This is only a preview of the August 1988 issue of Silicon Chip. You can view 40 of the 96 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Articles in this series:
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By JOHN CLARKE & GREG SWAIN
Crook in bed with a bad back, mate?
Want someone to bring you a nice hot
cup of tea? Don't shout - just press the
button on a small keyring transmitter to
attract attention.
The traditional way for someone
confined to bed to attract attention
is to shout at the top of his (or her)
lungs. But when one of our staff
members (who had best remain
nameless) tried that recently, there
was no response. His pathetic
whimperings were muffled by two
sets of closed doors and by a TV set
going full-bore in the family room.
That started him thinking (and
not before time). What was needed
was a unit that would sound a bell
or a chime in the family room in
response to the press of a button on
a small transmitter in the bedroom.
Aha! Why not base the circuit on
56
SILICON CHIP
the UHF Remote Alarm Switch
described in the March issue for
switching car burglar alarms on
and off?
And that's exactly what we've
done. We simply stripped the
Remote Alarm Switch receiver of
any unnecessary circuitry and married it to a two-tone chime IC, the
SAB0602. So, whenever you press
the button on a small keyring
transmitter, the receiver picks up
the signal and sounds a pleasant
two-tone chime.
For want of a better name, we've
called the finished project the
Remote Alert. It works like a
charm. And it eliminates enraged
yells, fist banging on walls, and the
aforesaid pathetic, self-pitying
whimperings.
Another ideal use for this project
is as a wireless doorbell. In this version, the transmitter PCB can be
housed in a small plastic project
box, together with a 9V battery and
a pushbutton switch. Depending on
circumstances, the completed unit
should have a range of 12 metres or
more.
By the way, you could have as
many "bell pushes" as you wanted.
Just build as many transmitters as
necessary (one for each door?) and
code them all exactly the same.
To keep costs low and simplify
construction, we've designed a new
printed circuit board for the
receiver. This PCB leaves out any
unnecessary circuitry from before
and includes the two-tone chime IC.
No changes are necessary for the
transmitter circuit so we have
simply reused the original PCB.
Transmitter
Fig.1 shows the transmitter circuit. It consists of a digital encoder
integrated circuit (IC1) and a UHF
oscillator operating at 304MHz.
ICl is a Motorola MC145026 9-bit
trinary encoder. Trinary code is
like binary code except that it uses
three logic states instead of two (ie;
high, low or open-circuit). Only one
transmitter code word (one 9-bit
word) is possible, as selected by
connections to the IC's nine address
inputs, Al fo A9.
In keeping with the trinary states
just mentioned, each of the nine address inputs can be connected to
the + 12V rail, to ground or left
open circuit. In our application,
because of the particular decoder
used in the receiver, the A9 input
must either be connected to the
+ 12V rail or to ground.
The 9-bit code word appears as a
series of pulses on pin 15 of ICl.
The frequency of these pulses is set
by the two resistors and the
capacitor connected to pins 11, 12
and 13. For our circuit, the frequency is about 2kHz.
Pushing the Transmit button (St)
causes the IC to deliver the coded
word from pin 15. This is used to
key the UHF oscillator Qt on and
off at a rate of 2kHz. When pin 15 is
high ( + 12V), Qt oscillates.
Qt is a BFR91A which is a surface mounting transistor intended
for use in UHF and microwave
amplifiers. Inductor 11 and the
2-6pF capacitor form a tuned circuit load for the collector while the
base is grounded (to AC signals) by
a 470pF capacitor. Stray capacitance between the emitter and
collector of Qt provides positive
feedback which causes it to
oscillate at 304MHz.
To increase the oscillator's output, the emitter deger ,: rntion
resistor is bypassed with J 1.5pF
capacitor, which is critical in value.
The value of this capacitor cannot
be too large because this would
reduce the positive feedback and
stop oscillation.
The transmitter is powered from
a 12V lighter battery (VR22, EL12,
GP23 or equivalent), but will work
from voltages down to as low as
4.5V (a 9V battery is used in the
doorbell transmitter version). The
When you press the button on the small keyring transmitter, the receiver picks
up the signal and sounds a pleasant two-tone chime. You can also build the
transmitter into a larger case for use as a doorbell.
+
LOW
14
IC1
MC145026
13
12
TE11
...
100k
.0022
+
220k
E
TYPE
MARKING
8
...
L1 : 32mm OF 0.71mm TINNED COPPER WIRE FOR SMALL VERSION.
LARGER VERSION USES PCB TRACKS.
toN LARGE VERSION ONLY
UHF REMOTE TRANSMITTER
SC03-1-288
Fig.1: the transmitter uses an MC145026 trinary encoder IC to key UHF
oscillator Qt on and off. A1-A9 are connected to give the address code
(see text).
battery is bypassed by a 0.1µ:F
capacitor located near IC1 and by a
0.047µF capacitor near the tuned
circuit for Ql.
When S1 is closed the current
drawn by the circuit is a few
milliamps, the exact figure depending on the code word selected at
Al to A9. The current through LED
1 is abou ~ 7mA. When St is open,
AUGUST 1988
57
4,70
r - - - -.....- ......--~-'Wl,,,.....--------~~----.----.....---+sv
ANTENNA
L4
L1
.,..
.,.
12VDC +
PLUG-PACK
47
+
100 +
16VW+
16
-_i 16VWL
,.
.
1
INPUT
A1
2 A2
3
220
A3
OUTPU
4
5
A4
11
IC2
MC145028
A5
4
39k
7
10
12 A9
.,.
DECODING
OPTIONS
~SPEAKER
L1 : 15T OF 0.63mm ENAMELLED COPPER WIRE
ON 3.2mm DIA. FORMER
L2 : 1.5T OF 0. 71 mm TINNED COPPER WIRE
ON 5mm DIA. F29 FERRITE CORE
L4 : BT OF 0.63mm ENAMELLED COPPER WIRE
ON 3.2mm DIA. FORMER
13 AB
SC03-1-DBBB-1
8
1sv~ so
.0068t
.022+
REMOTE ALERT
l
0 33
·
6
15 A6
14
IC3
SAB0602
,.
,.
0.1+
Fig.2: the receiver circuit can be divided into four sections: an RF input amplifier and detector (Ql, Q2), a
tuned 2kHz amplifier (ICl), trinary decoder IC2, and a chime generator circuit (IC3). Power comes from a 12V
DC plugpack transformer.
the current drain is less than
0.2µA.
The transmitter can be built into
one of two cases, one of which is
small enough to be attached to a
key ring. We'll talk about these
later in the section on construction.
Receiver
The receiver circuit is shown in
Fig.2 and is identical to the circuit
used for the Remote Alarm Switch
as far as the output (pin 11) of ICZ.
It consists of four sections: an RF input amplifier and detector (Qt and
QZ), a tuned ZkHz amplifier (IC1),
trinary decoder IC2 , and the chime
generator circuit (IC3).
The transmitted signal is picked
up by the antenna which is loaded
58
SILICON CHIP
by inductor 11. The signal is then
coupled via a .001µF capacitor to
the base of Qt , which is an RF
amplifier with a tuned collector
load.
Signal from the collector of Qt is
fed via a voltage divider consisting
of a 2.7pF and a 22pF capacitor to
self-oscillating detector stage QZ .
Whenever signal is received, this
stage oscillates at 304MHz. When
no signal is received, the circuit is
quiescent.
The detected signal from QZ appears across the .001µF capacitor
connected to its base. This
capacitor bypasses the 304MHz
signal but not the ZkHz pulse
modulation which is superimposed
on the signal fed to the antenna.
This ZkHz pulse signal is coupled
via 14 and a 2.ZµF capacitor to
ICla, an inverting op amp with a
gain of about 470. L4 is there to
keep RF energy out of this op amp
stage.
ICtb is a Schmitt trigger. It
squares up the amplified signal
from ICla before feeding it to ICZ,
the trinary decoder.
ICZ is an MC145028 decoder
which is compatible with the
MC145026 used in the transmitter
circuit. It is set up to respond only
to the unique code word sent by the
transmitter. This is done by connecting the address inputs Al to A9
in exactly the same way as for IC1 .
in the transmitter.
When IC2 detects a correct code
1=' -~,:~®:~~
T ~/0.,
{
~
.0470
1Al-H1G_"- -~
~--Lo_w
A7•
•
•A6 A9
AS•
•A4
A3•
eA2
<<at>)t2·6pF
~ctlOOk
Al•
~i220k
+
Fig.3: parts layout for
transmitter PCB. Substitute a
battery snap connector for the
battery clips and leave out the
LED and PC-mounting switch if
you are building the doorbell
version.
Fig.4: to code the transmitter
each A1-A8 input is connected
to the high rail, the low rail, or
left open circuit. A9 must he
connected high or low; it
cannot be left open circuit.
The transmitter PCB fits neatly into
the small keyring case which is
available from Dick Smith Electronics.
Power comes from a 12V lighter
battery.
Fig.5: mount the parts on the transmitter PCB as shown here. The
RF transistors (Qt & Q2) are installed from the copper side of the
PCB, with the part number visible from the component side.
Winding details for the coils are shown on Figs.2 & 6.
from the transmitter, the output at
pin 11 goes high. This triggers the
two-tone chime IC (SAB0602) which
produces a rich, heavily modulated
chime sound. Note that the chime IC
only sounds once and will not
retrigger if the transmitter button is
held down (ie, if pin 11 of IC2 stays
high).
IC3 drives an 80 loudspeaker via
a 330µF capacitor. The O.lµF
capacitor and 33k0 resistor on pins
6, 7 & 8 set the chime tone frequency, while the 0.33µF capacitor on
pin 3 provides a degree of high frequency filtering.
Power for the circuit is derived
from a 12V DC plugpack transformer . This feeds a 7808
3-terminal regulator and several
filter capacitors to produce a
regulated + 8V supply. This supply
regulation is necessary to prevent
false triggering.
Transmitter construction
Construction of the transmitter
mainly involves the assembly of a
small printed circuit board. This
board measures 46 x 33m and is
coded SC0312882. Fig.3 shows
where all the parts go.
The keyring transmitter case
r
L2 FORMATION
DIMENSIONS IN MILLIMETRES
Fig.6: L2 is made by winding
0.71mm tinned copper wire. on a
5mm plastic former. The former
is fitted with an F29 ferrite core
for tuning.
from DSE is supplied complete with
PC-mounting battery clips. If you intend installing the PCB in the larger
case for use as a doorbell, delete
the battery clips and wire in a battery snap connector instead.
You must install the IC with pin 1
towards the battery clip side of the
PCB as shown. The RF transistor
(Ql) is mounted on the copper side
of the PCB (see Fig.1 for pinouts).
The body of the transistor fits into a
hole in the PCB with the type marking uppermost and the leads then
soldered to the copper tracks
(which should be pre-tinned).
Note that some of the resistors
are mounted flat on the PCB while
others are mounted end on as
shown in Fig.3. All the capacitors
are mounted flush against the PCB
except the .0022µF greencap which
is bent to lie flat over the top of the
IC (see photo).
Pay particular attention when installing the switch (S1). It must be
installed so that the flat side of the
switch body is towards the battery
terminal end of the PCB. The LED
must be mounted so that the top of
its body is about 13mm above the
PCB surface.
Delete both the LED and the PCmounting switch for the doorbell
version in the zippy case. In this
AUGUST 1988
59
r:
0
1--
a:
0
w
0
...J
<(
0
0
w
0
1--
0
0
0
~
w
0
L..:
.:J
o]
0
SC03 -1-288-2
The front panel and printed circuit board
artworks are shown here for the
convenience of those who prefer to make
their own. Alternatively, you can buy
ready made boards and panels from the
usual suppliers (see page 96).
version, the PCB is soldered to a
panel-mounting pushbutton switch
via two short lengths of tinned copper wire. It will be necessary to
angle the board as shown in one of
the photographs so that there will
be enough .room to mount the
battery.
Receiver construction
The UHF receiver and chime cir60
SILICON CHIP
cuits are combined on a single PCB
coded 03107881 and measuring 87
x 8 7mm. This is designed to clip into
a plastic utility case measuring 159
x 96 x 51mm.
Fig.5 shows the parts layout on
the receiver PCB. Begin construction by installing all the low profile
components such as the resistors,
ICs and the wire link adjacent to
IC2. When these have been install-
ed, you can mount the capacitors
and the 3-terminal regulator.
The BFR91 transistors (Ql and
Q2) are mounted on the underside
of the PCB. As in the transmitter,
the bodies of these RF transistors
fit into holes, with the part number
visible from the component side of
the board. You will find it easier to
solder the leads if you tin the copper tracks first.
Transmitter
1 transmitter case (DSE Cat.
H-2497, 31 x 58 x 17mm)
1 PCB, code SC0312882,
46 x 33mm
1 PC-mounting pushbutton
switch, DSE Cat. S-1200
1 3mm LED
1 12V lighter battery (VR22,
EL 12, GP23 or equivalent)
Semiconductors
1 BFR91 NPN UHF transistor
1 MC145026,SC41342
trinary encoder
Capacitors
1
1
1
1
1
1
0.1 µF miniature polyester
.04 7 µF ceramic
.0022µF metallised polyester
4 70pF ceramic
1.5pF ceramic
2-6pF ceramic trimmer
Resistors (0.25W, 5%)
1 X 220k0, 1 X 100k0, 1 X
4 7k0, 1 x 1 OkO, 1 x 1 .5k0, 1 x
1 kO
PARTS LIST
Receiver
1 plastic utility case, 158 x 96
x 54mm
1 PCB, code SC03107881,
87 x 87mm
1 mini 80 loudspeaker
1 1 2V DC plugpack supply
Semiconductors
1 MC145028,SC41344
trinary decoder
1 SAB0602 dual-tone chime
1 LF353, TL072 dual FET
input op amp
1 7808 8V 3-terminal regulator
2 BFR91 NPN UHF transistors
Capacitors
1
1
1
2
1
1
3
1
1
220µF 16VW PC electrolytic
1 OOµF 16VW PC electrolytic
4 7 µF 1 6VW PC electrolytic
1 OµF 1 6VW PC electrolytic
2.2µF 16VW PC electrolytic
0 .33µF metallised polyester
0. 1µF metallised polyester
.022µF metallised polyester
.01 µF ceramic
1
4
1
1
1
1
.0068µF metallised polyester
.001 µF ceramic
22pF ceramic
3.3pF ceramic
2. 7pF ceramic
2.2pF ceramic
Inductors and wires
L 1 190mm 0.62mm enamelled
copper wire
L2 65mm 0.71mm tinned
copper wire, 5mm former
DSE Cat. L 1010, F29 ferrite
screw core
L3 3.3µH RF choke
L4 100mm 0.62mm enamelled
copper wire
1 300mm-length of 1mm solid
core insulated wire (for
antenna)
Resistors (0.25W, 5%)
2 X 4.7MQ, 1 X 470k0, 1 X
180k0, 1 x 150k0, 1 x 1 OOkO, 2
X 47k0, 1 X 39k0, 1 X 33k0, 1 X
22k0, 1 x 18k0, 5 x 10k0, 1 x
1 kO, 1 X 4 700, 1 X 2700, 1 X
4.70
11 is made by winding a 190mm
length of 0.63mm enamelled copper
wire around a 3.2mm (1/8-inch) drill
bit. Wind on 15 turns and strip the
insulation from the ends with a
sharp knife before soldering the
coil to the PCB.
14 is wound in exactly the same
fashion as 11 but requires only 8
turns.
12 is wound on a 5mm plastic
former, the base of which is pushfitted into a hole in the PCB. The
winding details are shown in Fig.6.
Don't forget to screw in the F29 ferrite core. 13 is a standard 3.3µH inductor that can be purchased ready
wound (eg, DSE Cat. 1-1765).
The antenna is simply a 300mm
length of single-core hookup wire
soldered to the antenna input pad
on the board (see Fig.5).
Testing and alignment
Both the transmitter and receiver
must be coded before they can be
tested.
Initially, to allow testing, we
recommend that only the A9 input
on both the transmitter and
receiver be coded. This input must
be connected to either the high or
The receiver PCB clips into slots in the side of the standard plastic case. For
best range, you should make the antenna about 300mm long.
low rails (ie, it must not be left open
circuit). The Al-A8 inputs can all
be left open circuit for the time
being.
The transmitter frequency can
now be set to 304MHz by using a
frequency meter. Temporarily connect pin 15 of IC1 to the positive
AUGUST 1988
61
111r . . .
·. · . ·
·~- - -[ir
For the doorbell version, the transmitter PCB is soldered
to the pushbutton switch on the case lid via two short
lengths of tinned copper wire. Angle the PCB as shown to
allow room for the battery.
(ie, for maximum reading on the
DMM). You may need to move the
transmitter away from the receiver
to obtain the setting for maximum
sensitivity.
If everything is working correctly, the chimes should now sound
whenever the transmitter button is
pressed. In fact, you may wish to
disconnect one of the speaker leads
during the adjustment procedure to
prevent the chimes from continually sounding.
When the correct setting has
been found, drip some molten candle wax into the screw core of 12.
This will prevent it from moving
and thereby detuning the receiver.
rail. This will set the oscillator in
operation. Now hold the transmitter near the input of the frequency
meter and adjust the trimmer
capacitor for a reading of 304MHz.
In some cases it may be necessary to connect a coil of wire between the input and ground of the
frequency meter to obtain a
satisfactory reading.
Once the frequency has been set,
remove the temporary connection
to pin 15.
Now connect the receiver to a
12V power supply and check that
the output of the regulator is at
+ BV. If everything is OK, switch
off and connect a multimeter set to
read DC volts between pin 7 of IC1
and ground.
Apply power and wait 10
seconds for the 2.2µF capacitor on
the base of Q2 to charge. Adjust the
slug in 12 for maximum signal when
the transmitter switch is pressed
Coding
Because this project is not being
used in a security application,
there's really no need to worry too
much about working out an
·elaborate security code. The main
eA1
eA2
eA3
eA4
A&•
A7•
AB•
A9•
With this doorbell, you don't have to run connecting
wires between the bell push and the chimes unit.
Because the current drain is so low, the battery should
last for its normal shelf-life.
I
eA5
LOW
Fig.7: connect the Al-A9 receiver inputs to exactly match the transmitter
code. Al·A8 can be high, low or open circuit; A9 must be tied high or low.
62
SILICON CHIP
thing to watch out for here is that
both the transmitter and receiver
codes are made identical.
For example, you may wish to
code only the Al, A2, A3, A4 and
A9 inputs (A9 must be tied high or
low as mentioned above). The remaining A5-A8 inputs could all be
left open circuit.
Coding is achieved simply by connecting each A1-A8 input to the
high rail or to the low rail, or by
leaving it open circuit. For example,
you could bridge Al to the high rail,
A2 to the low rail, leave A3 open
circuit, bridge A4 high and so on.
Figs.4 and 7 show the Al to A9
code inputs on the copper side of
the PCB for the transmitter and
receiver respectively. It's a good
idea to write your selected code
down on a piece of paper before actually making the necessary
connections.
Once coding has been completed,
the receiver PCB an be clipped into
the plastic case and the wiring to
the speaker and plugpack transformer installed as shown in Fig.5.
You will have to drill holes in the
side of the case to allow access for
the supply leads and for the antenna wire.
The loudspeaker is affixed to the
lid of the case using an epoxy
adhesive (eg, Araldite). Before
mounting the speaker, attach the
adhesive label to the lid and drill a
4mm hole at each marked location
to let the sound out. Deburr the
holes using an oversize drill.
~
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