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UHF Remote Dual 2
If you’re looking for a mains “switch” which can be operated from
some distance away, this low-cost, simple project could be just what
you want. With a UHF remote control and two switched “channels”,
it needs very little power thanks to the use of latching relays.
T
here are countless applications – especially with colder
weather coming on – where it
would be nice to turn mains devices
on and off remotely.
Imagine being able to switch something on and off without having to go
close to it – outside in the wind and
rain, for example. Imagine being able
to control two devices, completely
independently.
You’re imagining exactly what this
device does. It has an IEC mains input
connector (so uses a standard IEC
power cable) and two standard 3-pin
mains sockets, into which any mains
devices (up to 2300W total) can be
plugged and controlled.
It’s housed in a standard plastic
case with the only other control an
on/off switch.
The number of channels you construct is optional. The prototype was
made with two channels but if your
application only needs one channel,
you simply leave a relay and a few
other components out.
It’s a true remote “switch” – you
press one button on the pre-built UHF
remote control keyring transmitter to
turn one of the relays on, then press
another to turn it off. The transmitter
has four buttons on it, therefore it can
control two channels.
It has a nominal range of up to
about 80 metres, perhaps more and
this should be more than enough for
most applications. But this range can
be significantly extended with an optional module, which we will look at
a little later.
And speaking of relays, they’re not
your garden-variety types. Each has
a contact rating of 80A – much more
than is available from a standard
power outlet (10A).
But more importantly, they’re
latching relays which only require a
short-term power pulse to turn them
on or off. The advantage of this is that
once actuated, no power is required
to keep the relay “pulled in” – so you
don’t waste a lot of power if something
needs to be left on for a length of time.
We explain how latching relays work
in a separate panel.
Like the transmitter, the UHF receiver module is pre-built, thus avoiding
any setup problems.
How it works
The UHF keyring transmitter has
four pushbuttons, labelled A, B, C
and D. When any of these are pressed,
a pulse train is sent to the receiver
module, it is decoded and the corresponding receiver output will go high.
While the pulse train is sent while ever
the transmitter button is pressed the
The three main
components of our
UHF Mains switch:
(left) the UHF receiver
module, already
attached to the main PC
board and (right) the
UHF transmitter module
with its case behind.
80 Silicon Chip
siliconchip.com.au
30V Power Switch
By Ross Tester
Design by Branco Justic#
receiver circuit only
needs a very short
“high” to actuate the relay.
When the receiver output goes
high, a time-delay circuit comprising a 10μF capacitor and
22kΩ resistor feeds a pulse
of around 50ms to the gate
of the Mosfet connected
to it. This momentarily
turns the Mosfet on, grounding the end of the latching relay to
which it is connected.
Each end of latching relay is also
connected to 12V via a 47Ω 1W resistor, so when a Mosfet is turned on, the
coil is energised.
You might think that a 1W resistor
is not enough in this application but
it hardly raises a sweat, due to the fact
that the turn-on and turn-off pulses
are so short.
Note that a Mosfet is connected to
each end of the latching relay and only
one of the two Mosfets will have any
effect at any given time, depending on
which way the latching relay is set.
If, for example, the latching relay is
in the “off” position and button “A” is
pressed, the Mosfet will enable current
siliconchip.com.au
to flow through
the relay coil and
it will pull the
relay into the “on”
position, where it will
stay. Further pressing of the “A”
button will have no effect.
However, pressing button “B”
turns the Mosfet on connected to
the opposite end of the latching relay,
so current flows through the coil in
the opposite direction. This causes
the relay to switch over to the “off”
position and stay there – and again,
further pressing of button “B” will
have no effect.
Transmitter buttons “C” and “D”
and the second relay operate in exactly
the same way.
The circuit is powered by a halfwave rectifier (D1 and C1) circuit
running from an on-board 9V AC
transformer. This transformer has a
split primary and can therefore be
connected for 230V or 115V operation.
For 230V operation the transformer
primaries are connected in series;
for 115V they would be connected in
parallel.
The circuit can also be powered
from 12V DC via a pair of terminals
or, as we have done, 12V DC can be
taken out from this point to power a
LED (inside the on/off switch).
A second diode (D2) isolates the
12V supply, which powers the latching relays, from the 5V supply, which
powers the UHF receiver module. This
is to ensure that the module always has
enough power to operate even when
the relay coils are actuated.
Normal (standby) current is around
14mA but for the 50ms or so that the
relays are being powered, the current
rises to around 600mA.
One thing we haven’t mentioned
is that the UHF receiver must be
“trained” to recognise the specific
transmitter you are using, otherwise
they won’t work together. We’ll do
this as part of the testing procedure
a little later.
Construction
With the exception of the mains
input/output sockets and the power
switch/LED, all components mount on
a double-sided PC board coded K231A
# Oatley Electronics Pty Ltd
May 2009 81
REG1
78L05, 7805
+5V
ANTENNA
WIRE
173mm
LONG
OUT
IN
GND
D2 1N4001
A
K
1000 F
16V
10 F
+12V
2200 F
16V
A
10 F
22k
Q1-4:
STU432S
MOSFETS
10 F
D
22k
D0
S
47
1W
S
47
1W
Q4
10 F
G
22k
C
B
RLY1*
80A
D
RLY2*
80A
K
7805
A
SC
2009
78L05
IN
GND
OUT
IN
STU432S
D
GND
G
OUT
S
2-CHANNEL UHF MAINS SWITCH
and measuring 79 x 73mm.
The entire project is housed in a
standard all-plastic utility box measuring 95 x 157 x 53mm. It’s a fairly
tight fit in this box but it will all go
in, as our photos show.
As normal, make sure your PC board
is up to scratch – there should be no
shorted or broken tracks. They’re
unlikely these days but it pays to
check.
The first thing to do before starting
construction is to carefully remove
the braided output leads from the two
latching relays. Unfortunately, they
are not insulated nor are the really
long enough to do much with.
We cut them very close to the relay
terminals with sharp sidecutters, then
(later) used the remaining copper
braid as a handy solder point for the
mains wiring.
We also found it necessary to bend
the right-angle terminals back about
45° so the board would easily fit into
the case later on. Take care when cutting these off and bending because
the terminals are relatively easy to
damage.
That done, you can now proceed
82 Silicon Chip
230V AC
INPUT
MAINS OUTLET
1
N
A
E
MAINS OUTLET
2
N
A
#LED1 AND ITS SERIES RESISTOR ARE
INSIDE CASE OF POWER SWITCH S1
1N4001
E
N
LINK B-C
FOR 230V
* BOTH RELAYS ARE
LATCHING TYPES
Q3
G
D1
47
1W
S
D
G
GND
47
1W
S
Q2
22k
A
+12V
D
G
10 F
IEC INPUT
SOCKET
S1#
D
A
Q1
UHF
D2
RX
MODULE
9V
AC
K
#OPTIONAL
D3
A
K
T1
POWER
Rs
#
LED1
#
V+
D1 1N4001
E
Fig.1: the UHF receiver module D0-D3 outputs go
high as buttons A-D on the transmitter module are
pressed. These in turn control Mosfets which can
energise the coils of latching relays RLY1 or RLY2.
If the contacts are closed, they will open
and if open, they will close.
to assemble the PC board. Start with
the resistors and small capacitors
(watch polarity – all the electrolytic
are polarised) then the diodes and 5V
regulator. While the regulator is specified as a 78L05, we used a standard
7805 – either may be supplied in the
kit and both are fine.
The UHF receiver module plugs
into a 9-pin header socket mounted on
the top side of the PC board – solder
the socket in now but don’t plug in
the module yet.
Also solder in the three 2-way
terminal blocks (it snaps together to
form one 6-way). Make sure the wire
connection side goes towards the edge
of the PC board.
Solder the two larger capacitors
(2200μF and 1000μF) and the power
transformer in at the same time. The
two capacitors are a fairly tight fit and
may need a bit of juggling to place
alongside the power transformer. The
transformer will only go in one way
– the primaries towards the edge of
the PC board.
When the transformer is soldered in,
it’s a really good idea to glue a strip of
insulating plastic over the top of the
transformer primary solder connections – just in case!
The last components on this side
of the PC board are the two latching
relays. These have three pins to solder
in – two connect to the coil but one is
for stability only.
You should have only four components left – the Mosfets. These are very
small – in fact, they’re surface-mount
devices but fortunately the spacing is
quite wide so these should present no
problems in soldering.
The close-up photo shows best how
these devices are mounted. Use a clean
hot iron but don’t keep apply heat for
any longer than necessary.
Finally, solder a 173mm length of
thin insulated hookup wire to the
“antenna” position on the UHF module and then plug the module into its
header-pin socket on the PC board.
Training and testing
It’s easiest to check the project
before mounting it in its box – and
it’s quite safe to do so because we
will check it with a 12V DC power
supply. And while we’re about it, we
will “train” the receiver to work with
siliconchip.com.au
# – 4x STU432S MOSFETS
SOLDERED ON UNDERSIDE OF PCB
* --- KST-RX902A UHF RECEIVER
MOUNTED ABOVE TOP SIDE OF PCB
1000 F
+
GND
c
22k
22k
S
G
#
D
D
#
D
5x10 F
S
G
#
G
+
*
+
D
+
C
T1
+
B
REGNAD
LAHTAEL
EGATLOV
A
+
TP
CS
VT
D3
D2
D1
D0
+5V
–
22k
22k
T1 PRIMARY CONNECTIONS
D AND A; LINK B AND C
12VDC
REG1 2x
7805 47
1W
+
+
D1 D2
2200 F
S
G
S
#
D
2x
47
1W
K231A
RLY1
JMX-94F-A-Z
RLY2
JMX-94F-A-Z
Fig.2: the component layout for the double-sided PC board, complete with the pre-built UHF receiver module which
mounts on a header pin socket above the board. Compare this with the same-size photo at right and the completed project
wiring diagram overleaf (Fig.3). The terminals pointing down need to be bent to the left at about 45°.
your transmitter.
Before doing so, however, it’s wise
to give the board a thorough examination, checking for bad solder joints,
misplaced or mis-oriented components, etc. In fact, it’s even better to
have a second person do this for you
because you’re likely to see what you
want to see!
If satisfied everything is correct,
connect a 12V battery or power supply to the upper two terminals on the
terminal block (+ towards the edge of
the PC board).
There is a white pushbutton on the
UHF module – push it and hold it
down until the red LED on the UHF
module goes out. Now press button
“A” on the keyring transmitter until
the red LED flashes.
Your keyring transmitter is now
matched to your receiver.
When you press button A or C on
the transmitter two things should
happen: (1), you should see the red
“acknowledge” LED on the UHF
receiver module flash, and (2) you
should hear a quite distinct “thunk”
from one or other of the relays as it
switches over.
Pressing the B or D buttons should
achieve exactly the same result as the
relay releases.
If you connect a multimeter (low
Ohms range) across one of the relay
contacts, you should be able to confirm
it closes and opens as you press buttons A then B. If it doesn’t, try buttons
C and D – you might be across the
wrong relay!
If everything checks out, you’re
ready to mount the PC board in its
case, along with the input/output
connectors and on/off switch.
If not, you need to go back over
your component placement and soldering.
If the red acknowledge LED lights
when you press a transmitter button
that suggests the power supply is
fine but if you don’t hear the relay
Above is the area of the top side of the
board normally hidden by the UHF
receiver module. This also shows the
row of header pin sockets into which
the UHF receiver module plugs. Note
the regulator (top of pic) is in this case
a 7805 – a 78L05 could also be used.
A close-up of the underside of the same
section of board, showing the mounting
of the four STU 432S Mosfets.
It’s a good idea to glue some heavy
plastic insulation over the mains
terminals of the PC board . . . just in
case. It’s not just leathal, it’s lethal!
siliconchip.com.au
May 2009 83
MAINS OUTLET 1
CASE
12VDC
+
+
+
–
C
ANTENNA
WIRE –
ENSURE FREE END
IS SECURED UNDER
CABLE TIE AND NO
COPPER IS VISIBLE
(USE HEATSHRINK
SLEEVE IF IN DOUBT)
+
D
A
(UHF RX
MODULE)
+
E
RE G NAD
LA HTAEL
E GATL OV
A
B
+
N
+
+
IEC MAINS INPUT PLUG
K231A
c oatleyelectronics
HEATSHRINK
SLEEVING ON
ALL SPADE
CONNECTORS
CABLE TIES
RLY1
LED1
CONNECTIONS
S1
UNDER
POWER ON/OFF
CUT OFF
EXISTING BRAIDED
WIRE ON RELAY
CONNECTIONS AND
USE AS NEW WIRE
SOLDERING POINTS
RLY2
HEATSHRINK
SLEEVING ON
ALL RELAY
CONNECTIONS
MAINS OUTLET 2
Fig.4: follow this wiring diagram exactly – it’s important for your safety. If you
don’t want to use a power switch, run one of the brown wires from the IEC Active
terminal directly to the D terminal on the PC board – and also leave out the 12V wiring to the LED.
thunk, the problem is either in the
time delay R/C network, the Mosfets
or the relay.
If the red acknowledge LED doesn’t
light at all, the problem is in either the
5V regulator section or in the UHF
receiver module itself.
Mounting in the case
We used a 95 x 157 x 53mm (UB1)
ABS case which is available from a
number of suppliers. Ensure you get
the all-plastic variety (including lid),
ie, don’t use one of these cases with
an aluminium lid.
It’s a pretty tight fit in this box but
it does all go in, as our pictures show.
The PC board mounts in the bottom
of the case with the input and output
connectors above it.
Four holes need to be made in the
case. On one end, only a few millimetres down from the case top edge, are
the IEC mains input connector and the
on/off switch with its integral LED.
On each side of the case, at the opposite end to the input, are the 3-pin
mains outlet sockets. These mount
as close as practical to the end of the
case to give as much room as possible
84 Silicon Chip
inside for wiring.
Use photocopies of the cutout diagrams (Fig.5) as templates for drilling
the holes. That’s exactly how we cut
the appropriate sized and shaped
holes – we glued photocopies of the
diagrams to the case, then drilled a
number of fine (say 2-3mm) holes on
the inside of the lines. We then pushed
the middles out and smoothed the
holes with small files.
The 20mm hole for the on/off switch
is round, so this was drilled as large as
possible then enlarged with a tapered
reamer (although the above method
would work just as well).
You’ll also need to drill two 3.5mm
holes alongside the IEC connector for
its mounting screws (use the IEC connector itself to ascertain their position)
and four more in the bottom of the case
for the PC board mounting screws.
The actual PC board position is
quite critical because it must allow
room for the other components inside
the case. It actually mounts under
the sockets, sitting on four nuts to
raise it up enough for the Mosfets
soldered underneath. It also sits hard
up on the edge of the case so that the
relay terminals will fit in (bent back
45°, as mentioned earlier). We used
the PC board itself to carefully mark
the mounting hole positions but as a
guide, if you put the first hole 20mm
from the left (inside) edge and 5mm
down from the case wall, with the
remaining four holes on a 73 x 36mm
rectangle, you should be pretty-well
spot on!
In all cases, the PC board and the
IEC mounting screws are Nylon to
maintain insulation between inside
and outside of the case. The nuts
inside may be either Nylon, steel or
brass. But don’t put the board in the
case just yet – you need to connect
wires to the relay terminals first.
Connecting it up
Start by wiring from the relay terminals back to the input and output
sockets, as these are the hardest to
do. Use 10A, mains-rated (250VAC)
wire as you are switching the Active
power lines. You’ll need one length
around 100mm long and one around
200mm long.
Incidentally, the easiest way to get
such wire is to strip it from a dissiliconchip.com.au
The completed project, ready for
the lid to be screwed on. Note
the generous use of heatshrink
insulation and cable clamps;
also the routing of the
antenna wire as much as
practical away
from the mains wiring.
carded mains
lead!
Bare about
35mm or so of
insulation from
one end of each
wire and wrap each
around the right-hand
terminals of each relay. The
shorter wire goes to the closest
socket.
The wires should be mechanically
secure on the terminals (ie, they won’t
fall off!) before soldering.
Fortunately, the relay terminals are
quite easy to solder to but you will
require a reasonable amount of heat to
adequately solder the wires on.
The other two terminals (bent 45°)
are wired in parallel with another
length of brown mains wire, prepared
and soldered in a similar way.
Ideally, the terminals and wiring
should be insulated – we used a length
of large diameter heatshrink, slit down
the middle, which we wrapped around
the two relays (and their terminals) be-
fore shrinking. It’s not
perfect but its better than
nothing.
While you have the brown mainsrated wire at hand, cut off a short
length (~25mm) and bare 5mm at each
end. Assuming you’re wiring for 230V,
one end is secured in terminal B of the
six-way terminal block and the other
in terminal C.
Make sure no bare strands poke out
of the terminal block.
Put the PC board aside for a moment
while you wire the IEC input and the
mains switch (if fitted) plus the mains
outlets.
The IEC connector and mains
switch need to be fitted to the case
before you connect to them but the
two outlets can be done outside the
case – in fact, they have to be to gain
access to the grub screws.
Follow the wiring diagram exactly,
including the heatshrink insulation
over the various spade connectors.
Start with the IEC connector earth terINPUT PLUG
IEC MAINS
CUTOUT FOR
IEC MAINS
INPUT
CONNECTOR
14
A
5
6
5
9.5
A
5
18
6
A HOLES: 3mm
B
14
33.5
CUTOUT
FOR 3-PIN
MAINS
OUTLET
The end-on shot of the case shows the mounting of the IEC mains input
connector and the on-off switch with its internal LED. Be sure to use Nylon
screws for the IEC connector (as well as for the PC board mounting) to ensure
insulation integrity is maintained. For the same reason, an all-plastic switch is
used.
siliconchip.com.au
10.9
16.75
B HOLES: 4.5mm
Fig.5: same-size cutout details for the
IEC connector (as seen at left) and the
3-pin mains sockets (as seen in above
photo).
May 2009 85
14
Parts List –
UHF Remote Power Switch
1 double-sided PC board, code
K231a, 79 x 73mm*
1 UB1 (157 x 95 x 53mm) ABS utility
case with ABS lid.
1 TX01 UHF receiver/decoder
module*
1 TX9 4-button UHF keyring rollingcode transmitter
2 JMX-94F-A-Z SPST 80A latching
relays*
1 PC-mounting mini mains
transformer, 9V secondary*
3 2-way pc-mounting screw terminal
blocks (forms 1 x 6-way)*
1 9-way male header pin strip*
1 9-way female header pin socket
strip*
1 IEC mains input socket, screwmounting type
1 mains lead with IEC plug
2 surface-mount 3-pin mains outlets
1 250V 1A switch with integral LED
and resistor (optional)
1 500mm length 10A brown mainsrated hookup wire
1 500mm length 10A blue mains-rated
hookup wire
1 500mm length 10A green/yellow
mains-rated hookup wire
1 175mm length hookup wire (for
antenna)
1 50mm length red/black mini figure-8
(or individual red and black –
for LED
6 10mm M3 nylon screws
6 M3 nuts
3 6.4mm crimp-type spade connectors
2 6.4mm piggy-back spade
connectors
2 4.8mm spade connectors
5 mini cable ties
1 piece of rigid plastic, 20 x 30mm, for
PC board insulation
lengths of heatshrink tubing
Semiconductors*
1 7805 or 78L05 Regulator (REG1)
2 IN4001 power diodes (D1,2)
4 STU432S power Mosfets (Q1-Q4)
Capacitors*
1 2200μF 16V electrolytic
1 1000μF 16V electrolytic
5 10μF 10V electrolytic
Resistors*
4 22kΩ 1/4W
4 47Ω 1W
* These components form K321B Kit
86 Silicon Chip
minal – it has two green/yellow earth
wires crimped inside one spade connector. These other end of these two
wires screw into the earth terminals
on the mains outlets.
You will note that we used a couple
of “piggy back” 6.4mm spade connectors on the Active and Neutral IEC
connector terminals. The Neutral has
three blue wires, one screwing into
each of the mains outlets “N” positions
and one terminal “A” on the terminal
block on the PC board. Brown wires
connect the IEC Active terminal to the
on/off switch and to the paralleled
relay terminals.
Another brown wire connects from
the other terminal of the switch to
terminal “D” on the terminal block on
the PC board, while the + and – LED
terminals on the switch connect to the
+ and -12V terminal block positions.
There is a series resistor inside the
switch so the LED can be connected
directly to 12V.
Again, follow the wiring diagram
exactly and you shouldn’t go wrong.
Incidentally, the reason we are specific
about which terminal is wired with
which wire is that connecting the blue
wire (Neutral) to terminal “A” keeps
the brown wire (Active) as far away
from the 12V supply as possible.
Because mains wiring is involved,
all of the spade connectors really
need to be crimped with a ratchet
crimper – the “plier” type of crimper
really doesn’t apply enough pressure
to adequately crimp the cables. If you
don’t have a ratchet crimper, it’s a good
idea to solder the wires to any spade
connectors (as well as crimp them).
Before you get too far down the
track, you will need to insert the PC
board into the case, along with the
two mains outlet sockets, to complete
the wiring.
Dressing the cables
Where mains wiring is involved,
we must assume the worst-case scenario where, somehow, a wire lets go
(eg, it unsolders due to heat, or is not
screwed in properly, etc). This being
the case, we must assure the wire cannot flail around and contact something
it shouldn’t.
Therefore, all of the wiring within
the case needs to be routed along the
edges and fastened together with small
cable ties. Small cable ties are very
cheap ($2 a bag at bargain stores!) so
don’t scrimp on them
There are a couple of handy mounting holes on the mains outlets (which
we don’t use here as the outlets
“sandwich” around the case) which
make handy cable tie anchors – see
the photos.
The UHF receiver requires a short
length (173mm) of hookup wire for its
antenna. Ideally, this wire should also
be mains-rated.
One thing that would concern us
about this wire is if there were any
strands of copper poking out the end.
Just to be on the safe side, we covered
the end of the wire in a short length
of heatshrink and made doubly sure
it was secured properly.
You will note in the photographs
that this antenna wire is also kept
away, as much as possible, from the
mains wiring. This is not just for safety
reasons; keeping the antenna wire
separate will also give the receiver its
maximum sensitivity and therefore
greatest range.
When you are satisfied that the project is wired as shown in our diagrams,
place the lid on the box and screw it
in place.
Now connect the power and turn it
on. The LED inside the switch should
glow, indicating you have 12V – and
when you press “A” or “C” on the
transmitter you should again hear that
“thunk”. Press “B” or “D” to turn it
off, then connect a mains device such
as a lamp or other easy-to-recognise
device to either of the power outlets
and check that you can turn it on and
off via the transmitter.
Finally, remember that turning off
the power switch will not turn off
any device which is being switched
– it stays in its current state until you
switch it with the key transmitter. The
power switch only disconnects power
to the UHF Switch.
Where from, how much?
This project was developed by
Oatley Electronics, who retain
copyright on the design & PC board.
The K231B Kit, which includes the
UHF receiver and all on-board
components sells for $49.00 inc GST
The TX9 transmitter, including keyring case, sells for $16.00 inc GST
Freight is $7.00 per order
web: www.oatleyelectronics.com.au
or (02) 9584 3563
siliconchip.com.au
What is a latching relay?
These shots are of the type of latching
relay used in this project, with the
one on the right removed from its case
so you can see what makes it click!
The two braided leads
welded to the terminals
should be cut off as
they are not used.
This explanation comes from our December 2006 issue but we thought it
would be opportune to repeat it, as a latching relay is not something that you
come across every day. In fact, even those “in the trade” may not understand
the operation nor purpose of a latching relay.
First, a conventional relay operation: this has an electromagnet, formed by
a coil wound on a laminated iron core. While current flows through the coil, a
magnetic field is created which attracts a spring-loaded steel armature towards
the iron core. The armature either pushes or pulls electrical contacts towards
or away from each other, making or breaking a circuit (and in most relays,
both – breaking one circuit then making another). When the current stops, the
magnetic field collapses, so the armature springs back and the contacts revert
to their normal state.
A latching relay is much the same, except that once the armature has switched
over to the opposite position, it will stay there, even when the current through
the coil stops. It will only switch back the other way when told to by the controlling circuit. You could even disconnect the latching relay from the circuit
completely and it would still stay in the last-set position.
A good analogy is a standard switch: you push the lever one way and it stays
there until you push it the other way. The difference is that instead of a finger
pushing or pulling a lever, you have the magnetic field pushing or pulling the
armature. The armature may be held in place by a permanent magnet or it
may be mechanically latched, based on a spring and detent system (which,
incidentally, is how most switches stay in the selected position).
Another analogy is a bistable multivibrator or flipflop – it has two stable
states, neither of which has any pre-eminence over the other.
Latching relays may have two coils – one switching to one position, the
second switching to the other – or it may have a single coil, where the current
is reversed through the coil to switch to the opposite state. This is the type of
latching relay used in this project.
It is a common misconception that latching relays do not consume power
when energised. Although current is not required through the coil to hold the
armature in position, current will still flow if applied, negating the reason for
using a latching relay over a conventional relay. Therefore, a short pulse of
current is normally used to actuate it, just as in this project.
Where conventional relays have “normally open” (NO) and “normally closed”
(NC) positions, latching relays with changeover contacts don’t – because there
is no “normal” position. In our case, the relay is a SPST type so, like a switch,
the contacts are either open or closed (off or on, if you like).
Finally, no relay coil suppression diodes can be used on a single-coil latching
relay because of the polarity reversal. Therefore the voltage rating of any switching transistor (or Mosfet in this case) must be high enough to safely handle the
sp‑ike which occurs when current ceases and the magnetic field collapses.
siliconchip.com.au
Want really
long range
(2km or so!)?
Oatley Electronics have
available a tiny (27 x 20mm)
add-on transmitter module
which is claimed to increase the range of the TX09 transmitter from tens of
metres to kilometres.
It’s the TX-03 module,
which also operates under
Australian LIPD (licenceexempt) regulations.
There are only
three connections required
– data (which
can be taken
from the antenna output),
power (3V or 5V)
and ground. It will operate from
315MHz - 433.92MHz and from -40°
to +80°. Oatley’s RRP is $16.00
The manufacturer of the TX-03
states that the transmit power is
15dBm, which equates to 32mW.
Presumably this is at the upper end
of the specified operating voltage
range (3-12V).
The maximum legal output power
of LIPD devices in the 433MHz band
is 25mW, so (again presumably) the
transmitter would need to be operated at the lower end of the supply
voltage range to remain legal. Indeed,
Oatley Electronics warn that operating at 5V may exceeed the legal limit.
Therefore, we suggest operating only
from 3V. Oatley claim a range of
2km+ at 3V and 4km+ at 5V.
Naturally, the TX-03 module will
not fit inside the keychain transmitter case so you will have to make
other arrangements to mount it and
also power it. The telescopic whip
antenna can be unsoldered from the
TX-01 PC board and a short wire
used to connect that point to the
“Data” input on the TX-03. While the
TX-01 has a 12V battery, using this
would result in too much transmitter
power, as described above.
Unfortunately, despite extensive
searching, we have been unable to
obtain any further specifications for
the Chinese-made TX-03.
SC
May 2009 87
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