GSM REMOTE CONTROL - ElettronicaIn

Hi-tech
GSM REMOTE CONTROL
MODULAR AND LOW-COST
N
owadays, creating a remote control is much
simpler and safer than it used to be, even if
it is to be used from the other side of the world;
what made this possible was the advent of cell
phones and the availability of low-cost GSM/GPRS
modules that could be easily integrated in standalone
solutions, capable of handling both calls and
A modular solution
allowing for the use
of a single circuit for
numerous functions,
such as gate opening,
bidirectional telecontrol,
and many others. It
adopts a newly-conceived
low-cost GSM/GPRS module.
by BORIS LANDONI
SMSs. The use of SMSs allows one to know the
exact output of the commands that have been given,
since it is enough to set sent messages to confirm a
relay commutation in order to be sure that a given
command was properly executed. Precisely because
of the possibilities offered by a cell-phone based
remote control, and encouraged by popular demand,
Elettronica In ~ Aprile 2010 51

[Complete ELECTRIC scheme]
54 Aprile 2010 ~ Elettronica In

identify the bi-tones involved in
the phone selection found within
modern telecommunication
systems –bi-tones which every
phone emits and sends out on
phone lines when its keyboard
keys are pressed. The identifier is
necessary in order for the DTMF
functionality to work.
Having said that, let’s take a
better look at the electric scheme:
power is supplied by continuous
voltage, not always stabilized (applied
to PWR + and -) at a value
between 5 and 32 V; such voltage
is filtered at the bottom by the
diode protecting against polarity
inversion (D1) through condensers
C1 and C2. Fuse F1 enables us
to protect both the circuit and the
power source in case of a short
circuit in the integrated regulator
discussed below, which is necessary
to obtain the 4 volts needed
for the rest of the circuit to work.
The switching regulator is based
on a MC34063 chip, utilized in
the classic configuration of series
PWM regulators charged by
inductance, whose output voltage
depends on the energy stored in
L1; the regulator is stabilized by
the component demoted from
resistive divider R2/R3, which is
needed to set at 4 V the component
leveled at the top of both C4
e C5. The 4 volts at the bottom of
the abovementioned condensers
are sufficiently filtered by other
condensers placed on the power
lines of the microcontroller and of
the GSM module; which presents,
during transmission, absorption
peaks compensated for by C7, C8,
C13, C14, C15 e C16, thus avoiding
that an impulsive current
request may cause the microcontroller
to be disturbed.
The microcontroller used to
handle the whole system is a
powerful PIC18F46K20-I/PT by
Microchip, which we use in its
configuration with an internal
clock oscillator; both the scheme
and in the printed circuit are nevertheless
equipped with external
quartz, which we included for
those who may want to modify
the firmware and develop specific
applications requiring an external
oscillator.
Once the I/O lines have been
initialized, the microcontroller
verifies the logical state of the
opto-isolated inputs at voltage
level (RB4 and RB5) as well as
that of lines RC4, RC5, RD0,
RD3, RX, which are needed to
receive the main notifications
from the cellular module; more
specifically, RD3 is used to detect
incoming calls (it interfaces with
RI of the cellular module), while
RC4 controls the GSM’s reception
led , whose output (dubbed
STATLED) pulsates at a frequency
of 1 Hz when the module
is searching for the radio-mobile
network, and supplies impulses
at logical zero, lasting 0.5 seconds
and followed by a 2-second pause,
when the module has grasped
the signal. The frequency and
duration of the impulses enable
the PIC to understand the
conditions of the radio-mobile
network range and to behave
accordingly; for example, if the
opto-isolated input goes off and
it needs, therefore, to send SMSs
or make calls, but detects that the
cellular module has no reception,
it waits for the Telit module to get
reconnected to the GSM/GPRS
network before making any calls.
The attempt to make calls or send
SMSs is repeated only three times,
after which the device gives up.
The microcontroller contains
a UART accessible via pins 44
(transmission) and 1 (reception)
which it uses in order to communicate
with the cell phone; more
precisely, through the first pin
(TX), it cyclically questions the
module in order to check whether
any SMSs have been received,
whereas both TX and RX are
used to communicate with the
GSM module when making calls
and receiving or sending messages.
Regarding the UART, it is
important to note that the following
control signals are used: CTS
(Clear To Send), RTS (Request
To Send) and DCD (Data Carrier
Detect), which correspond to
those of the cellular module being
used. They complete the set of I/
Os destined to the cell phone, the
RC5 and RD0 lines: the former
controls the turning on and off
of the GSM (though a transistor
placed in the small board of the
cell phone), while the latter takes
care of resetting the cell phone.
The button for locally handling
this device’s working modality
is read through line RA3, configured
as input and equipped with
an external pull-up resistor (R11) –
and therefore active at a low level.
Inputs are read through lines
RB4 and RB5, both of which are
configured as input and equipped
with an internal pull-up; each one
of them reads the state of the output
transistor of the corresponding
photocoupler (the optos used
here are TLP181). Each of the two
available inputs (IN1 and IN2) is
active when under a voltage between
3 and 30 V. When a 3-volt
voltage (at a minimum) is applied
to input IN1, the photocoupler’s
LED is switched on and the output
phototransitor is in conduction
state; therefore, the collector
(pin 5) is at about zero volt during
I/O configuration, due to the
fall on the resistor of the internal
pull-up that was configured. If
the input is not polarized, the
opto-isolator gets inhibited and
its pin 4 shows a high level.
As for the relays, they are controlled
by the microcontroller’s RC0
and RE2 lines, through two NPN
transistors driven by current am-
Elettronica In ~ Aprile 2010 55

[Base unit: ASSEMBLING]
plifiers; line RE2 controls transistor
T1, while line RC0 controls
transistor T2. A high logical state
causes the transistor to saturate,
thus determining the amount
of current flowing in the coil of
the corresponding relay. Each
instance of activation is signaled
with a LED, powered along with
the coil. In order to protect the
transistors’ collector junction as it
goes from saturation to inhibition,
56 Aprile 2010 ~ Elettronica In
when the relay’s coil inductance
generates peak inverse voltage,
we connected a diode parallel to
the coil, and such diode eliminates
unwanted impulses. The
entire relay exchange is available
so as to allow for the handling
of circuits requiring a normally
closed contact or a normally open
one. Still regarding the relay, we
should note that, although it is a
5-volt-coil type, in our circuit it
Parts List:
R1: 0,1 ohm 1W (1206)
R2: 2,2 kohm (0805)
R3: 1 kohm (0805)
R4: 100 kohm (0805)
R5: 4,7 kohm (0805)
R6: 4,7 kohm (0805)
R7: 330 ohm (0805)
R8: 330 ohm (0805)
R9: 4,7 kohm (0805)
R10: 10 kohm (0805)
R11: 4,7 kohm (0805)
R12: 10 kohm (0805)
R13: 330 ohm (0805)
R14: 330 ohm (0805)
R15: 1,5 kohm (0805)
R16: 1,5 kohm (0805)
R17: 330 ohm (0805)
R18: 4,7 kohm (0805)
R19: 4,7 kohm (0805)
R20: 1 kohm (0805)
R21: 2,2 kohm (0805)
R22: 4,7 kohm (0805)
R23: 330 kohm (0805)
R24: 39 kohm (0805)
R25: 56 kohm (0805)
R26: 100 kohm (0805)
R27: 100 kohm (0805)
C1: 100 nF multilayer (0805)
C2: 1000 µF 25 VL electrolytic
C3: 100 pF ceramic (0805)
C4: 100 nF multilayer (0805)
C5: 100 µF 16 VL electrolytic
C6: 100 nF multilayer (0805)
C7: 100 nF multilayer (0805)
C8: 470 µF 6,3 VL tantalum (CASE-D)
C9: unused
C10: unused
C11: 100 µF 16 VL electrolytic
C12: 100 µF 16 VL electrolytic
C13: 100 nF multilayer (0805)
C14: 470 µF 6,3 VL tantalum (CASE-D)
works on just 4 volts; this is possible
because the model we chose
can prompt relay exchange even
at less than 3.5 volts.
The room temperature is detected
by the microcontroller with the
aid of an extremely accurate
probe DS1820 produced by
Dallas Semiconductor, which is
capable of taking temperatures
in the range of -5÷150 °C with an
accuracy of just ± 0,5 °C (in field

C15: 470 µF 6,3 VL tantalum (CASE-D)
C16: 470 µF 6,3 VL tantalum (CASE-D)
C17: 10 pF ceramic (0805)
C18: 100 nF multilayer (0805)
C19: 10 pF ceramic (0805)
C20: 100 nF multilayer (0805)
Q1: (see text)
Q2: quarzo 3,579545 MHz (HC49/4H SMX)
U1: MC34063AD
U2: DS18B20+
U3: 24FC256-SN
U4: TLP181
U5: TLP181
U6: PIC18F46K20-I/PT (MF857)
U7: MT88L70AS
D1: 1N4007
D2: 1N5819
D3: 1N4007
D4: 1N4007
T1: BC817
T2: BC817
LD1: LED 3 mm red
LD2: LED 3 mm red
LD3: LED 3 mm yellow
LD4: LED 3 mm yellow
LD5: LED 3 mm green
L1: Inductor coil 20 µH
RL1: Relay 5V 1 one way
RL2: Relay 5V 1 one way
P1: Microswitch
F1: Fast fuse 2 A (1206)
Varie:
- Screw connector 2 poles (2 pz.)
- Screw connector 3 poles (2 pz.)
- DC plug
- Male strip 6 poles
- Female strip 3 poles
- Female strip 16 poles
- Female strip 4 poles 90°
- PCB
-10÷85 °C), and then of digitizing
and making them available on a
unidirectional serial line connected
to the microcontroller’s RC0
pin. This probe communicates
using protocol One-wire, which
allows for bidirectional dialogue
on a single thread; for this to be
possible, pull-up R19’s resistance
must be present.
The PIC is programmed in-circuit,
through the ICSP connector,
which is attached to lines /MCLR,
PGU and PGC; the microcontroller’s
power and mass are also
connected to the ICSP. But we
didn’t think that was enough, so
we included a serial communication
interface enabling users to
program data related to the various
functions (e.g., list of phone
numbers, handling of input
levels, text of SMSs sent by the
circuit following commands, etc.)
through a PC: this allows users
to program their remote control
before activating it, thus avoiding
having to send configuration
SMSs, which can be used at any
time, but are best used only once
the system has been installed in
situ. Since the UART is already
busy communicating with the
cellular module, serial communication
occurs via lines RE0 and
RA5, exploited, respectively, as
Elettronica In ~ Aprile 2010 57

iPhone becomes CDMA
The rumor is spreading that Apple is working on a new iPhone
model, for the time being called generically 4G; this phone is
a hybrid UMTS/CDMA which can work with any carrier in the
world. Thanks to a new hybrid chip produced by Qualcomm, it
should compatible not only with networks and dealers based on
protocol CDMA2000 (operators in America, Japan, and Korea)
such as Verizon and Sprint Nextel in the U.S., but also with those
using 3G, common in Europe. Consequently, this new product can
be used worldwide in all those areas covered by either UMTS or
GSM, exploiting at a maximum the CDMA (Code Division Multiple
Access) system’s potentialities. This new phone will be produced
by Pegatron Technology, which will release its first samples
in September, just in time to counter its competitors, such as
Symbian, Windows Phone 7 and Android.
TX and RX; AN3, assigned to the
internal A/D converter, is needed
to detect the presence of the 5
volts and therefore the connector
insertion. The serial interface
is at a TTL level and can easily
be connected to a USB converter
USB such as FT232 by FTDI, in
order to interface the microcontroller
with a PC equipped with
USB. For the interface, you can
use FT782M, a small module
produced by Futura Elettronica
(www.futurashop.it), already
58 Aprile 2010 ~ Elettronica In
equipped with a pin-strip connector
at a pitch of 0.10 inches
that can be directly inserted in
our circuit, on the TTL connector,
a female SIL at a pitch of 0.10
inches.
The PC can be programmed
through a specific utility we have
included which was specially
designed for Windows: it is a
simple program that can downloaded
free of charge from our
website www.elettronicain.it and
which, once it has been installed
and launched, provides access to
a control panel from where the
user can enter and modify data
by simply clicking on the various
boxes.
Still regarding the device’s configuration,
it is important to note
that the corresponding data are
not saved in the microcontroller’s
EEPROM, but in an external
memory chip, dubbed 24FC256-
SN; it is a 256- kbit EEPROM
CMOS with serial access, and
with an I²C-Bus interface. In order
to communicate with such chip,
the microcontroller initializes
its I/O lines RD4 and RD5, used,
respectively, as SDA (data line)
and (clock line). Moving the remote
control’s configuration data
into an external memory allows
us to exploit the entire internal
EEPROM to enhance its available
functions. Let’s now take a
look at the section related to the
identification of DTMF bi-tones,
performed with the aid U7, an
MT8870 chip capable of discerning
standard 16 bi-tones thanks to
a complex scheme of active filters
put together through the clock
signal generated by its internal
oscillator, which is connected to
pins 7 and 8 as well as to quartz
Q2. Whenever it identifies a
bi-tone, U7 displays, in binary
format, the bi-tone’s corresponding
number on outputs Q1 (least
important bit) Q2, Q3 e Q4 (most
important bit); all the numbers
from 0 to 9 are represented with
binary combinations from 0000
to 1001, while A, B, C, D, * and #
are expressed with the remaining
combinations, going from 1010
to 1111. The MT8870 chip has an
output latch that helps maintain
the logical combination corresponding
to the last read bi-tone
on Q1÷Q4; in order to avoid that
the device may be deceived in the
event that two like bi-tones are
received consecutively, the chip

is equipped with an STD line (pin
15), which changes from one to
a logical zero when a bi-tone is
identified. Our microcontroller
reads the STD through line RD1
so that it can tell whether like
bi-tones have been received consecutively
and correctly identify
possible codes sent out by the
cell phone. Note that the MT8870
input has been connected to the
GSM audio output precisely
because those bi-tones transmitted
from a landline phone or cell
phone travel on phone channels
rather than on the data line.
PRACTICAL REALIZATION
Now that we are familiar with
the structure and functions of the
base circuit, we can look at how
it is built; let us state in advance
that its realization presupposes a
double-faced printed board with
metallic holes –a choice we made
in order to be able to reduce the
overall size of the device. As a
result, you need to have a 20÷25
W solder with a very fine point,
as well as thin alloy thread (max.
0.20 inches in diameter). As for
the printed circuit, you can find
the traces corresponding to the
two faces on our very site www.
elettronicain.it; if you should
be unable to create it on your
own, keep in mind that there are
various companies that can do it
for you at a fairly low cost (e.g.,
MD srl www.mdsrl.it). The first
component to be assembled is the
microcontroller, which should be
placed centrally on relevant pads
and secured by lightly soldering
one electrode on each side; while
soldering, care must be taken in
order not to cause the neighboring
pins to short-circuit (the space
is quite small). Use the same
technique for the other integrated
circuits as well, but only assemble
those you actually need; note that,
unless you intend to use all its
functionalities, this card should
be assembled after you have
read the articles that illustrate its
specific functions. However, keep
in mind that, overall, you need
DS18B20 only if you want to use
the thermostat function , while
8870 is necessary in order to use
the remote control via DTMF bitones
(DTMF key function).
Once all the soldering has been
done and you have made sure no
pads were joined by mistake, you
can take care of the components
to be assembled in a standard
way, that is, rectification diodes
and LEDs, button, pin-strips, and
relays, starting with those with
lower profile and working according
to the polarities indicated
by the assembling drawings
illustrated on these pages. Once
everything has been set up, you
need to insert deep in the relevant
connectors the module containing
the GSM cellular (which we will
discuss next month). In order for
the remote control to work, there
must be a power generator able
to supply between 5 and 32 volts
- 800 mA, or a lithium-ion battery
of 800÷1.000 mA/h.
Please stay tuned for the next
issue! g
where to buy
All the components used for this
project can be easily found. However,
in the next few months, it will be possible
to find kits for the various applications.
For the time being, those who
want to create their own systems can
get the “heart” of the device, that is,
the GSM/GPRS module SIM900
quadriband (850/900/1800/1900
MHz) for 48 Euro, including VAT. This
module doesn’t need any connectors
and can be soldered to the circuit just
like a standard integrated module in
SMD.
The material can be requested at:
Futura Elettronica, Via Adige 11,
21013 Gallarate (VA)
Tel: 0331-799775 • Fax: 0331-792287
http://www.futurashop.it
Elettronica In ~ Aprile 2010 59