This is only a preview of the November 1994 issue of Silicon Chip. You can view 29 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. Items relevant to "A Novel Alphanumeric Clock":
Items relevant to "80-Metre DSB Amateur Transmitter":
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|
Keep tabs on your car or boat
with this
UHF RADIO
ALARM PAGER
This UHF alarm pager is ideal for keeping tabs
on a boat that’s moored near your home, or on a
car parked in your driveway or in a nearby carpark. When triggered, it transmits a signal that
activates a buzzer in a small receiver unit.
By BRANCO JUSTIC
Car and boat theft is a common
problem but unfortunately conventional alarm systems are not always
the complete answer. You don’t have
to be too far away from the vehicle to
be out of earshot and, of course, most
people ignore alarms due to the high
incidence of false triggering.
That’s the main problem with conventional alarms. Despite the fact that
the car (or boat) is not far away, it’s
quite possible to miss the alarm if it
goes off. This particularly applies if
the car is parked in the street and you
live at the back of a block of units, or
36 Silicon Chip
if you visit an office block or shopping
centre and the car is in an adjacent
carpark.
This unit overcomes that problem by
paging you if it detects an intrusion,
although any such incident should
always be investigated with due discretion. It has a range of about 300 metres in open air and about 150 metres
in a built-up area or if you are inside
a building. Note that these figures
were obtained with the transmitter
placed on the dashboard of a car and
will vary depending on the individual
installation.
As can be seen from the photos, the
Alarm Pager consists of two separate
units: (1) a PC board which carries
the sensor/transmitter circuitry; and
(2) a compact receiver unit built into
a plastic case with a keypad.
The transmitter board mounts
inside the car (or boat) and is powered by the existing 12V supply. It’s
designed to be automatically armed
when the ignition is switched off,
which means that you cannot forget to
switch the unit on. It has two sensor
input channels and can be triggered
using vibration detectors (ie, piezoelectric transducers), high or low-going
alarm sensors (eg, reed switches), or
a combination of both.
The receiver circuit is built into a
small plastic case which is fitted with
a clip so that the unit can be worn on
a belt. It is controlled by a keypad
which has the following func
tions:
Off, On, Battery Test (Batt.), Test and
Reset. This unit is powered from a 9V
alkaline battery which should have a
life of about 400 hours.
When a valid paging signal is received from the transmitter, a buzzer
inside the receiver briefly “beeps”
every five seconds or so and continues
until the receiver is manually reset
(by pushing the Reset button). This
internal buzzer also provides audible
feedback when the other keys are
pressed. For example, pressing the
On key gives a short “beep”, while
pressing the Off key gives a much
longer “beep”. Pressing the Test key
gives the paging sequence (ie, a brief
beep every five seconds), while a continuous “beep” results if the Batt key
is held down (provided of course that
the battery is OK).
By making some simple decisions
during construction, you can customise the alarm pager to suit your
requirements. One option is to use the
unit with an existing alarm system, so
that it is triggered by an existing sensor. It could even be switched on and
off using the existing alarm’s remote
control.
However, for the purposes of this
article, we’ll assume that you intend
arming it via the ignition switch.
Vibration sensor
Ideally, we recommend that you
trigger the unit using sensors mounted
inside the front doors or adjacent to
the door pillars. A vibration detector
consists of a piezoelectric disc with
a threaded rod and nut assembly
soldered close to the rim – see photo.
This arrangement provides excellent
sensitivity to bumps and knocks but,
since the resonant frequency is set to
about 70Hz, avoids false triggering due
to low-frequency vibrations (eg, from
wind gusts).
By using this arrangement, the unit
pages you each time you get out of the
car and shut the door (assuming that
you are using the ignition to activate
the unit). The resulting sequence of
beeps from the receiver assures you
that the unit is working correctly and
is a useful test feature.
Similarly, the unit will page you
when you enter the car but will be
disarmed as soon as the ignition is
turned on.
How it works: transmitter
Fig.1 shows a block diagram of the
alarm/transmitter cir
cuit. It’s really
several circuits all rolled into one.
Starting at the left, the alarm in-
ENABLE/
DISABLE
(IGNITION
SWITCH)
ANTENNA
SWITCHED
MODE
+15V TRANSMITTER
+15V SUPPLY
IC4, Q8
IC2
SENSOR
INPUT 1
SENSOR
INPUT 2
P1
ALARM
INPUTS
8-SECOND
MONOSTABLE
Q1-Q3
IC1c
IC1d
Q7
PT
VIBRATION
SENSORS
Fig.1: block diagram of the UHF Alarm Pager. When an input is detected, an
8-second monostable turns on Q7 via IC1d & starts a switched mode power
supply (IC2). This in turn “fires” up the transmitter circuit (IC4 & Q8).
puts can be triggered by the above
mentioned vibration detectors or
by some other sensor with a pulsed
output (either positive or negative-going). When triggered, the input circuit
(Q1-Q3) triggers an 8-second mono
stable and Q7 turns on (via IC1d) for
the duration of the monostable period.
When Q7 turns on, a switched
mode power supply (IC2) “fires up”
and supplies power to the transmitter
circuit (IC4 & Q8). As a result, the
transmitter broadcasts a pulse-coded
RF signal for eight seconds and this
signal is picked by the receiver and
processed to pulse the internal buzzer
on and off.
Fig.2 shows the complete circuit
details for the alarm/transmitter. It
uses two virtually identical input
channels, one for low-going sensors
(Input 1) and one for high-going
sensors (Input 2). The only real difference between the two channels is
that Input 1 includes inverter stage
IC1a to invert the low-going input
•
pulse. Associated with this is an extra
clamping diode to protect the inverter
inputs (pins 1 & 2) plus a 47kΩ pullup
resistor.
Let’s take a closer look at how this
input operates.
Normally, no signal is applied to
the input and so pins 1 & 2 of IC1a are
pulled high. Pin 3 of IC1a will thus be
low and FET Q1 and transistors Q2 &
Q3 will all be off.
However, when a low-going pulse
is applied to the input, pin 3 of IC1a
switches high and forward biases D4.
As a result, a voltage of about 0.6V
appears across D4 and a sample of
this is applied to the gate of FET Q1
via VR1.
Alternatively, the signal for FET Q1
can come from piezo transducer P1.
When this vibrates (eg, when a door
closes), it generates an AC output voltage. This voltage is clipped to about
0.6V p-p by back-to-back diodes D4 &
D5 and applied to the gate of Q1 via
VR1 as before.
Features
Function: detects intrusion into parked vehicles, moored boats or
a building and transmits a paging signal to a receiver.
•
Range: approximately 300 metres in open air, reliable 150-metre
range in normal building locations. Note: these figures were obtained
with the transmitter placed on top of a plastic car dashboard.
•
Transmitter power supply: 9-14V DC operation from a car battery,
a plugpack supply or from a battery pack (eg, about 800 hours from
eight series C-size alkaline batteries). Current consumption is 3.5mA
quiescent or 50mA during transmission.
•
Receiver power supply: 9V battery (about 100 hours from a zinc-carbon battery or 400 hours from an alkaline type). Current consumption
negligible when “off” or about 2.5mA when “on”.
•
Battery test: battery checks as OK if above 5.4V. During this test,
the battery is continuously loaded by the buzzer.
November 1994 37
38 Silicon Chip
R11
10k
D8
1N914
D2
1N914
R1
10k
D1
1N914
ZD1
15V
1
2
7
R12
10k
P2
PIEZO
TRANSDUCER
D9
1N914
OUT
C13
.0033
D10
D5
D3
1N914
R3
10k
+8V
GND
IC1a
4093
14
3
P1
PIEZO
TRANSDUCER
R2
47k
C12
470
16VW
IN
IC3
7808
D11
2x1N914
D4
2x1N914
C14
10
VR2
1M
VR1
1M
C9
0.47
R30
220
5
C4
10
C3
680pF
C2
10
C1
680pF
+8V
C7
100
UHF ALARM PAGER-TRANSMITTER
SENSOR
INPUT 2
SENSOR
INPUT 1
GND
FROM
BATTERY
+12V
R31
D20
1N4007 22
1W
G
G
6
R14
680k
R13
39k
S
D
S
Q4
2N5484
D12
1N914
D6
1N914
Q1
2N5484
C8
.0015
2.2k
R28
D
R4
39k
2
R5
680k
4
7
R27
470
IC2
MC34063
R26
1
3
8
1
E
C11
0.47
R9
22k
R10
100k
R35
10k 13
R34
10k 12
R33
10k 11
R32
10k 10
R17
100k
R16
2.2k
R15
470
R8
100k
C
E
B
Q5
BC558
R18
22k
E
Q6
BC548
C
R19
100k
D13
1N914
E
Q2
D7
BC558
1N914
C
Q3
R7
BC548 C
2.2k
B
B
R6
470
C10
100
D19
SR103
L1
+15V
5
6
A12
15
16
17
R36
1M
4
E
R20
100k
D14
1N914
I GO
+8V
IC1b
R25
1k
B
12
13
C5
11 10
D18
1N914
D15
1N914
C
G S D
VIEWED FROM BELOW
C E
BC--B
R24
10k
ICId
C E B
A
10
+8V
C18
6.8pF
C16
4.7pF
2SC3355
8
9
Q8
2SC3355
VC1
2-7pF
R22
1M
E
C
R21
10k
R40
82
B
*
L2
*ETCHED ON PC BOARD
C6
100
IC1c
D16
1N914
C15
.001
C17
.001
* ANTENNA
D17
1N914
R39
2.2k
R37
6.8k
D47
1N4148
LED1
K
ENABLE/DISABLE
VIA IGNITION
SWITCH
9
IC4
AX5026
A11
A10
A9
18
A
R38
120
K
Q7
2N2219
R23
1k B
R29
10
304MHz
SAW
FILTER
E
C
The FET amplifier stage (Q1) is
normally biased at close to its cutoff
point due to the high value of source
resistance used (R5 = 680kΩ). Similarly, transistor Q2 is normally biased off
by R4 and so Q2’s collector normally
sits at 0V.
However, when a sensor is triggered (or vibrations are detected),
Q1 conducts and charges capacitor
C2. While C2 charges, sufficient base
current flows to turn Q2 on and this,
in turn, switches Q3 on and pulls pin
6 of IC1b low via D7 (note: pin 6 of
IC1b is normally held high via R10).
Q2 then switches off again as soon as
C2 is charged, since the voltage across
R4 is now too low to provide sufficient
forward bias.
IC1c and its associated parts form
the 8-second monostable. Normally,
pins 12 & 13 of IC1c are held low
via R20 and so both sides of C5 are
high, pin 10 of IC1d is low and Q7
is off. However, when pin 6 of IC1b
goes low, its output at pin 4 switches
high and pulls pins 12 & 13 of IC1c
high via D14
Pin 11 of IC1c now switches low and
so the positive side of C5 also goes low.
Pin 10 of IC1d thus switches high and
this turns on Q7 and the transmitter
(by switching on its power supply),
as described previously. At the same
time, D15 latches pins 12 & 13 of IC1c
high to ensure the correct monostable
timing period.
C5 now charges via R22 and R21
and, after about eight seconds, pulls
the inputs of IC1d high again. Pin 10 of
IC1d thus switches low again and Q7
turns off. At the same time, the high
on pins 12 & 13 of IC1c is released and
so the monostable is ready for a new
timing cycle.
Note that when pin 10 of IC1d
switches high (to turn on Q7), C6
charges via R24 and D18 and the
voltage across C2 is pulled high via
D6. Similarly, the voltage across C4
Fig.2 (left): this diagram shows the
complete circuit details for the alarm/
transmitter. It uses two virtually
identical input channels, one for
low-going sensors (Input 1) and
one for high-going sensors (Input
2). When triggered, Q7 turns on &
the transmitter section (IC4 & Q8)
broadcasts a coded RF signal, as set
by address lines A9-A12.
The pre-built UHF front-end module in the receiver must be installed with its
component side towards the AX528 decoder IC, as shown here. Sockets were
used to mount the ICs in the prototype but these can be considered optional.
in the other channel is held high via
D12. This effectively disables the two
input channels during the 8-second
monostable period and for some time
afterwards since FETs Q1 & Q2 are
biased off.
When pin 10 of IC1d switches low,
it takes about 30 seconds for C6 to
discharge via R5 & R14. This means
that the 8-second monostable can only
be retriggered some 30 seconds after
the previous cycle has ended. This
prevents false triggering at the end of
the monostable period.
The other sensor input channel
accepts signals from sensor 2 and/or
piezo transducer P2. As mentioned
previously, it works in virtually identical fashion. When triggered, Q6 pulls
pin 5 of IC1b low and so pin 4 swit
ches high and triggers the 8-second
monostable as before.
IC2 and its associated parts forms
the switched mode power supply.
This is enabled whenever Q7 is on
(ie, during the 8-second monostable
period) and supplies power to the
transmitter circuit.
IC2 is an MC34063A DC-DC converter IC and is wired here in standard
step-up configuration. It accepts an
8V input from 3-terminal regulator
IC3 and steps this up to provide an
output of 15V across C10. R28 & R30
set the output voltage, while C8 sets
the frequency of the internal switching
oscillator.
This arrangement is used to provide
a stable +15V supply rail for the transmitter. It ensures frequency stability
with varying input supply voltages
and also ensures that the transmitter
board can be used with supply voltages
from 10-14V.
Transmitter circuit
The transmitter circuit is based on
an AX-5026 trinary encoder IC. When
power is applied, this IC generates a
sequence of pulses at its output (pin
17). The rate at which these pulses
are generated is set by a 1MΩ timing
resistor (R36), while the code sequence
is set by resistors R32-R35. These resistors pull the A9-A12 address lines
low, while the remaining address lines
are left open circuit.
The coded output from IC1 appears
at pin 17 and drives RF transistor Q8.
This transistor is connected as a Hartley oscillator operating at 304MHz, as
set by a tank circuit consisting of L2
(etched on the PC board), VC1, C16 and
C18. In addition, a SAW resonator is
used to provide a narrow-band feedback path. Its lowest impedance is at
its resonant frequency of 304MHz and
thus the tuned collector load must be
set to this frequency in order for Q8
to oscillate.
The SAW resonator ensures frequency stability and makes the transmitter
easy to align. It ensures that the oscillator will only start and pulse LED
1 when the tuned circuit is virtually
dead on frequency.
November 1994 39
ON
S5
7
IC3d
K
BATTERY
TEST
S3
R15
27k
K
R8
47k
D1
1N914
R7
470k
11
C3
22
8
10
1,3,6,8
10,11,12
UHF ALARM PAGER-RECEIVER
TEST
E S1
A12
12 A11
A10 A9
11 10 9
R4
4.7k
2
Q1
BC548 C
B
UHF
RECEIVER
MODULE
2
7
5
R2
1M
16
13
IC1
AX5028
17
R3
10k
C1
0.47
18
14
15
R1
220
1
11
RESET
S2
13
IC3b
12
9
11
4
3
CONNECTION NUMBERS ON SWITCHES
REFER TO KEYPAD HEADER SOCKET
LED2
R11
4.7k
A
BUZZER
7
R10
14 4.7k
10
IC2d
4093
9
IC3a
4093
8
R6
4.7k
R5
4.7k
IC2c
IC2a
1
Fig.3: the coded signal from the transmitter is processed by the UHF front-end module & decoded by IC1. When a valid
signal is received, Q1 turns on & oscillator stage IC2a drives Q2, the buzzer & LED 2 to deliver the paging signal.
C
E
7
R14
10k
R13
10k
A
Q2
BC558 E LED1
R12
B
22k
K
E
C
Q3
B
BC558
C
R9
10k
C4
0.47
C2
22
13
6 12
5
2
IC2b
4093
ANTENNA
40 Silicon Chip
VIEWED FROM
BELOW
B
D5
1N914
D4
1N914
D3
1N914
D2
1N914
+9V
R17
1k
R16
1k
A
C5
22
C6
100
4
IC3c
4093
3
How it works – receiver
6 12
0FF
S4
5
2
1
14
R18
4.7k
R19
4.7k
4
B1
9V
VC1 is used to adjust the centre frequency of the
tuned circuit. This point corresponds to maximum
current consumption and is found by adjusting VC1
to obtain peak brightness from the LED 1.
Fig.3 shows the circuit details of the receiver.
This is based on a factory-built “front-end” module
that’s accurately aligned to the transmitter frequency (304MHz). It uses surface mount components to
give a compact assembly and is fitted with a pin
connector along one edge so that it can be plugged
into a PC board.
In operation, the front-end module picks up the
coded RF pulses from the transmitter via a short
antenna. The received signal is then processed via
an internal bandpass filter, an RF preamplifier, a
regenerative detector, an amplifier and a Schmitt
trigger. When a valid signal is received, a digital
pulse train appears at pin 5 and this is fed to pin
14 of IC1.
IC1 is an AX-528 Tristate decoder and is used
to decode the signal generated by the transmitter.
As with the AX-5026 encoder, this device has 12
address lines (A1-A12) and these are connected to
match the transmitter code (ie, pins 10-13 are all
pulled low).
If the code sequence on pin 14 of IC1 matches
its address lines, the valid transmission output at
pin 17 switches high and turns on transistor Q1.
This in turn triggers an S-R flipflop based on gates
IC3a & IC3b.
Normally, the flipflop is in the reset state and so
pin 10 of IC3a is low. However, when Q1 turns on,
pin 8 of IC3a is pulled low and so pin 10 goes high.
This high enables a Schmitt trigger oscillator stage
based on IC2d. Its timing capacitor (C3) is charged
via R7 each time pin 10 goes high and discharges
via D1 and R8 when pin 10 goes low.
This arrangement means that IC2d operates with
a duty cycle of about 10:1. As a result, a pulse train
that’s high for about five seconds and low for 0.5
seconds appears at pin 10 of IC2d. This pulse train
is applied to the base of PNP transistor Q2 and this
in turn drives the buzzer (B1) to produce a brief
sound every five seconds. It also flashes LED 2
which is wired in parallel with the buzzer.
Test switch S1 bypasses transistor Q1 and is used
to check that the buzzer circuit is working correctly.
Once the buzzer is activated, the circuit can only
be reset by pressing S2. This pulls pin 13 of IC3b
low and resets the S-R flipflop.
The receiver circuit is turned on and off by
switching power to the UHF front-end module and
to decoder IC1. This is done by pressing switches S4
& S5 and these in turn toggle a second S-R flipflop
based on IC3c & IC3d.
When S5 (ON) is pressed, pin 6 of IC3d is pulled
low and the output at pin 4 goes high and pulls pin
2 of IC3c high. Pin 3 of IC3c thus switches low and
this low is inverted by parallelled inverter stages
IC2a-IC2c to supply power to the UHF front-end
module and to IC1.
INPUT 1
ENABLEDISABLE
10uF
22k
D13
100k
470
D12
2.2k
100k
39k
82
IC4
AX5026
1M
Q6
Q5
1
10k
.0033
100uF
.001
2.2k
D47
6.8k
A
K
120
10k
D18
Q8
SAW
LED1
10k
D19
D6
.001
4.7pF
1 .0015
0.47
10k
2.2k
22k
D7
100k
680pF
100k
Q2
Q4
10k
470uF
IC3
7808
D20
+12V
22 1W
GND
10k
D8
D9
470
39k
680 k
10uF
680k
D4
D5
D11
D10
VR2
Q3
6.8pF
100uF
10k
10W
1k
1k
1M
680pF
10uF
INPUT 2
D16
D17
10k
D15
IC2
34063
VR1
P2
220
1
470
L1
2.2k
1
Q1
P1
VC1
Q7
D14
IC1
4093
10k
47k
D2
D1
10k
D3
0.47
10uF
100k
ZD1
Fig.4: install the parts on the transmitter PC board as shown here, taking care to
keep all component leads in the UHF transmitter section (around IC4 & Q8) as
short as possible. The enable/disable input is wired to the ignition switch.
Pressing S4 (OFF) has the opposite
effect. This pulls pin 1 of IC3c low
and so the output at pin 3 goes high.
The outputs of inverters IC2a-IC2c
thus switch low and remove power
from the front end of the circuit. The
remainder of the circuit draws negligible current in the quiescent state
and so is permanently powered from
the 9V battery.
The RC timing circuits connected
to the outputs of IC3c & IC3d set the
on and off indication periods. When
S5 (ON) is pressed, pin 3 of IC3c goes
low and this takes the negative side of
C5 low. C5 now immediately begins
charging via LED 1, the base-emitter
junction of Q3, R13, D3 & R16. As a
result, Q3 turns on while C5 charges
and briefly flashes LEDs 1 & 2 and
sounds the buzzer. A similar sequence
of events occurs when S4 is pressed
except that this time C6 charges via
R14, D5 & R17.
Diodes D2 & D4 ensure that the
positive sides of C5 & C6 can not rise
more than 0.6V above the positive
supply rail.
Finally, the circuit includes a battery
test feature based on S3, R12 & R15.
Because of the values chosen for R12
& R15, Q3 will only be biased on when
S3 is pressed if the battery voltage is
greater than about 5.5V. This means
TABLE 1: RESISTOR COLOUR CODES (TRANSMITTER BOARD)
❏
No.
❏ 2
❏ 2
❏ 5
❏ 1
❏ 2
❏ 2
❏
10
❏ 1
❏ 3
❏ 3
❏ 3
❏ 1
❏ 1
❏ 1
❏ 1
❏ 1
❏ 1
Value
1MΩ
680kΩ
100kΩ
47kΩ
39kΩ
22kΩ
10kΩ
6.8kΩ
2.2kΩ
1kΩ
470Ω
220Ω
120Ω
82Ω
10Ω
1Ω
22Ω
4-Band Code (1%)
brown black green brown
blue grey yellow brown
brown black yellow brown
yellow violet orange brown
orange white orange brown
red red orange brown
brown black orange brown
blue grey red brown
red red red brown
brown black red brown
yellow violet brown brown
red red brown brown
brown red brown brown
grey red black brown
brown black black brown
brown black gold gold
red red black brown
5-Band Code (1%)
brown black black yellow brown
blue grey black orange brown
brown black black orange brown
yellow violet black red brown
orange white black red brown
red red black red brown
brown black black red brown
blue grey black brown brown
red red black brown brown
brown black black brown brown
yellow violet black black brown
red red black black brown
brown red black black brown
grey red black gold brown
brown black black gold brown
brown black black silver brown
red red black gold brown
November 1994 41
No particular order need be
followed for the transmitter
board assembly but make sure
RECEIVER
0.47
that all polarised parts are cor
FRONT END
1
rectly oriented. In addition,
22uF
220
be sure to keep all component
1M
leads as short as possible in the
Q1
1
transmitter circuit (top right
0.47
IC1
AX528
hand corner of the board).
1
Note that the flat side of trimmer
capacitor VC1 should go
4.7k
LED1
towards Q8. The SAW resonator
K
A
Q2 Q3
IC3
should be mounted flat against
470k
4093
47k
the board, while transistor Q8
K A
1100uF 22uF
D1
should only stand about 1mm
LED2
22UF
proud of the board. Be careful
BUZZER
with the orientation of the LED
– its anode lead is the longer
D2
D4
of the two. It can be mounted
close to the PC board since it is
27k
only used during the setting-up
10k
procedure.
10k
The large inductor (L1) is
KEYPAD SOCKET
B1
supplied
pre-wound and can
1
be left until last. Clean and
Fig.5: this is the layout for the receiver
tin the ends of its leads with
PC board. Note that pins 10-13 must be
solder before mounting it on
connected to the 0V rail via short wire
the board. When this is done,
links to match the address code in the
check the board carefully to
transmitter (see text).
ensure that the assembly is correct – it only takes one wrong
that the buzzer will sound and the component value to upset the circuit
two LEDs will light only if the battery operation.
is OK.
The receiver board is equally
straightforward to assemble but again
Construction
keep all leads as short as possible.
Fig.4 shows the wiring details for Install the parts exactly as shown in
the transmitter board, while Fig.5 Fig.5, leaving the receiver module till
last. This component must be installed
shows the receiver layout.
4.7k
22k
D3
D5
1k
1k
4.7k
4.7k
4.7k
10k
4.7k
IC2
4093
10k
4.7k
250mm ANTENNA
The completed receiver can be fitted
with a clip so that it can be worn on a
belt. Note that the keys on the keypad
must be labelled exactly as shown
here; ie, key 1 = Reset, key 3 = Test,
key 5 = Battery Test, key 7 = Off & key
9 = On.
with its component side towards the
1MΩ resistor. The 13-pin keyboard
connector is mounted at the other end
of the board – see photo.
It’s optional as to whether the two
LEDs are hidden inside the case (in
which case there will be no visible paging or on/off indication) or mounted on
the end of the case near the keypad. If
you elect to hide them inside the case,
they can be mounted directly on the
The keypad is
connected into circuit
by plugging it into a
keypad socket at one
end of the receiver PC
board. Take care to
ensure that the buzzer
is oriented correctly
& don’t forget to fit a
250mm-long antenna
to the designated pad
near the front-end
module.
42 Silicon Chip
PC board and all indication will be
via the buzzer.
On the other hand, you might wish
to have a silent pager, with indication via the LEDs only (just leave the
buzzer out).
The antenna consists of a length of
insulated hook-up wire about 250mm
long. This is soldered to a pad which
connects to pin 2 of the front end
module.
The A9-A12 address line of AX528
decoder IC must now be tied low to
match the address programmed into
the AX5026 encoder in the transmitter. This simply involves connecting
pins 10-13 to the adjacent earth track
that runs along the outside edge of
these pins.
Note: if you wish, you can alter the
coding in both the transmitter and
the receiver by tying selected address
pins high or low or leaving them open
circuit. That way you can have your
own unique code, although it is not
really necessary for this project.
For example, you might tie A9 high,
leave A10 open circuit, and tie A11
& A12 low. Short wire links can be
used to make these connections in the
receiver but note that you will have
to scrape away the solder mask from
the adjacent rails at each connection
point so that the track can be soldered
(the positive rail runs adjacent to the
inside edge of the address pins in the
receiver).
What ever you do, make sure that
the transmitter code exactly matches
the receiver code otherwise the remote
control won’t work.
Once completed, the receiver board
can be installed in the bottom of the
case and secured to the integral standoffs using a couple of self-tapping
screws. This done, plug the keypad
into its connector and secure it by
A piezo disc is turned into a vibration detector by soldering a threaded rod &
nut assembly (made of brass) close to its rim to give a resonant frequency of
about 70Hz. The opposite edge of the disc is then soldered to a piece of scrap
PCB material as shown here. The wiring connection should be run using
shielded cable (centre conductor to the centre of the piezo disc, shield to the PC
board).
peeling away its backing paper and
carefully affixing it to the top of the
case.
The keys on the keypad should
be labelled exactly as shown in the
photograph; ie, key 1 = Reset, key 3 =
Test, key 5 = Battery Test, key 7 = off
and key 9 = on.
Test & alignment
To test the receiver, connect a 9V
battery and carry out the following
checks:
(1). Press Test and check that the
buzzer beeps and LED 2 flashes every
five seconds or so. Check that the
circuit can be reset by pressing Reset.
(2). Press On and check that the
buzzer briefly sounds and that both
LEDs briefly light. If so, press Off and
check that the buzzer sounds and both
LEDs light for about three seconds.
(3). Press Batt and check that the
buzzer sounds and that both LEDs
light for as long as the key is held
down.
If all these checks are OK, then
most of the receiver circuit is working
correctly and the case assembly can be
completed. Before doing this, however, a small channel must be filed in the
end of the case adjacent to the battery
compartment to serve as an exit point
for the antenna. The case can then be
clipped together and secured using
two self-tapping screws at the battery
compartment end.
We now come to the transmitter
TABLE 2: RESISTOR COLOUR CODES (RECEIVER BOARD)
❏
❏
❏
❏
❏
❏
❏
❏
❏
No.
1
1
1
1
4
7
2
1
Value
1MΩ
470kΩ
47kΩ
27kΩ
10kΩ
4.7kΩ
1kΩ
220Ω
4-Band Code (1%)
brown black green brown
yellow violet yellow brown
yellow violet orange brown
red violet orange brown
brown black orange brown
yellow violet red brown
brown black red brown
red red brown brown
5-Band Code (1%)
brown black black yellow brown
yellow violet black orange brown
yellow violet black red brown
red violet black red brown
brown black black red brown
yellow violet black brown brown
brown black black brown brown
red red black black brown
November 1994 43
PARTS LIST
Transmitter Board
1 PC board, code OE93/
PAGERTX
1 304MHz SAW filter
1 prewound inductor (L1)
2 piezo discs
2 1MΩ trimpots
Semiconductors
1 4093 quad Schmitt NAND gate
(IC1)
1 MC34063 switched mode
supply IC (IC2)
1 7808 3-terminal regulator (IC3)
1 AX-5026 trinary encoder (IC4)
2 2N5484 FETs (Q1,Q2)
2 BC558 PNP transistors
(Q2,Q3)
2 BC548 NPN transistors
(Q3,Q4)
1 2N2219 NPN transistor (Q7)
1 2SC3355 NPN RF transistor
(Q8)
1 15V 1W zener diode (ZD1)
1 SR103 Shottky power diode
(D19)
1 1N4007 power diode (D20)
19 1N4148 or 1N914 signal
diodes (D1-D18, D47)
1 red LED (LED1)
Capacitors
1 470µF 16V electrolytic
3 100µF 16V electrolytic
4 10µF 16V electrolytic
2 0.47µF monolithic
1 .0033µF ceramic (3n3)
1 .0015µF ceramic (1n5)
2 .001µF ceramic (1n)
2 680pF ceramic
1 6.8pF ceramic
1 4.7pF ceramic
1 2.7pF trimmer capacitor (VC1)
Resistors (0.25W, 5%)
2 1MΩ
2 1kΩ
2 680kΩ
3 470Ω
5 100kΩ
1 220Ω
1 47kΩ
1 120Ω
2 39kΩ
1 82Ω
2 22kΩ
1 10Ω
10 10kΩ
1 1Ω
1 6.8kΩ
1 22Ω 1W
4 2.2kΩ
Receiver
1 PC board, code OE/93/PAGER
1 case with battery compartment
1 keypad
1 PC-mounting keypad socket
1 304MHz front-end module
1 9V buzzer
Semiconductors
1 AX-528 Tristate decoder (IC1)
2 4093 quad Schmitt NAND
gates (IC2,IC3)
5 1N4148 signal diodes (D1-D5)
2 red LEDs (LED1, LED2)
1 BC548 transistor (Q1)
2 BC558 transistors (Q2,Q3)
Capacitors
1 100µF 16V electrolytic
3 22µF 16V electrolytic
2 0.47µF monolithic
Resistors (0.25W 5%)
1 1MΩ
4 10kΩ
1 470kΩ
7 4.7kΩ
1 47kΩ
2 1kΩ
1 27kΩ
1 220Ω
Where To Buy The Parts
A kit of parts for the UHF Alarm Pager is available from Oatley Electronics,
PO Box 89, Oatley, NSW 2223. Phone (02) 579 4985. Prices are as follows:
Transmitter (includes PC board plus on-board components): $49.00.
Receiver (includes PC board, on-board components, case & keypad): $52.00.
Please add $4 for postage with each order.
Note: copyright © of the PC boards associated with this project is retained
by Oatley Electronics.
alignment. To do this, temporarily
solder a link between the collector
and emitter of Q7 and apply power
(12V to +12V & GND). This will start
the switchmode supply based on IC2
44 Silicon Chip
and “fire up” the transmitter for as long
as the link is in place. All you have to
do now is adjust VC1 using a plastic
tool until LED 1 begins to flash. When
this happens, the oscillator is working
and you can tweak VC1 for maximum
transmitter output (ie, maximum LED
brightness).
Finally, the completed transmitter
board can be tested by removing the
link across Q7, then re-applying power
and pulling sensor input 1 low (ie,
by connecting the input to ground).
When you do this, the transmitter LED
should flash for about three seconds.
If the receiver is on, it should immediately begin paging you (ie, you should
hear a brief beep every five seconds).
Sensor input 2 can be checked in
a similar manner by connecting it to
the positive supply rail. Just remember
that after each transmission, you will
have to wait at least 30 seconds before
the transmitter can be reactivated (this
is the time it takes for C6 to discharge,
as described earlier). In fact, it’s best
to wait for about 60 seconds after the
transmitter LED goes out before attempting to retrigger the unit.
Installation
Finding a convenient location to
mount the module is probably the
greatest challenge in installing the
unit. On top of the rear parcel shelf
is probably the best location in a car,
with power derived from the supply
to the boot lamp. The enable/disable
input should be connected to the
switched side of the ignition switch
and this will involve running a lead
back to the front of the vehicle (eg,
you can tap into a suitable point in
the fusebox).
If you elect to switch the unit using
a UHF remote control (eg, as part of
an existing alarm), just remember that
pulling the enable/disable input high
(ie, to +8V) disarms the circuit. The
vibration detectors can be installed
inside small plastic cases and these
can be mounted next to the door
pillars.
Finally, note that this unit can be
easily adapted for use as a 12-channel
paging system and that is why provision has been made on the transmitter
board for diodes D21-D42 (bottom
right
hand corner). When combined
with suitable switches, these diodes
change the coding of the transmitter
and you can build individual receivers
with unique matching codes.
Full details on how to convert the
unit to a 12-channel pager will be
supplied with a kit from Oatley Electronics and this kit will also include
SC
the extra diodes (D21-D42).
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