Car speed Detector Project

In this project I am going to show you how to measure the speed of running car (or man) from outside. Police department uses this type of system to prevent over speed of vehicles. Our project is a little different from the system used by police but the overall car speed detector concept is same.

In this project, two IR sensors are placed apart on one side of road. When any vehicle crosses the sensors, the internal timer of Arduino counts the time between activation of sensor. Now speed is measured by using simple distance time relationship.


Both IR sensors are connected to the interrupt pin of Arduino, and they detect the falling wave. The purpose of using interrupt is that, it improves the efficiency of system. A LCD is connected to Arduino and measured speed is shown on LCD.

When car moves in front of the first sensor, it gives the output signal to Arduino, Arduino detects the falling wave, now internal timer of Arduino is started and when car moves in front of second sensor timer is stopped.

Now Arduino measures the speed of car which is measured by distance time relationship

                          Speed = Distance ÷ Time 
  • Speed: Car’s speed
  • Distance: Distance between sensors
  • Time measured by Arduino

IR Sensor:

The IR sensor includes an IR LED and a phototransistor. When an object passes between the sensors, light reflects from the object and falls on the phototransistor. An operational amplifier IC (LM358) is used and the phototransistor is connected to it. When object come in front of sensor, it sends a logical HIGH signal to Arduino.


An Arduino Nano is used as the controlling unit, You may use any other Arduino variants according to your choice.

Components Used

IR Sensor2
Power Adaptor12 Volt1

Car Speed Detector Circuit

Circuit diagram is shown in the figure below. If you are going to make circuit on breadboard or general purpose PCB (or Zero PCB), the figure below is useful.

Car Speed Detector Circuit Using Arduino
Car Speed Detector Circuit Using Arduino


For PCB etching,

Car Speed Detector using Arduino - PCB Design
Car Speed Detector using Arduino – PCB Design


Car Speed Detector Circuit Using Arduino
Car Speed Detector Circuit Using Arduino


In the Arduino pins D2 and D3 are interrupt, where D2 is INT0 and D3 is INT1. Output pins of the IR sensors are connected to these pins.

Circuit to measure speed of car
Circuit to measure speed of car


LCD is connected to Arduino’s D4 to D9 pins, where D4 is connected to EN, D5 is connected to RS and D6 to D9 of Arduino are connected to D4 to D7 pin of Arduino. A preset is connected to third pin of LCD, which is contrast control. Pin 15 and 16 are used for backlight of LCD.




Download Program/Code

At the beginning of the code a header file is declared by name “LiquidCrystal.h”, which is used for LCD display. In the next line pins of LCD are declared in the function “LiquidCrystal lcd(4,5,6,7,8,9)”, number in the bracket shows the pins of Arduino that are connected to the LCD.

In the line 4 and 5 two integers are declared by the name sensor1 and sensor2, these are pins of Arduino pins which are connected to IR sensors.

After it 4 integers are declared by name Time1, Time2, Time, and flag. Where “Time1” is the measured time when “sensor1” is activated and “Time2” is the measured time when “senaor2” is activated. The integer Time is the difference of “Time1” and “Time2”, which is equivalent to time taken by car to go from “sensor1” to “sensor2” or “sensor2” to “sensor1”

Now a float is declared by name “Speed” that is measured speed of the running car.

At the starting of “void setup()” two interrupts are attached by function “attachInterrupt(0, fun1, Falling)” that means when “interrupt0 (INT0)” detect the falling wave, fun1 will run.

Now the LCD is begins by using “lcd.begin(16,2)”  function. LCD is cleared by using the “lcd.clear()” function and “SPEED MEASUREMENT” is printed on the LCD by using function “lcd.print”

In the “void fun1()” will run when “interrupt0 (INT0)” activated, in this function current time is measured by using “Time1 = millis()”. After it an “if else” condition is used, it makes the “flag” 1 when it is 0 and makes 0 when it is 1.

After it “void fun2()” is used, that is the exact same as  “void fun1()” but it runs when “interrupt1 (INT1)” is activated.

In the “void loop()” first of all Time is measured by using “Time1” and “Time2”, the “Time” should be positive so “if else” is used for making it positive. But this loop runs when flag is equals to zero so an “if” condition is used. If “Time1” and “Time2” are equals “Speed” will be zero.

In the line 47 an condition activated when speed is equals to zero, at this time “….OK….” is printed on second raw of LCD, that indicates, system is ready to use.

In the line 51 speed is printed on LCD after printing “Time1” and “Time2” will become zero.


In the measurement of speed distance, time relationship is used. So distance between sensors is very important, so we need to calibrate the system.

In the line an integer is declared by name distance that is equal to 27, this is the distance between both sensors. This distance can be varying according to your arrangement, so measure the distance in Centimeter and change here.



  1. would it be possible for me to see how u did ur code? im still quite confused as to how to do the coding

  2. It’s too good sir

    Make more sensor and mechatronics project please sir.

  3. aditya aherkar

    there is no code for arduino project of car speed monitoring
    Sir can you please send me the code

  4. Mark Joseph Lizardo

    Hello. Can I measure the acceleration using this too??

  5. Hi John,

    I would like to clarify with you, how do I go about in continuing your program code, such that 2 LEDs will light up after the object has gone over a certain speed limit?

    Thank You.

  6. How much voltage that arduino needs ? 5 volts ? 18 volts ? Tell me

  7. Brent Martin

    Can I use this Arduino UNO as my motherboard for this project? Because I was thinking to create a speed detector for my thesis. 🙂

  8. Sir,
    Your project is too good, sir in your code i would like to ad servo for over speed. and two more ir can you do this for me please. i am only 11 years old.i would like preset this project in my school.

    another 2 ir for other side of cars, and servo for over speed.

  9. David 70154

    Hello. really nice project here. Thank you for uploading. I tried simulating it on proteus but i am wondering what to use as my IR sensor on the proteus. And what is the use of the flag in the code?