Archive for category Projects

Servo motor, Serial COM and 8051

In this project I control the angular position of a servo motor by using the serial communication ability of the 8051 microcontroller. To state it a bit more simply ..I control the angle by which the shaft of the servo motor rotates by pressing keys on my laptop. This project might be helpful to you, if you are planning to build a robotic arm someday and want to control it using your computer.

List of things I am using here:

  • 8051 microcontroller board.
  • A Servo motor.
  • Connecting wires.
  • Computer (..obviously :P)

The connections are really very simple. Choose any of the 32 input/output pins available on your microcontroller board and connect it to signal wire of the servo motor.

A servo motor has just three wire. One for Vcc, one for GND and a signal wire. They are generally colour coded. On the motor which I have, the colour codes are as follows:

  • Red – Vcc
  • Brown – GND
  • Yellow – Signal

In case you have any confusion regarding the colour codes for your motor,  it is always a good idea to just google the model of your motor and clear your doubt.

Once you are done with these connections, thats it  .. now all that needs to be done is to program the microcontroller.

In case you have used an Arduino board before, you might know that there already exists a library for controlling the Servo motor which makes the task trivial. But in the case of 8051 the it is a bit more complicated. Its easy but some math needs to be done.

The position of the servo motor depends on the width of the pulses that it receives on the signal wire.And these pulses need to have a time period of approximately 20ms. After doing some research on the web i found the “on” time of the pules required for various positions(.. applicable to my servo).

  • 0.388 ms – 0 degrees
  • 1.25ms – 90 degrees
  • 2.14ms -180 degrees

Next we need to create a loop that runs for a fixed amount of time and depending upon what is the width of the pulses required we can run it multiple number of times. The following code accomplishes it.

To give an example of how i use this code, say I want to set the angular position of the servo to 0 degrees. In that case i need to provide the pulses with the width of 0.388 ms.So i do it this way.

  • Set the output 1
  • Call set_timer(8) 8 * 50 = 400usec = 0.4ms ~0.388ms
  • Set the output 0
  • Call set_timer(400 – 8) // because 400* 50us = 20000us = 20ms.

And since the calling function is itself inside a loop that goes on forever unless you receive a character serially, we are able to provide the pulses every 20ms on the signal line that sets the servo at angular position zero.

The following function receives the angle that needs to be written on the servo and calls the set_timer() function shown above from within.

0,90 & 180 are the constants denoting different angles.

RI = 0 when no data is received  on the serial line and RI = 1 when data has been received successfully. So once data is received, it breaks out from the loop and exits to return to the main() function, as the write_pos() was called from main.

Now in the main function we do all the initializations, like setting the baud rate etc needed for serial communication.

To decide what value needs to be put in TH1 for a particular baud rate there is the following formula if the mode is 1:

TH1 = 255 – ((Crystal freq / 384)/baudrate).

Thats it then..when you load this program onto the microcontroller you ll be able to control the position of servo motor using the keys ‘1’, ‘2’ and ‘3’ on your keyboard.Check out the video here

Here is the final code:

//Servo motor position control using serial    communication

#include <reg52.h>
#include <stdio.h>

void write_pos(int position);
void set_timer(int x);

sbit servo_signal1 = P1^0;

int main (void){

unsigned char ch;

SCON = 0x50; //
TMOD = 0X22; //
TH1 = 244;   //
TR1 = 1;     //
TI = 1;      //
RI = 0;      //

ch = SBUF;
RI = 0;

printf(“%c “,ch);
while (1){

case ‘1’:
write_pos(0); // Set Servo position to 0 degrees
case ‘2’:
write_pos(90); // Set Servo position to 90 degrees
case ‘3’:
write_pos(180); // Set Servo position to 180 degrees
while(!RI); // wait till you get a valid input
RI = 0;

ch = SBUF;
printf(“%c “,ch);



void write_pos(int position){
int msec;
case 0:
msec = 8;
servo_signal1 = 1;
servo_signal1 = 0;
set_timer(400 -msec);}
RI = 0;
case 90:
msec = 25;
servo_signal1 = 1;
servo_signal1 = 0;
set_timer(400 -msec);}
RI = 0;
case 180:
msec = 40;
servo_signal1 = 1;
servo_signal1 = 0;
set_timer(400 -msec);
RI = 0;

void set_timer(int x){

int i;
// To run the loop given below once it takes approx 50us
// Since I have already used the timer 1 for serial communication, I use
// the timer 0 to give PWM signal to the servo motor.


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Android controlled Arduino bot

This is a project which me and a friend of mine ( attempted long time back.

It is a robot which is controlled by a cell phone.

It makes use of the Arduino and the android platform. It’s a simple bot which has an arduino board, powered by the Atmega328 microcontroller, as the brain. However the microcontroller receives its command from an Android phone.

The inspiration for the project was to acquire the ability to affect the physical environment around us using a device to which everyone has access to ….Cell phone.

The following components were used in making it:

  • Arduino boards
  • RF Transmitter/Receiver
  • L293D motor driver
  • Motors
  • Laptop
  • Android phone

So….this is how it works..

The first stage of this project is to have a graphical user interface with which the people can interact. As the robot, at this stage, only needs to maneuver in two dimensions, the GUI is not as impressive as it would need to be for more advanced applications. Currently it is just a group of 5 buttons namely…front, back, right, left and stop.

The real action is however the one that cannot be seen. This was the tough part actually. Since none of us was well versed with java and Android programming, it took a fair amount of searching and learning in order to get the communication set up. Right now only the bare basics of communications have been included in the project.

When the application is run, it first checks to see if the Bluetooth on the phone is enabled. If not it asks permission to enable it.

Next the list of the devices with which the phone had previously paired with is presented. Since the app is in a very crude stage, the ability of searching for new devices has not been included.

Once the device is selected it establishes a link with it. Here the other device with which the phone communicates is my laptop. The laptop acts as a server and has a python script running which bridges the link between the Bluetooth and the USB serial port.

Once the link is established, the user can press one off the buttons to maneuver the bot. Behind every button click there is a listener which then takes a byte code representing that button and sends it to the server.

The server is a Python script running on the laptop. It is nothing but a tunnel which directs the input from the bluetooth to the serial port.

The server then forwards this number to the serial port.

The serial port has an arduino board attached to it which is further connected to the RF transmitter.

The transmitter was in turn attached to another arduino board. To work with RF communication we used the VirtualWire library which is very well documented and made the job really easy.

The transmitter then forwards this code to the receiver on the bot. On getting the data from the RF receiver the microcontroller calls upon the appropriate functions which set the inputs of the Motor driver ic L293D to HIGH & LOW values correspondingly to maneuver the robot.

As you might have noticed, we had to take the longer route here. Since we dont have a bluetooth module for direct contact with the arduino, we had to use the laptop as an intermediate (with the module the laptop wont be required at all).

You can check out the video of what we ended up with at: Here ‘s the video

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