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Science project idea

Two-gate speed meter that times a toy car with infrared sensors

Speed is distance divided by time. Two infrared beams a known distance apart start and stop a timer as a car passes, so its speed can be found to a good accuracy and compared with a stopwatch.

Make it yourself8 numbered steps, the materials and the science, all free.Go to the stepsGet a ready-made kitReady-made models, kits and spare parts in the store under Physics.Browse PhysicsAsk on WhatsAppAsk the head office whether a model or kit for this idea is available.Send a message

What it shows

Speed is distance divided by time. Two infrared beams a known distance apart start and stop a timer as a car passes, so its speed can be found to a good accuracy and compared with a stopwatch.

The science

Each sensor output goes low when something blocks its infrared beam. The sketch stores micros() when the first gate triggers and again at the second gate. With the gates 50 cm apart, speed is 0.5 m divided by the time in seconds. The average speed between the gates is what is measured, and with a ramp the car speeds up, so it is the speed at the middle of the gates that is found.

Materials

Cost: under Rs 1,500. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.

  • An Arduino Uno or compatible board
  • Two infrared obstacle sensor modules
  • A 16 x 2 I2C LCD
  • A toy car and a ramp (a board and books)
  • A metre ruler and tape
  • A breadboard and jumper wires

Safety

For this project: USB power only.

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 rules

Step 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.

  1. Fix the two sensors to the sides of the track exactly 50 cm apart, using a ruler, facing the path of the car.

  2. Connect each sensor to 5 V and GND, and the outputs to pins 2 and 3.

  3. Connect the LCD to 5 V, GND, A4 and A5.

  4. Use attachInterrupt on both pins so that the exact time is saved even if the car is fast.

  5. Calculate time as the difference between the two stored values and speed = 0.5 / time in metres per second.

  6. Print time and speed on the LCD and wait for reset by a button.

  7. Release the car from three different ramp heights and record the speeds.

  8. Plot speed against ramp height and compare with the theory that v = square root of 2 g h, less friction losses.

What you should see

The LCD shows car times and speeds, with higher ramps giving higher speeds that agree roughly with the energy sums.

Viva questions

A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.

1What is being measured?

The average speed over the 50 cm between the two gates.

2Why use interrupts?

They record the time exactly when the beam is broken, even if the program is busy.

3Why is measured speed less than the ideal?

Friction and the energy in the turning wheels take away some of the energy.

4What happens with a longer gap?

The speed is averaged over more of the motion and the timing error is a smaller fraction.

Free ebook: Viva Questions for Science Projects

Take it further

Use three gates to measure the acceleration of the car on the ramp.

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.

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