Electronics Projects

Sending SMS using AT Commands

2012-03-17T00:26:00.000-07:00

This article describes sending a SMS from a Microcontroller through a GSM modem using AT Commands. through RS232. AT commands can be sent to a GSM modem via a computer serial port or from the serial port of a 8051 Microcontroller.

Computer serial port

1. Connect your GSM modem to the computer serial port. New systems nowadays dot not have serial port, hence you would need to buy a USB to serial converter and connect the GSM modem to it.

2. If you are using Windows XP® OS, open Programs -> Accessories -> Communications -> HyperTerminal

3. Select the COM port you have the modem connected to.

4. Check the port settings, make sure that the baud rate matches with that of the GSM modem and and also that the Flow Control is set to "None".

5. Type the AT Commands below in the HyperTerminal Editor

AT+CMGF = 1 and Enter
AT+CMGS="+919449XXXXXX" and Enter
"Electronicprojs.Blogspot.com" and Hit CTRL+Z

6. In Windows 7® OS though, there is no hyperterminal program inbuilt. You will have to download a similar one. There are many good free programs available.

7. You would need to note that in this case we are connecting the serial port cable from Female pin (GSM Modem) to Male pin (Computer). Hence no crossing is required. In other words, we should use a straight cable.

8051 MicroController Serial port

1. 8051 SFR's are programmed for a baud rate of 9600.

2. Send the AT commands in #5 above through code. For Enter use escape character "\r" and for CTRL+Z use ASCII 0x1A.

3. The serial port cable is connecting from Female pin (GSM Modem) to Female (8051 Board). So you need a have a female to male converter. Also, make sure that the Rx and Tx inside in the converter are crossed. In otherwords, the Rx pin of GSM Modem should go to Tx pin of 8051 microntroller. This is called a crossed cable.

4. Below is the C program using Keil® Compiler for 8051. We added infinite while loop after the sendsms() routine because otherwise we found that the code compiled was sending the SMS in a infinite loop.

Note : Beginners always complain about their code working through serial port of a computer but the same not working through chip. It is very essential to understand the difference between crossed cable and straight cable as well as the pins configurations of male and female connectors before starting.

C program

#include <REGX51.H> #include <AT89X51.H> unsigned char *command_AT = "AT\r"; unsigned char *command_CMGF = "AT+CMGF=1\r"; unsigned char *command_CMGS = "AT+CMGS=\"+919449XXXXX\"\r"; unsigned char *message = "electronicprojs.blogspot.com"; unsigned char CTRLZ = 0x1A; void puts(unsigned char* ptr); void putc(unsigned char chr); void sendsms(void); void initialize(); main() { initialize(); sendsms(); while(1); } void initialize() { SCON = 0x50; /*SCON: mode 1, 8-bit UART, enable receive */ TMOD |= 0x20; /*TMOD: timer 1, mode 2, 8-bit */ TH1 = 0xFD; /*TH1: for 9600 baud */ TR1 = 1; /*TR1: timer 1 run */
} void sendsms() { puts(command_AT); puts(command_CMGF); puts(command_CMGS); puts(message); putc(CTRLZ); } void puts(char* p) { char *temp = p; /*temp pointer so that the actual pointer is not displaced */ while(*temp != 0x00) { putc(*temp); temp++; } } void putc(unsigned char chr) { SBUF = chr; while(TI==0); /*Wait until the character is completely sent */ TI=0; /*Reset the flag */ }

Global Positioning System (GPS) - Introduction

2012-03-06T06:43:00.003-08:00

Global positioning system (GPS) is a satellite based navigation system that provides the user with accurate position and time information. Today we find GPS receivers in almost every field – right from car navigations systems to missile launches, mobiles to aircraft - GPS is becoming an integral part of our lives. Here is an attempt to make the readers better understand the GPS system and to some extent, the techniques involved in its working

GPS consists of a constellation of satellites (at least 24) that are deployed into orbits of 20000km. The data transmitted by the satellites is a Direct sequence spread spectrum (DSSS) signal with each satellite having a unique PRN (pseudo random noise) code of its own. The satellites are identified by this unique code that is a part of the signal that they transmit. The satellites have Ids from 1 to 32.

The orbits are inclined at an angle of 55 degrees to the equatorial plane. The choice of orbit is such that there are at-least 5 satellites visible to a GPS receiver (with an open sky view) from any point on earth. The satellites orbit the earth with an orbital period of 12hrs. Hence the visibility scenario at any point on earth repeats itself every 24 hrs. It simply means that, if at a particular point on earth, I see satellites with Ids 2, 4, 10, 17, 25, and 20 then I would see the same set of satellites tomorrow at the same time.

The basic idea of GPS positioning – Trilateration

Trilateration is the basic theory behind GPS positioning. Let me explain this in very simple terms.Consider a planar (2D) system in which I have three reference points or base stations (BS) which are transmitting signals and a mobile station (MS) or a receiver capable of receiving and decoding the signal from these base stations. The positions of these BS are fixed and are known to the MS.

Let us assume that the data sent by each of the BS has sufficient information such that the MS is able to calculate the accurate range or distance from the BS i.e. if MS decodes signal from BS1, it can obtain its exact distance from BS1.

Now, I know my distance (d1) from BS1 and as said earlier, I know the location of BS1. This means that the position of the MS is anywhere on the circle centered at BS1 and with a radius d1.

Similarly if the MS can decode the signal from BS2 and obtain the distance d2 from it, then the ambiguity associated with the position of MS reduces to the intersection of the two circles (with BS1 and BS2 as centers and d1 and d2 as radius respectively). Hence the position is either point A or point B as shown in the figure. Extending this idea to one more BS reduces the ambiguity to a point that is the intersection of all the three circles thus formed.

Now that you are convinced with the position determination in 2D space, we can extend the same to position determination in 3D space with MS being your GPS receiver and BS being the satellites in orbit. The range information from one satellite would give you a sphere, two satellites would give you a circle and three satellites would give you two points. One of these points, the true position will be towards the surface of the earth and the other will be on the other side of the satellite orbit and is discarded.

NOTE:

Even though it seems that only three satellites are essential for position determination, in practice 4 are required due to the low precision clock on the receiver (more about this later)
The positioning can also be thought of as a linear system of three unknowns (position co-ordinates x, y, and z) of the receiver, which requires at-least three equations to solve. These three equations are obtained by three satellites.
The fourth unknown called the user clock bias requires the range information from the fourth satellite.

Ultrasonic Transmitter and Receiver

2012-03-04T04:53:00.001-08:00

Introduction

Ultrasound is a form of cyclic sound pressure with a frequency greater than the upper limit of human hearing, this limit being approximately 20KHz. Some animals – such as dogs, dolphins, bats, and mice – have an upper frequency limit that is greater than that of the human ear and thus can hear ultrasound. Ultrasonic waves, however, being inaudible to humans, have little or no effect on the ear even at high intensities. Ultrasonics have found wide industrial use like nondestructive testing an object is irradiated with ultrasonic waves; variation in velocity or echo of the transmitted waves indicates a flaw. Fine machine parts, ball bearings, surgical instruments, and many other objects can be cleaned ultrasonically. Metals can be welded together by placing their surfaces in contact with each other and irradiating the contact with ultrasound. The molecules are stimulated into rearranged crystalline form, making a permanent bond. Ultrasonic whistles, which cannot be heard by human beings, are audible to dogs and are used to summon them.

Transmitter

Working

The 555 IC is used here as an astable multivibrator
It oscillates at a frequency of about 40kHz. An ultrasonic transmitter transducer is used here to transmit ultrasonic sound very effectively
A 10k pot is provided to vary the frequency of the astable multivibrator to adjust for maximum performance
During the positive half cycle npn transistor SL100 is turned on while during the negative half cycle pnp transistor SK100 is turned on effectively doubling the peak to peak voltage fed to the ultrasonic transducer

Design

f = 1 / (0.693 * ( Ra+ 2Rb)*C))
f = 40kHz which is the center frequency of most ultrasonic transducers
Let C = 680pF
R_b = 10kOhm

Let R_a be made up of two resistors, fixed R_a1 and variable V_Raso that
Ra = R_a1+ V_Ra

Let R_a1= 4.7kOhm
Then V_Ra = 16.47kOhm so that Ra ≈ 22kOhm
Practically we use V_Ra = 20KOhm variable resistor and tune it

Note : In our case, the closest capacitor to 680pF available in the market was 560pF. We replaced the same in the circuit and it worked with tuning of V_Ra

Receiver

When there is an obstacle the ultrasonic sound is received by ultrasonic receiver transducer. It converts it to electrical variations of the same frequency

Ultrasonic receiver also uses a two-stage amplifier, a rectifier and an operational amplifier in non-inverting mode. Output of op-amp is connected to a buzzer

These signals are amplified. The amplified signals are then rectified and filtered

The filtered DC voltage is given to non inverting pin of high frequency op-amp CA3140. The inverting pin of op-amp is connected to a variable resistor

The 9v ouput voltage of comparator is made microcontroller compatible by using a 7805 voltage regulator

Circuit Diagram

Ultrasonic Transmitter and Receiver Circuit

People Counting Machine through GSM

2012-03-04T02:15:00.011-08:00

The main objective of this project is to design a system which counts the number of people present inside a premises with the count being sent automatically as an SMS upon dialing the phone number.

It consists of 3 modules

1. Infrared Transmitter and Receiver
There are two of them say IR1 and IR2. Breaking the beam of IR1 first and then IR2 will increment the count by 1 where as breaking the beam IR2 and then IR1 will decrement the count. The output of IR1 and IR2 are connected to microntroller interrupts INT0 and INT1 respectively.

2. Microncontroller with LCD
The microcontroller used here is 89c51. INT0 and INT1 of the micronctroller is connected to output of IR1 and IR2 respectively. It is connected to a GSM module via a serial port. And the microncontroller is also connected to a LCD which displays the total count at any given point of time. The microntroller increments or decrements the count based on weather INT0 interrupt occurs first and then INT1 or vice versa and continuously displays on the LCD. Micronctroller will also receive a serial interrupt when there is an incoming call in whch time it will read the phone number, disconnect and send the total count as a SMS

3. GSM Module
The microcontroller sends AT commands to GSM module to read the phone number, disconnect and send an SMS to the calling number. Below are the AT commands used.

1. Disconnect a call
ATH and Enter

2. Read the phone number of an incoming call

Set the below AT command only once

When there is a incoming call, the data from GSM will be similar to below for default settings

+CLIP: "+919449XXXXXX",145,"","",0

RING

+CLIP........

Read the phone number above into an array and send the SMS

3. Send an SMS

AT+CMGF=1 and Enter
AT+CMGS="+919449XXXXXX" and Enter
"Hello World" and CTRL+Z

During programming CTRL+Z requires to be sent as 0x1A Ascii character while the escape character for Enter is '\r'. For better understanding on sending a SMS and C code, refer to this link

Burglar Alert through SMS

2012-02-25T07:29:00.002-08:00

The main purpose of this project is to design a system that would automatically send SMS to a predefined number up on the Infrared beam is broken between transmitter and receiver.

It consists of 3 modules

1. Infrared transmitter and receiver
2. Microncontroller
3. GSM Modem

It consists of a 38Khz infrared transmitter while the receiver is TSOP1738. The transmitter has burst cycles between 10 and 70 cycles as prescribed by TSOP1738 datasheet. The 8051 microntroller is programmed to receive a high to low edge triggered interrupt when the beam between transmitter and receiver is broken. The interrupt routine will send a SMS to a predefined number.

Light Following Robot

2012-02-24T21:14:00.007-08:00

The main purpose of this project is to design a robot which would follow the path of higher luminescence across all directions.It is based on the principle that resistance of a LDR changes with the amount of light falling on it.

It consists of 3 modules

Sensor Circuit

It consists of 4 LDRs' arranged at each side of the robot (front, back, left, right). The resistance of an LDR will decrease with the increase in the luminescence. This variation is fed to a comparator with a reference voltage set. We use LM324 IC which conveniently has 4 comparators on a single chip. In our case we found that each LDR was having different response the same luminescence. For example, for the same indoor daylight the LDR resistances were respectively 6k, 8k, 9k, 10k. To offset this variation, we are using seperate pot for reference voltage for each LDR. You can use a single pot and use the same reference voltage for all 4 LDR's if you find that LDR's are having the same response (i.e shorting pins 3,5,10,12 in LM324). The 4 outputs of comparator is fed to a port of Microcontroller 8051 via port P0.

Sensor Circuit

Microcontroller

The microntroller used here is 89c51. The microcontroller gets the sensor inputs via port P0. The micrcontroller will drive the robot only when one of the LDR's resistance (i.e one of front, back, left, right) is below the threshold and consequently the output of the comparator is logic HIGH. The microcontroller will send the signal for the robot to drive in specific direction via port P1. One thing to note is that the robot can move either forward or backward. In case it moves forward and then right, the robot turns right looking into forward direction. Similarly, if the robot moves in the opposite direction i.e reverse and then right, the robot needs to move right looking into reverse direction (or left looking into forward direction).

DC motors with H-Bridge

The DC motor driver or H-bridge IC used here is L293D. The H-bridge converts the logic voltage 5v into DC motor specific 9v and also takes care of back emf when the DC motor direction reverses. The microcontroller send the signals to H-bridge via P1 based on which the robot moves in the specific direction (or stops). The DC motors used here is 150RPM, considering that there is not much load, higher RPM motors can be used to get higher speed.

Microcontroller H-Bridge circuit

Path Following Robot with Obstacle Detection

2012-02-24T20:16:00.010-08:00

The main aim of this project is to design a robot that can follow a black path without collisions. The black path below the car can be determined using the principle that black color absorbs all radiations and the presence of an obstacle is detected using ultrasonic waves.

The project comprises of four main blocks

Ultrasonic Transmitter and Receiver
Microcontroller
DC motors with H- Bridge
Infrared Transmitter and Receiver

In order to drive the robot, DC motors are used. These DC motors are controlled by the micro-controller- Atmel 89c51.The microcontroller used here is an 8-bit, single supply +5volts operated, HMOS technology, 40 pin IC. To drive the motors we use an H-bridge-L293d which boosts the microcontroller voltage to 12 volts required to drive the DC motors.IR transmitter and receiver is used to track the black path under the car. Comparator IC LM324 is used to interface these sensors to microcontroller.The presence of obstacle in the path is detected by the ultrasonic transmitter and receiver module. If an obstacle is detected in the direction of motion of the car, the microcontroller is signaled accordingly.The microcontroller via the port P1 reads the status of the IR sensors and the obstacle detection system and a decision is made whether to proceed in the same direction or take a left turn or right turn.On reaching the end of the path,the car would rotate until it encounters the black path and continues to follow the line.