The $40 Line Follower - Step 3B: Electronics (Building the Circuit)

The Electronics

  The circuit will be mounted on two different boards- The main circuit board and the sensor board.

First, cut the PC board such that the main circuit board and the sensor board have a dimension of 8cm x 8cm and 8cm x 1cm respectively. 

The Main Circuit Board

Drill four holes at the corners of the 8cm x 8cm PC Board (main circuit board). These holes will be used to attach the circuit board to the base of the robot with the help of the 2cm spacers.


The 8x8 PC Board


Solder a 100uF capacitor as shown in the figure. It is always a good practice to use capacitors whose voltage ratings is twice the voltage in the circuit. Since we will be using 9V in the circuit, I used a capacitor rated as 25V. You may also use a 16V capacitor, anything less than that is not recommended.


Solder the red and black wire of the battery cap to the positive and negative lead of the capacitor respectively. The 100 µF capacitor provides power supply decoupling, that is, it prevents spikes in the circuit. The capacitor plays a very important role because we will be dealing with analog signals in our circuit. 


Next place a 270 ohms resistor in the board in such a way that its leads are inline with the positive lead of the capacitor.



Once the resistor is soldered, connect its lead, which is closest to the capacitor to the red wire of the battery cap.



Then solder a Green LED to the board such that its positive lead is inline with the other end of the 270 ohms resistor. This green LED will be used as a power indicator. Whenever the circuit is powered the green LED will glow. If the LED does not glow it is a clear indiacation that something in the circuit is wrong(most of the time it is a short circuit). The LED becomes really useful during debugging.


Be carefull while soldering LEDs, they are easily damaged by heat

Connect the LED’s positive lead to the end of the resistor closest to it and connect its negative lead to the black wire of the battery clip(Gnd).



Plug in your battery to the battery cap, and if you have done everything right, the green LED will glow.



Unplug the battery from the battery cap and place the DIP socket into the board as shown and solder it. The DIP socket is used as a base to the L293d motor driver IC. Since ICs are damaged by heat, you will have to first solder the DIP socket into the board and then place the IC in the socket. By using DIP sockets in your circuits, you can use the same IC in different circuits.


The next step is to solder pins as shown in the follow images. Refer to the graphical representation, given later in the page, if you are not sure about the connections.




Once the above connections are made, insert four male header pins as shown in the next image(refer to the graphical circuit diagram given later in the tutorial). 


Connect each header pin to the DIP socket’s pin closet to it. 


In the next step we will solder the two 20k presets. The preset we use has three pins. The two pins that are together are the track pins. The pin that is separate from the track pins is the wiper.The presets will be used to adjust the sensitivity of the sensors. 


After soldering the presets to the PC board, connect one track pin of one preset to the track pin of the other. 


Then connect the joint to the red wire of the battery cap. 


Then place two BC548 transistors such that each of their bases is inline with one of the two presets wiper.


  Be careful while soldering transistors, they are damaged by heat.


The next step is to connect the two emitters of the transistors together. I used red wire for connecting the two emitters. 



Similarly connect the two collectors of the transistors together and finally connect the joint to the track pin of the preset.

   Similarly solder two more presets and transistors on the other side of the DIP socket. And connect them as you did previously for the other two presets and transistors.




Then connect the wipers and bases of transistors as shown below with the help of blue wires.




You will have to insert two 1K resistors into the board as shown below, once you have soldered all the four transistors and presets as shown above. Solder the resistor such that its leads are inline with the emitter of the transistor. Connect one end of the resistor to the emitter of the transistor. 



Next insert two red LEDs such that their anodes are inline with 1K resistor’s leads. Solder their anodes to the other lead of the 1k resistor and connect their cathodes to the black wire of the battery cap. 


Once done, cut five 5 cm long wires and add molex connectors to each using the method given at




Solder two of these wires to the 1K resistors. From these wires we will get high or low inputs from the sensors to drive the motor 


Solder the remaining two wires to the red wire of the battery cap.
With this the circuit board is complete. The graphical soldering and wiring diagram is given below.


For polarized components - Capacitors and LEDs – the red pin is the positive lead and black is the negative lead.


The Sensor Module

The Sensor Module comprises of four LEDs to emit light and four LDRs to sense the intensity of light.
For the sensor module we will make use of the 8cm x 1cm PC Board which we cut at the beginning of the tutorial. Normally, 1cm of PC Board would have four rows of holes. Just for the sake of convenience I named the four rows A, B, C and D.



Insert the four blue LEDs into the module such that their anode is in row B and cathode in row C.

Note that all the anodes & cathodes of the LEDs are inserted in the same row.

Don’t cut the leads but bend them as I have done here.


Once the leads have been bent solder them as show below.What I have done here is connected the LEDs in parallel.  


After soldering the LEDs, insert the LDR next to a LEDs such that its leads are in the rows A and D (outermost rows). 


Solder the remaining LDRs in the same manner. 



Once done, solder 10cm long blue wire to one lead of each LDR 


Connect the other leads of all the LDRs together and solder it to a 10cm long black wire. All that we are doing here is connecting one lead of each LDR to the Ground through the long Black wire.



In the nest step, solder 10 cm long red and black wires in the rows B and C respectively. These wires will provide current for the LEDs.



Once soldered to the board connect the red wire to the anode and the black wire to the cathode of a LED.



The sensor module is now complete. A graphical representation of the sensor module and its wiring diagram is given below.




Hi, thank you for your nice tutorial. I have a question.

In your picture, you connceted the joints of the emitters to the right and left of the circuit to the track pin of the presets to right and left of the circuit respectively. However, in your diagram you only connected the joints of the emitter to the right circuit to the track pin of the preset ro right circuit. So, which is the correct way?


and also what program did you use to make the breadboard circuit diagram, Thank you so much ,Eric

Program used for breadboard circuit diagram

I used CorelDraw (yes you read it right, I used coreldraw)