INDEX

Here you can see that the voltage is varied by using the resistors. The voltage,Vout=1.25(1+R2/R1). Here R2 will become R2//R21 if first sensor output will become high. You can remove R2 from the circuit. But in that case voltage will become low. When R21=1 then the voltage should drop to
some 9V. Assume that R2=9R1. So when no sensors in black strip Vout=12.5 and motor will run at the maximum speed. When R21=1, ie first sensor is at black then voltage should drop some less say 9V. i.e, Req=R2//R21=R2R21/(R2+R21) is such that Vout=1.25(1+Requ/R1).
 

So adjust the Req so that the desired speed reduction is obtained for each sensors. Suppose you make Vout=9V when first sensor go out, Vout=6V for second and Vout=4.5 for third one and Vout=2.5 for the last sensor. You first decide the value of R1 and R2 and from that decide the values of R21,R22,R23,R24. If your line follower is still loosing the line decide which point it is loosing line and which sensor is going to dark line. Adjust Vout for that sensor. You adjust the resistance values such that line follower won't loose the line and it will go with maximum speed. Suppose if two sensors are in dark(such condition normally won't exist), then Req=R2//R21//R22(assume first two sensors are in black). If you have want speed and good braking then add one more sensor with a relay such that good breaking occurs. Suppose if S4 is in black and due to speed or some problems your line follower leave the line,then add one more sensor such that its output is driving a relay and the
common of relay is connected to the one end of motor(ground end shown in above diagram) and the other ends are connected to ground and Vcc(12V). The default position of the relay should be ground so that motor should run properly. Suppose the S4(sensor 4) left the black line, then sensor connected with relay enters the black line and it activates the relay and Vcc(12V) will reach the ground end of the motor. At that time the voltage at other end(LM317 output) will be around 2.5V and will become 12V because Sensor 4 left the black line. Thus both ends get 12V and this is
(1,1) condition of H-bridge and braking of the motor occurs.