How to make a Light Seeking Robot

Why wanna make a light seeking robot ?

1. Easy to make.
2. Basic electronics knowledge is needed.
3. Low cost.
4. Programmable or non-programmable.
5. My first year's project in engineering school.

The robot above tends to move to the source of light. It will also turns to the left only when the left sensor was triggered. The same thing goes to its right motion too. I would like to present two prototypes which i have made previously.

The design above is made during my first year's mini project. 

The list of components given by my instructor are :

1. Simple 6-V DC motor (2 pieces)

2. Resistor 330-ohms (2 pieces)

3. Resistor 100-ohms (2 pieces)

4. LED (2 pieces)

5. Bipolar Junction Transistor 2N3904 (2 Pieces)

6. Light Dependent Resistor (2 pieces)

7. 9-V Battery holder

The circuit schematic which I have simulated using Proteus Professional is shown below :

Finally it works ! Yet it is not as perfect as I expect.

After troubleshooting it, I found it seems to have some problems in the circuit schematic and its overall design. Here's the problems listed :

• The robot moves with a slow pace or doesn't move at all. I believe that must be caused by overweight chassis loaded with extra items like battery. Both of the motor moves at high speed when the whole body lifted up. I put this as the major obstacle as it prevents the overall movement. The solution here is to add in a gearbox set.

•  The gearbox set above could overcome the problem by producing greater torque  against its high speed motor. The gear to gear ratio is the pacemaker in speed                reduction. Hence,regardless the mass of the chassis itself, the movement of the robot      will not be affected.

• The usage of BJT 2N3904 is not practical at all. What I've neglected in the whole project is to inspect the datasheet of every elements in the circuit schematic. The reason is that 2N3904 is unable to produce a large amount of current from the collector to the emitter, which failed to drive the motor when triggered. Besides, it tends to burn out easily when high voltage battery is used. In other words, it failed work perfectly as a switching component in this project.

• When small amount voltage is used to power up the entire circuit, the leds would drain out the current  supplied to the motor. The leds used is to indicate the On- and Off-states of the partial circuit. However, I  felt the usage of leds (especially the super-bright ones) would reduce the current flows to the motor.

After completing the project, I have decided to construct a new circuit schematic of the same scope. Hence, this is the list of components I've used :

1. 2 simple 6-V DC motors
2. 4 Bipolar Junction Transistor 2N2222 (BC 547 or BC 548 can be used as well)
3. 2 6-V DC SPST relays
4. 2 Light Dependent Resistors (LDR)
5. 2 10-kOhm resistors 
6. 2 100-k trimpots
7. 2 regular diodes 1N4148
8. Battery 9-V x 2 with holders

Construct the circuit on breadboard using the circuit schematic below:

       

Next, solder it ! 

Then that's the final product after combining with the new chassis and gearbox set.

The major modification made to the circuit is mainly the model of transistors and its arrangement. I choose to replace 2N3904 (general purpose) with BC 547. Arrangement of Darlington transistors could amplify the output current. Once the first transistor get its current signal at its base, more electron flows into the base of the second transistor. Hence, it could trigger the SPST relay with a small amount of output current.

Next, I remove the indicator leds. Add a red color led if you want to.

Why do I need a set of relay in this circuit ? It gives much more precise output signal than a transistor alone. A device which functioned with basic electromagnetic concept. Whenever there's current flow through the solenoid, the armature will in-turn attracted to the magnet and allow the current in secondary circuit pass through. 

The concept of using of relay is quite similar to Logic Gates. Otherwise, operational amplifier too could be used as comparator (LM 324). Much more changes have to be made on the circuit again.