Measure RPM - Optical Tachometer

This Instructable will show you how to make a Portable Digital Optical Tachometer using an Arduino Uno.

Instead of a slotted sensor , it has a reflection based sensor. So :

1. You don't have to worry about the thickness of the rotor

2. The number of blades won't change your readings

3. It can also read the RPM of drum style rotors which slotted sensor can't

What is a tachometer ?

A tachometer is a device used to measure the RPM or Revolutions Per Minute of any rotating body. Tachometers can be contact based or non-contact ones. The non-contact or contact-less optical tachometers usually use laser or Infrared beam to monitor the rotation of any body. This is done by calculating time taken for one rotation.

FEATURES

It can measure RPM over 20k

Sensor range extends upto 7~8 cm

Displays Maximum RPM when left Idle

Automatically toggles modes from "Idle" to "reading"

Can be adjusted to match the ambient lighting conditions

It is comparatively cheap and easy to build

Can work without an LCD

Programmable and supports customization

Connect an SD card to the Arduino to keep logs

Downloads

LCD_TACHOMETER.ino

Part List :

Electronics

Arduino
Resistors - 33k , 270 ohm , 10k potentiometer
LED - blue
IR LED and Photodiode
16 x 2 LCD
74HC595 shift Register
Ribbon cable ( 3 wire )
Perfboard and headers

Tools and Hardware

Soldering Iron
Solder
Pins
Screws
Motors and DC fan

Build the Sensor

For the sensor you'll need an IR LED and a Photodiode.

1. Start by sanding the LED and photodiode to make it flat ( do not sand it too much or you'll destroy it ).

2. Then fold a strip of paper sheet as shown. Make two such Structures so that the LED and Photodiode fit tightly into it. Joint these together by glue and paint them black.

3. Insert your LED and Photodiode in them in such a way that the positive ( longer ) lead of the LED is right above the shorter lead of the photodiode.

4. Glue them into the cover using superglue and solder the positive ( longer ) lead of the LED to the shorter lead of the photodiode.

5. Solder the 3 wire ribbon cable to the remaining leads

In my case :

1. Orange wire --> LED's positive pin and photodiode's shorter lead

2. Yellow wire --> photodiode's longer lead

3. Green Wire --> LED's ground pin

You're ready to make the board >>

Making the Sensor Board

Take a small piece of Perfboard and place the components according to the schematics.

The resistor values may vary depending on what kind of photodiode are you using.

The potentiometer helps in reducing or increasing the sensitivity of the sensor.

Finally solder the sensor wires as shown and solder 3 headers.

The headers ( in order ) are shown on the left side of the schematic.

make a cuboidal paper tube whose length is equal to the sensor wires.

The 3-pin LCD

This method uses a 8-bit shift register 74HC595 with a 16 x 2 LCD. Normally this LCD uses 6 pins but using a shift register reduces the pin requirement by 3.

The full instruction guide and the library can be downloaded from THIS WEBSITE !

## Recent Update : The library provided on the website has a lot of errors and conflicts. I've uploaded my version of enhanced ShiftLCD library. I recommend you to use the one attached below.

The only thing that I've changed is :

instead of going for (2, 4, 3) configuration I've used (8, 10, 9)

So be sure to change the pin mapping accordingly

Downloads

ShiftLCD.rar

Make the Box

You can use any type of case for this but I've used a piece of cardboard to make enclosure.

Cut the cardboard as shown and cut appropriate sized slits for the USB port , power jack and the sensor board.

Mount the Arduino on the platform using screws.

Attach the sensor and push it through the hole.

Connect the LCD to Arduino as shown.

Close the box and paint.

Finishing Touch

Make a small ( 5mm ) hole to fix the status LED. Solder a 270 ohm resistor to the LED and insert it into pin 12 on Arduino.

Fold the cardboard along the lines to complete the enclosure. Keep the folds in place by using pins.

Cover the sensor with a cubical paper tube to give additional mechanical strength.

Place the LCD module over the box.

Your device is ready for calibration and programming.

Program

Code

COPY THIS CODE INTO YOUR ARDUINO IDE

#include<ShiftLCD.h>

ShiftLCD lcd(8 ,10 , 9);    // DEFINE LCD PINS

volatile byte REV;       //  VOLATILE DATA TYPE TO STORE REVOLUTIONS
 
unsigned long int rpm, maxRPM;  //  DEFINE RPM AND MAXIMUM RPM
 
unsigned long time;         //  DEFINE TIME TAKEN TO COVER ONE REVOLUTION
 
int ledPin = 12;           //   STATUS LED
 
int led = 0,RPMlen , prevRPM;  //  INTEGERS TO STORE LED VALUE AND CURRENT RPM AND PREVIOUS RPM
 
int flag = 0;             //  A VARIABLE TO DETERMINE WHETHER THE LCD NEEDS TO BE CLEARED OR NOT

long prevtime = 0;       //  STORE IDLE TIME TO TOGGLE MENU
    
 void setup()
 {
     Serial.begin(9600);   // GET VALUES USING SERIAL MONITOR
     
     lcd.begin(16, 2);     // INITIATE LCD
     
     attachInterrupt(0, RPMCount, RISING);     //  ADD A HIGH PRIORITY ACTION ( AN INTERRUPT)  WHEN THE SENSOR GOES FROM LOW TO HIGH
     
     REV = 0;      //  START ALL THE VARIABLES FROM 0
     
     rpm = 0;
     
     time = 0;
     
     pinMode(ledPin, OUTPUT);
     
     pinMode(3, OUTPUT);           
     
     pinMode(4, OUTPUT);
     
     digitalWrite(3, HIGH);             //  VCC PIN FOR SENSOR
     
     digitalWrite(4, LOW);              // GND PIN FOR SENSOR
     
     lcd.print("TACHOMETER");           //   STARTUP TEXT
     lcd.setCursor(0, 1);
     lcd.print("- ELECTRO18");          //  THAT'S ME
     delay(2000);
     lcd.clear();
     
 }
 
 void loop()
 {
  long currtime = millis();                 // GET CURRENT TIME
  
  long idletime = currtime - prevtime;        //  CALCULATE IDLE TIME
    
    if(REV >= 5 )                  //  IT WILL UPDATE AFETR EVERY 5 READINGS
   {
     
             
     if(flag==0)                     //  CLEAR THE LCD TO AVOID ANY GARBAGE TEXT
     {
       lcd.clear();
       lcd.print("SENSOR MEASURING");
       flag=1;                          //   AFTER FLAG = 1 , THE LOOP WILL NOT EXECUTE AGAIN
     }
     
     rpm = 30*1000/(millis() - time)*REV;       //  CALCULATE  RPM USING REVOLUTIONS AND ELAPSED TIME
     
     if(rpm > maxRPM)
     maxRPM = rpm;                             //  GET THE MAX RPM THROUGHOUT THE RUN
    
     time = millis();                            
     
     REV = 0;
     
     int x= rpm;                                //  CALCULATE NUMBER OF DIGITS IN RPM
     while(x!=0)
     {
       x = x/10;
       RPMlen++;
     }       
          
     
     
     if(RPMlen!=prevRPM)                             // IF THE RPM FALLS TO A LOWER NUMBER WITH LESS DIGITS , THE LCD WILL GET CLEARED
     {
       lcd.clear();
       prevRPM = RPMlen;
       flag=0;
       lcd.print("SENSOR MEASURING");
     }
     
     lcd.setCursor(0, 1);
     lcd.print(rpm,DEC);                        //  PRINT RPM IN DECIMAL SYSTEM
     
     lcd.setCursor(6,1);
     lcd.print("RPM");
     delay(500);
     
     prevtime = currtime;                        // RESET IDLETIME
    
   }
   
   if(idletime > 5000 )                      //  IF THERE ARE NO READING FOR 5 SEC , THE SCREEN WILL SHOW MAX RPM
   {
     
     if(flag==1)                            // CLEAR THE LCD
     {
       lcd.clear();
       flag=0;
     }
     
     lcd.clear();
     lcd.print("MAXIMUM RPM");
     lcd.setCursor(0, 1);
     lcd.print(maxRPM,DEC);                     // DISPLAY MAX RPM
     lcd.print("   RPM");
     delay(2000);
     lcd.clear();
     lcd.print("IDLE STATE");
     lcd.setCursor(0, 1);
     lcd.print("READY TO MEASURE");
     delay(2000);
     prevtime = currtime;
   }
     
 }
 
 void RPMCount()                                // EVERYTIME WHEN THE SENSOR GOES FROM LOW TO HIGH , THIS FUNCTION WILL BE INVOKED 
 {
   REV++;                                         // INCREASE REVOLUTIONS
   
   if (led == LOW)
   {
     
     led = HIGH;                                 //  TOGGLE STATUS LED
   } 
   
   else
   {
     led = LOW;
   }
   digitalWrite(ledPin, led);
 }
//////////////////////////////////////////////////////////////  END OF THE PROGRAM  ///////////////////////////////////////////////////////////////////////

Explanation and Calculation

This program basically monitors the IR sensor's value constantly and with the highest priority using Interrupts.

The Arduino Uno has 3 interrupts and the Interrupt 0 is pin 2 on the arduino.

attachInterrupt(0, RPMCount, RISING);

This line attaches an interrupt to pin 2 on arduino in "RISING" mode. This means that whenever the sensor goes from LOW to HIGH , the function RPMCount(); is invoked.

This means that in one revolution , the function will be called twice ( REV++ ). Therefore actualREV = REV/ 2.

rpm = 30*1000/(millis() - time)*REV;

To calculate the actual RPM, we need the time taken for one revolution. And (millis() - time) is the time taken for one full revolutions.

In this case , let t be the time taken for one full revolution , so the total number of revolutions RPM in 60sec ( 60*1000 millisecond ) is :

rpm = 60*1000 / t * actualREV => rpm = 60*1000 / (millis() - time ) * REV/2

OR rpm = 30*1000 / (millis() - time) * REV;

Testing and Troubleshooting

Testing :

1. Take a DC fan and stick a white tape to one of it's blades. Place the sensor 2~7 cm from the blades

2. The readings will appear on the LCD

3. If the sensor gets no readings for 5 sec then it will automatically display the idle screen

4. The Idle screen will display the maximum RPM reached in that particular run.

TROUBLESHOOTING :

1. If the status LED is not blinking, try to adjust the potentiometer until the sensor is able to get readings

2. Ambient light may sometimes interfere with the sensor. Decreasing the sensitivity would eliminate the chance of getting false readings.

3. Check the polarity of the photodiode properly.

4. If everything fails , check your sensor manually by using :

Serial.println( digitalRead(2) ) ;

if your sensor doesn't show " 1 " when any object is placed in front of it then try increasing the value of 33k resistor.

Conclusion

Though there are many optical tachometers available in the market, this device is comparatively cheap and works quite well. I've tested it above 20000 RPM and it works every time ! Being open source and programmable , there arise infinite possibilities of customizing this project.

Feel free to ask anything about this project. Suggestions , queries , corrections and "grammatical errors" are welcome !

Happy Tinkering :)