Thank you all for your responses and suggestions!!!  I will replace the reed switch with a new one and check.

Thanks for your input  .  I have tested it with multimeter and confirm its a reed switch.  If the magnet is near to it, I can see the continuity in the multimeter.  Now the question is, are there any chances that it may go defective or malfunction at higher speeds?   

Yes, Its a treadmill and sensor has only two wires.  Is it a reed switch?

This sensor is fixed to the frame just next to the pulley and pulley has a small round magnet fixed to it.  Is it some kind of hall effect sensor? And what are the chances that it may go bad?  It controls the speed of the motor and the problem I am facing is the speed fluctuation.  Not sure if this sensor is the contributor to the problem or something else.  Any help is greatly appreciated and thanks in advance.

You need to drive the relay with a transistor.  You can find the driver circuit herehttp://arduino.cc/playground/uploads/Main/relays.pdf

Thanks for your quick response! In the data sheet Junction to case thermal resistance mentioned as 0.9 degrees C/W.  Considering its generating 5W, can I assume the heat it will produce is 0.9 * 5,  i.e 4.5 degrees centigrade?   is it correct? or I have to multiply with junction to ambient thermal resistance i.e, 50 degrees C/W? in this case the heat its going to produce is 250 degrees centigrade.

Hi There!

I will be using a high power TRIAC BTA41 to turn on/off a 1 HP motor.  This TRIAC is capable of handling the load up to 40 amps.  As per my calculation load current will be around 5 amps and watts will be 1100 (5amps * 220VAC).  What I would like to know is, do I need a heat sink attached to this TRIAC to handle this load?  If yes, how to calculate the dimensions of the heat sink.  Datasheet of BTA41 is attached. Thanks in advance!

My two cents..  You can monitor the current by using Arduino.  You need a shunt resistance in series with the power supply and measure the voltage across the shunt resistor (This is how the ammeters are build).  Of course you need to calibrate that to get the correct values.  You can refer the link http://www.reuk.co.uk/Make-a-Shunt-Resistor.htm if you want to build your own shunt.  Once your arduino starts reading the current flow you can turn on or off the relay at the desired values.

Thank you both for your detailed explanation, you answered what I need!!!

Hi there!  I never used the motors before so I need to know, What is the difference between Stepper and Servo motors and where we use them.  For making POV(persistence of vision) what kind of motor I need to use and what should be the minimum RPM of the motor is needed?

Your power source is not sufficient to drive the new motors.  Do you know the voltage rating of the new motors?

If its wired correctly, the two possibilities I can think are either first power supply is 30V or you are measuring the voltage between + and - instead of common.

Are you driving the relay with the transistor?  Please post your schematic.

Instead of using transistors to drive LED's,  ULN2803 is used.  Its easy and no complexity compared to using the transistors.

What are the voltages you are seeing at base of BC547 when its active and inactive?