What it shows
Each light dependent resistor switches the motor on the opposite side, so the buggy turns towards the brighter side and drives at a torch. This is a simple analogue form of phototaxis used in robots.
The science
For each side, the LDR goes from the supply to the transistor base, with a 10 kΩ resistor from the base to ground. In bright light the LDR resistance is low, the base voltage rises, and the transistor switches its motor on. A 1N4007 across each motor soaks up the voltage spike when it is switched off. The left LDR drives the right motor, so more light on the left speeds the right wheel and the buggy turns left, towards the light.
Materials
Cost: under Rs 500. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A cardboard or ice-cream-stick chassis
- Two 6 V toy gear motors with wheels and a front castor
- Two LDRs and two 10 kΩ resistors
- Two BC547 transistors and two 1 kΩ resistors
- Two 1N4007 diodes
- A 4 x AA holder with four AA cells (6 V)
- A slide switch, wires, tape and a torch
Safety
For this project: Cells only. Keep fingers away from the wheels. Do not let a motor stall for long: a BC547 handles only about 100 mA, so use small motors, or ask an adult to fit a BC337 if the transistors get warm.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Build the chassis with the two motors at the back and the castor wheel at the front, and fix the battery holder over the axle line.
Mount the two LDRs at the front, pointing forward and outward at about 30 degrees to each other, with a card divider between them.
For the left LDR, connect it from the positive rail to a row, and a 10 kΩ resistor from that row to the negative rail.
Connect that row to the base of transistor A through a 1 kΩ resistor, and the transistor emitter to the negative rail.
Connect the RIGHT motor between the positive rail and the collector of transistor A, with a 1N4007 across the motor, band towards the positive rail.
Repeat for the right LDR with transistor B driving the LEFT motor.
Switch on in a dim room and shine the torch from the left: the buggy should turn left and follow the beam.
If it turns away from the light, swap the motor wires or the motor sides; if it always runs, shade the LDRs with short tubes.
What you should see
The buggy turns towards a torch and follows it across the floor, speeding up as the beam gets brighter.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why does the left LDR drive the right motor?
Speeding up the right wheel makes the buggy turn left, towards the brighter side.
2What is the diode across each motor for?
It protects the transistor from the voltage spike when the motor current stops.
3What is the role of the 10 kΩ resistor?
It forms a voltage divider with the LDR to give the base voltage.
4What limits the motor size?
The BC547 can only carry about 100 mA.
Take it further
Reverse the wiring so the buggy runs away from light and hides in the dark.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
Robotics
Motors, Gears and Motor Drivers
Make things move: DC motors, servos, steppers and how to drive them