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COMM
UNICATIONS
Radio-Linked Caller
Identification System
With up to 16 caller units
By Dr. Pei An
pan@intec-group.co.uk
This article introduces a PC-based radio-linked caller identification system.
It consists of a central receiver and up to 16 caller units. The receiver is
connected to a computer via the Centronics port. Each unit has a push-to-
make button on it. When the button is pressed, the caller unit sends out
a unique code to the soundings. The receiver intercepts the code and the
computer decodes it and displays the number of the activated caller unit
on the screen. The operation distance is about 75 metres in buildings and
200 metres in open fields.
The system has a wide range of
applications. In hotels, restaurant
and shops, the caller unit can serve
as a ‘Call Assistance’ button used by
customers to call for assistance. It
also has applications in interactive
teaching environment in schools and
colleges.
The system utilises ‘2nd genera-
tion’ versions of FM radio transmit-
ter and receiver modules (TX2/RX2)
from Radiometrix Limited. The trans-
mitter is a low-power device (LPD)
type-approved to the Radiocommu-
nications Authority (RA) specifica-
tion MPT 1340 in the UK and Europe.
This avoids the need to submit the
project for final approval.
A Visual Basic 5 program has
been developed for the system,
demonstrating how the system is
integrated with software.
54
Elektor
1/2001
COMM
UNICATIONS
How it works
FM MODULATED RADIO
CARRIER SIGNAL
Figure 1a
shows the principle of the
caller unit. Inside a caller unit (trans-
mitter), an encoder IC (HT-12E) con-
verts a 12-bit parallel data into ser-
ial data. The first eight bits represent
a
system address
and the next 4 bits
are the
address
of the caller unit. The
system address ranges from 0 to 255
and is a value assigned to the sys-
tem. The address of a call unit
ranges from 0 to 15. The serial data
is fed into the Radiometrix TX2 radio
transmitter module, in which the ser-
ial data modulates a radio frequency
carrier signal (at 418 MHz or 433
MHz, depending on the TX2 version
used) using FM modulation. Next,
the radio signal is transmitted to sur-
roundings from an antenna.
The basic operation of the system
receiver is illustrated in
Figure 2b
.
Inside a receiver, an FM radio
receiver module (RX2) demodulates
the radio signal picked up by the
antenna. Once demodulated, serial
data is fed into a serial-to-parallel
decoder IC (HT-12D), which converts
the serial data back to parallel for-
mat. The address bits are compared
with the preset address of the
decoder. If they match, the 4-bit data
is latched to the output (which is the
address of the activated caller unit).
If the address does not match, the
decoder ignores the data. After a
dataword is successfully received,
the computer reads the data from the
decoder and displays the number of
the activated caller unit on the
screen.
ANTENNA
TX2
TRANSMITTER
FM RADIO
SERIAL ENCODED DATA
TO BE TRANSMITTED
PARALLEL-TO
-SERIAL DATA
ENCODER
12-BIT PARALLEL DATA
(8 BIT SYSTEM ADDRESS
FM MODULATED RADIO
CARRIER SIGNAL
AND 4 BIT CALLER UNIT ADDRESS)
SERIAL ENCODED DATA
RECEIVED
ANTENNA
THE SAME DATA
RX2
RADIO RECEIVER
MODULE
4 BIT DATA
SERIAL-TO-
PARALLEL DATA
DECODER
Computer
8-BIT PRESET
SYSTEM ADDRESS
Centronics port link
000108- 11
Figure 1. Principle of the Caller Identification system.
Table 1
Variant of TX2/RX2 radio link modules
Parameters
Description
Frequencies
418.00 MHz for UK use
433.92 MHz for European use
Supply voltages
5V (4-6V for TX2 and RX2)
3V (2.2V to 4V for TX2, 3 to 4V for RX2)
RX data transfer rate
-A: 7kHz baseband BW, slow data up to 14kbps
-F: 20kHz baseband BW, fast data up to 40kbps
VCC
3
TX2
DATA IN
RF OUT
20kHz 2nd order
SAW
Oscillator
band pass
filter
Radio transmitter and
receiver modules
The Radiometrix radio transmitter
and receiver modules (TX2 and RX2)
allow a digital radio link to be imple-
mented easily. The modules are a
SAW-controlled FM radio transmitter
and receiver which are specially
designed for radio telemetry and
tele-command applications (SAW =
surface acoustic wave). There are a
variety of TX2/RX2 modules that can
be used with the present project.
They are shown in
Table 1
.
A brief description of the trans-
mitter and receiver modules is
given here. Technical details of the
modules can be found in the data
2
Buffer
Low pass
filter
5
Stablised
100K
GND
GND
1
4
VCC
5
RX2
1st local
oscillator
2nd local
oscillator
DETECT
3
AF
6
Adaptive data slicer
+
RF IN
2nd mix
IF amp
demodulator
DATA
SAW
6 KHz low
pass filter
band pass
filter
Pre-amp
band pass
filter
Buffer
1
7
AF
-
1st mixer
GND
G
N
D
2
4
000108 - 12
Figure 2. Internal block diagram of the radio link modules.
1/2001
Elektor
55
COMM
UNICATIONS
sheets, Reference [1].
with a typical current consumption
of 13 mA at 3.5 V. The output digital
signal appears at pin 7 (RXD) with a
CMOS logic level. Pin 3 is the Carrier
Detect output. It may be used to
drive an external pnp transistor to
obtain a logic level carrier detect sig-
nal. If not used, it should be con-
nected to +5 V.
0.5 mm diameter enamelled copper wire close wound on 3.2 mm dia former
Transmitter Module type TX2
The pin functions of the transmitter
are given in
Figure 2a
. For the +5 V
and 433 MHz version, the operation
voltage ranges from 4 to 6 V DC. The
typical current consumption is about
10 mA at 5 V. For the +3 V and
433 MHz version, the operation volt-
age is between 2.2 V to 4 V DC with
a typical current consumption of 6
mA at 3 V. Digital data to be sent
(which should be a CMOS logic level
at the same power supply voltage) is
fed into pin 5. An antenna is con-
nected to pin 2. The block diagram of
the module is given in
Figure 2a
.
Helical type
Pin 2 (RF OUT)
418MHz: 26 turns, 433MHz: 24 turns
Pin 2 (RF OUT)
Loop type
4 to 10 sq. cm
inside area
1mm wide track
Antenna options
The antenna used with an LPD like
the TX2 module is usually one of
three versions: the helical type, the
loop type and the whip type (see
Figure 3
). The helical antenna has a
small size. It needs to be optimised
for the exact wavelength in use. The
loop antenna consists of a loop of
PCB track, which is tuned by a vari-
able capacitor. The quarter-wave
whip-type antenna is a length of
wire, a rod, a PCB track or combina-
tions.
Figure 3
shows how the dif-
ferent versions are constructed and
compares the performance amongst
various antennas. The above
Pin 1 (GND)
Capacitor 1.5 - 5 pf
Whip type
Wire, rod PCB track or combination
Pin 2 (RF OUT)
418MHz: 16.5 cm, 433MHz: 15.5 cm total from RFout pin
Antenna performance chart
Helical
Loop
Whip
Receiver Module type RX2
The pin functions of the receiver are
shown in
Figure 2b
. For the +5 V
version, the operation voltage ranges
from 4 to 6 V DC. The typical current
consumption is about 13 mA at 5 V.
For the +3 V version, the operation
voltage is between 3.0 to 4.0 V DC
Ultimate performance
Ease of set-up
Size
Immunity to proximity de-tuning
000108- 13
Figure 3. Antennas for the radio link modules.
Encoder
Decoder
Power on
Power on
Stand-by mode
Stand-by mode
Disable VT &
ignore the rest of
the word
No
Data in ?
No
-TE enable?
Yes
No
8 bits address
matched ?
Yes
serial encoded data
transmitted
Yes
Store 4 bits data
No
-TE still
enabled?
Match
Previous stored
data ?
No
Yes
Yes
serial encoded data
transmitted again
Latch 4 bits data
to output & activate VT
No
Yes
Triple check
completed ?
000180 - 15
Figure 4. Flow charts of encoder and decoder operation.
56
Elektor
1/2001
COMM
UNICATIONS
antenna considerations also apply to
the receiver.
12 bits pilot period
12 bits code period (A0-A7,D0-D3)
Heart of the system:
HT12-E/HT12-D
The HT-12E and HT-12D from Holtek
are a CMOS encoder/decoder IC
pair designed for digital code trans-
mission and receiving. They have a
wide operating voltage from 2.4 V
up to 12 V with a typical stand-by
current of 1 µA. They have an on-
board oscillator that requires one
external resistor. Brief descriptions
of the ICs are given here. For more
details, please refer to the manufac-
turer’s datasheet, Reference [2].
When pin numbers are mentioned
below, you may already refer to the
circuit diagrams. The operation of
these interesting ICs is conve-
niently described using flowcharts,
see
Figures 4a
and
4b
.
The
HT-12E encoder
encodes 12-bit
of parallel data into a serial data. It
transmits the data upon the receipt
of a falling
ed
ge at the Transmit
Enable pin (TE). The 12 bits of data
consist of eight bits of address (A0-
A7 connected to pins 1-8) and four
bits of data (D3-D0) connected to
pins 10-13). Therefore the total num-
ber of address combinations is 2
8
or
256. The external oscillator resistor
is connected between pins 15 and
16. The serial data is output from
pin 17.
The operation of the HT-12E is
that initially the encoder is in the
sta
nd-by mode. Upon receipt of a
TE signal (active Low), it begins a
4-word transmission cyc
le
and
repeats the cycle until the TE sig-
nal becomes high (see Figure 4a).
Each word contains two periods:
the pilot code period and code peri-
ods as shown in
Figure 5a
. The
pilot code period has a 12-bit
length period and is at logic Low.
The code period also has a 12-bit
length period and contains the ser-
ial encoded data. The encoder
detects the logic state of the 12 bit
inputs (A0-A7 and D0-D3) and
encodes the information into a ser-
ial datastream. The logic levels ‘0’
and ‘1’ are encoded as shown in
Figure 5b
. The order of the databit
transmission is from A0 to A7, then
from D3 to D0.
Start bit 1/3 bit
1
2
3
Clock signal of
the encoder
One data bit
Encoded data format
for logic ‘1’
Encoded data format
for logic ‘0’
1 data bit
000108 - 16
Figure 5. Data format of one transmission (a) and encoded serial data format (b).
The
HT-12D decoder
receives the 12-
bit word and interprets the first eight
bits as the address and the last four
bits as data. When the received
address matches the decoder’s pre-
defined address, the Valid Transmis-
sion (VT) output goes High and the 4-bit data
is latched to the data output pins. A triple
check scheme is used to enhance the reliabil-
ity of data reception (Figure 4b). The VT out-
put remains high until the right code is not
received anymore. The pre-defined address for
5V
ANT
K1
C4
R4
1
8x 4k7
R5
1
4x 10k
100n
R1
R2
C3
23456789
2345
15
16
18
OSC1
OSC2
100n
1
2
3
4
A0
A1
A2
A3
A4
A5
A6
A7
17
DOUT
IC1
13
D0
D1
D2
D3
5
HT12E
12
11
10
6
7
8
DATA IN
S1
16
15
14
13
12
11
10
9
S2
5678
14
9
C5
ON = 0
OFF = 1
ON = 0
OFF = 1
100p
1
2345678
4
3
2
1
system address
caller unit address
IC2
S3
TC55RP5002EZB
78L05
5V
+12V
78L05
TC55RP5002EZB
R3
BT1
12V
D
1
C1
C2
22µ
16V
22µ
16V
000108 - 14
Figure 6. Circuit diagram of the caller unit. Up to 16 of these may be used within a single
system address.
1/2001
Elektor
57
COMM
UNICATIONS
length, 33 mAh capacity). The volt-
age is regulated to +5 V by a low-
power and low-drop voltage regulator
type TC55RP5002EZB from Telcom.
Alternatively, an 78L05 may be used,
but unlike the Telcom IC it will not
allow the battery voltage to drop to
5.5 V before the transmitter becomes
inoperative. Also note the different
pinouts of the two regulators! S3 is a
push button switch (push to make
type). If the button is not pressed, no
voltage is supplied to the circuit. If it
is pressed, the caller unit immedi-
ately transmits the code. This
arrangement allows a maximum bat-
tery power saving to be achieved.
During a transmission, current con-
sumption is about 15 mA. A calcula-
tion indicates that an N-type battery
will last 2.3 hours of data transmis-
sion. Assuming that the activation
time of switch is 1 second on aver-
age, the battery will last for at least
2,300 calls.
5V
ANT
C3
R4
1
8x 4k7
100n
K1
14
DATA IN
A0
A1
A2
A3
A4
A5
A6
A7
18
K2
23456789
1
2
3
4
17
VT
P 11 (BUSY)
P 32 (ERROR)
P 13 (SELECT)
P 12 (PE)
P 10 (ACK)
P 19 (GND)
VAL
D0
D1
D2
D3
IC1
13
D0
D1
D2
D3
5
HT12D
12
11
10
6
7
8
OSC1
OSC2
15
16
9
S1
16
15
14
13
12
11
10
9
R2
R3
ON = 0
OFF = 1
1
2345678
system address
IC2
TC55RP5002EZB
S1
78L05
5V
5V5...10V
17mA
78L05
TC55RP5002EZB
D1
R1
C1
C2
22µ
16V
22µ
16V
COMPONENTS LIST
Caller Unit (TX)
000108 - 17
Figure 7. Circuit diagram of the central receiver unit.
Resistors:
R1,R2 = 1M
Ω
1%
R3 = 4k
7
R4 = SIL array, 8-way, 10k
Ω
R5 = SIL array, 4-way, 10k
Ω
Table 2. Some values of external
resistors for encoder and decoder.
HT-12E (encoder)
Ω
Capacitors:
C1,C2 = 22
F 16V radial
C3,C4 = 100nF, 5mm lead pitch
C5 = 100pF
µ
HT-12D (decoder)
R
F
osc
R
F
osc
1.1 M
3 kHz
62 k
150 kHz
Semiconductors:
D1 = LED, high efficiency
IC1 = HT-12E (Holtek), RS
Components # 854-100, Maplin
# AE17T, Conrad # 175420
IC2 = TC55RP5002EZB, RS
Components # 207-0045
750 k
4.3 kHz
33 k
240 kHz
the encoder is determined by the logic states
at pins 1-8 for A0-A7. The latched data is out-
put from pins 10 to 13. The serial data is input
at pin 14. The external oscillator resistor is
connected between pins 15 and 16. Pin 17 is
the valid transmission (VT) indicator output.
The external resistors required on the OSC
pins of the HT12D/E are 1% types.
Table 2
gives the resistance values for 3 kHz and
4.3 kHz clock frequencies, F
osc
(in the present
circuit, F
osc
= 3 kHz). For other frequencies,
please refer to the manufacturer’s data sheet.
switch block S1) and a 4-bit data
(caller address set by DIP switch
block S2) into a serial data form. The
serial data is outpu
t fr
om the Dout
pin, number 17. The TE input (Trans-
mit Enable, pin 14) is set perma-
nently to Low to enable data trans-
mission.
The serial data generated by the
HT-12E encoder is fed into pin 5 of
the TX2 radio transmitter module
which is plugged or soldered onto
connector K1. The transmitter signal
emerges from pin 2 of the TX2 mod-
ule and is radiated by an antenna.
The power supply to the caller
unit is an N-type 12-V alkaline bat-
tery (10 mm diameter and 28 mm
Miscellaneous:
K1 = Transmitter module,
RadioMetrix type TX2, Farnell #
722-4930 (418 MHz) or 722-
4953 (433 MHz)*
S1 = 8-way DIP switch
S2 = 4-way DIP switch
S3 = pushbutton, 1 make contact
Antenna, see text
BT1 = 12V N-size battery (L1028)
PCB, order code
000108-1
(see
Readers Services page)
ABS case, size 124 x 33 x 30 mm,
Maplin # FT31J
Circuit of caller unit
Figure 6
shows the circuit diagram of the
transmitter. The encoder, HT-12E, converts an
8-bit address (system address set by DIP
58
Elektor
1/2001
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