INDEX

Line Following Robot using PWM

For making PWM without any microcontroller is a tough task. We can use 555 for that.

Here the sensor output is the comparator outputs. Comparator output should be HIGH(5V) or LOW(0) for proper working, so that transistor switching takes place. If you use LDR , connect a series resistor and give this output to Lm324 with other input as a reference voltage,Vref so that
white and black strip are detected.
This is the circuit for one motor. You can connect the output of 555 as one input of H-bridge with other input as ground. But it is better to use other input as Vcc so that breaking will occur fast. I haven't used this circuit because PWM is not fast compared to varying voltage across motor. This is
because of the performance of the motor. The motor is an inductor, so there will be time for charging and discharging, if PWM is so fast then the motor won't get enough time to charge and the torque will be get reduced because it is getting enough time to charge to maximum voltage.
Remember in the above circuit
Ra1+Rb1=Ra2+Rb2=Ra3+Rb3=Ra4+Rb4=R
T=1.4RC2
Here the T=10ms(the one i used), but depends on motor quality, adjust according to your performance and needs. This is the circuit for one motor only. Here we are using four sensors for each side S1,S2,S3 and S4. If you use power transistor sometimes you need a base resistor (I haven't tested this circuit,but this circuit will work). Better to use a power mosfet mostly used one IRF540. Remember that here I assumed that only one sensor will be at the black strip. Suppose if two sensors are in black strip then Ra=Ra1//Ra2=Ra1Ra2/(Ra1+Ra2) and Rb=Rb1//Rb2 ,but Ra+Rb won't
be equal to R. So you should be through with this assumption. You should see all practical possibilities of your track before using this circuit. Thus this circuit uses 8LDR+2 IC555 +L293D as main components. Next way is to vary the voltage across the motor. This is one which is better than PWM. You can vary voltage using zener diodes and variable voltage regulator,LM317. See LM317 for more explanation. LM317 have a good current driving capability more than 1A which  is enough for one motor. The mostly available zener diode have ratings of .5W,1W etc.. Here
we use a DC motor of 250mA.So for a zener diode of 1W it can carry about 4V.