old organiser suitably
driven by SEROUT, could easily be
used instead.
siliconchip.com.au
idle capacity to “store and forward”
the temperature data.
The technique is akin to LEO (Low
Earth Orbital) “flying mailbox” satellites which take in weak ground
signals, when over a remote area, for
resending as they pass over a base station perhaps 20 minutes later.
When placed in an elevated RF
sweet spot (and perhaps solar-powered), enhanced signal broadcasting
results, allowing data gathering from
areas that may otherwise be UHF black
holes – a cave or well perhaps.
The small 230 hole (+ 40 supply
Put a receiver
and transmitter
module together
and what do you
get? A repeater, of
course! The code
for the Picaxe
control can be
found on Stan’s
website (address
at end of this
article).
With the use of the Picaxe WRITE
and READ commands, quite a stack
of these variables could be stashed in
EEPROM for later retrieval as well of
course, effectively making a wireless
temperature data logger.
Every school should make one to
explore and experiment !
A simple quarter-wave antenna
(~165mm at 433-4MHz), perhaps
spiraled somewhat for compactness,
should give a range through wooden
walls of about 50m.
For coverage beyond this, consider
antennas such as the Yagi “cotanga”
or magnetic pickup version described
last month (www.picaxe.orcon.net.nz/
yagi433.jpg) and if used at both ends
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perhaps 300m range may result.
In situations where the transmitter
signal is well shielded from the receiver behind metalwork, buildings,
hills or extensive vegetation you’ll
need a bit more ingenuity.
A repeater!
Taraaaa! You saw it here first – a
dead simple but effective Picaxe controlled 433MHz data repeater.
The Picaxe driving code is a breeze,
but keep in mind it’s only set up to
“store and forward “ a single variable,
so don’t expect WiFi bandwidth!
Since the baby 08M has spare I/O
channels and memory, it was tempting
(and indeed proved feasible) to use its
And here’s the breadboard layout of
the repeater. The long black object is
also an antenna – just a different type
than our curly wire version.
January 2006 101
rail) breadboards we used nicely fit
both a 433 Rx and Tx module alongside the Picaxe, and following simple
hookup wiring the repeater can be
programmed with www.picaxe.orcon.
net.nz/434rpt.bas
To show its action, green and red
LEDs (for awaiting receiving then retransmitting) connect via 1kW dropping resistors – much larger than really
needed but reducing battery drain to
just a few milliamps.
433MHz transmitters only come on
when data is fed to them but naturally
the sensitive receiver must be switched
off before the transmitter comes on,
otherwise it will be overloaded. Such
needs can again be easily handled via
our Picaxe, since each output has the
ability to provide (“source”) ~ 20mA
current when high.
Somewhat annoyingly, the Picaxe
SERIN command can’t be interrupted
or timed out but completely stops
processing until a suitable signal arrives, meaning the receiver can only
be switched off (and the program able
to continue) after such prescribed data
is received.
There’s a parallel here with fishing
and the discard of any less worthy
catches, as you’ll only go home when
a desirable barramundi (?) is in the
bag.
Note: the temperature data handling here is a simplex in nature, and
similar to a radio station sending out
programs. Extensive data massaging,
using CRC error detection or even half
duplex confirmation is rather beyond
this initial article so has not been
considered, although is mentioned in
the references.
If the receiver is close to both the
sender and the repeater, a double set
of data will show up on the screen as
the two signals are received.
Although you’ll obviously not need
the resending in such a strong signal
arrangement, normally position the
repeater where it can just reliably hear
the sender and the receiver can further
hear the repeater’s outgoing signal.
Perhaps initially reduce the SLEEP
to just a few seconds to speed up the
process, as the informative switching LED patterns will greatly help
positioning.
Solar power?
With hardware and gift/bargain
stores now displaying racks of solar
powered garden lamps at near throw102 Silicon Chip
Just to prove the point, here’s a version of the circuits on the DSE prototype
board. Ignore the 2 extra LEDs in the repeater circuit. Note the IC sockets
supporting the Rx module – they can also be trimmed for the programming lead.
away prices (often under A$5 each),
it’s tempting to power our modules
from the sun via parts salvaged from
such lamps.
Since each lamp usually has an
epoxy-covered four-wafer PC cell (delivering ~2V at 30mA) and a 600mAh
NiCd, a 3-PV array will be sufficient to
drive a module (probably the repeater)
and charge four NiCds.
Average current demands of the
Picaxe controlled units are around
10mA (much less when sleeping),
meaning ~eight hours of daylight will
be sufficient to run a setup and keep
the batteries at full charge.
To avoid oversupplying the Picaxe08M (which normally needs under 6V)
and prevent battery discharge via the
panel at night, a blocking diode should
also be fitted.
Although cheap, silicon diodes
waste 0.6V but conveniently the solar
garden lamps again come to the party
and provide a superior Schottky version (1N5817 etc) which drops only
0.2V.
Amazingly for the lamp price, further useful parts like an ultra-bright
white LED lurk in the device for later
projects – how can these things be
made so cheaply?
Footnote for sunbelt regions: just
as the photovoltaic (PV) panels need
sunlight, you need to ensure that the
repeater electronics aren’t cooked by
strong sun. It can happen!
Don’t mount the repeater in too
inconvenient a place either, as you’ll
no doubt need to access it for software
upgrades and occasional dirt removal
from the panels.
Birds naturally appreciate elevated
roosts but their droppings (especially
from seagulls) may be the weak point
in a pico PV-powered system like
this!
References:
For convenience these are hosted,
along with mentioned URLs and project software, at www.picaxe.orcon.
SC
net.nz/434rpt.htm
* s.t.swan<at>massey.ac.nz
siliconchip.com.au
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