Frequency and RPM Counter

Measuring frequency is one of the prime requirements for many applications. The most obvious method of measuring frequency is using CRO (or now a days using DSO). But this instrument is not handheld or available with all the students or hobbyists at any time. It is actually laboratory instrument and not the portable one. Also it is costlier. Also conventional CRO does not give the direct frequency value read out. One has to first set the waveform then find time/division and finely calculate time period and frequency. So it’s a long process that takes time. Another way is use frequency counter that will give us direct digital readout of frequency. In this no need of first adjusting the waveform into screen then find time / division and then calculate time period and frequency such as CRO. Just apply the signal input and get the read out of frequency. That’s why frequency counters finding their own place in measuring instruments.

RPM counter also finds its importance in so many industries as it measures current RPM of any motor. Measuring current RPM of the motor and based on that take decision like to increase RPM or decrease it, is the prime requirement. In certain application it is required to maintain the RPM of motor within desirable limits. So in that the RPM is continuously measured and taken as feedback. If this value deviates much out of the limits, immediate actions can be taken.

So here I am presenting a single circuit to measure frequency as well as RPM. This circuit can alternatively measures frequency or RPM using a switch. If switch is connected to

· direct signal input – it measures frequency

· Sensor (opto-interrupt) output – it measures RPM.

Also it measures either frequency or RPM only once or in continuous loop after every 5 second. Let us see some other features of this circuit

· measures frequency from 50 Hz to 65 KHz

· measures RPM from 5 to 9999

· uses non contact type RPM sensor

· digital output with text on 16×2 LCD

· option for continuous measurement

Let us first see the block diagram of the system

System block-diagram:

The major building blocks are wave shaping circuit, sensor, micro controller and LCD

Wave shaping circuit: – It converts any input signal to pulses (unipolar square or rectangular) wave. Its output is between 0 to 5 V. So actually it converts the input signal shape into the type of shape that can be recognized by micro controller.

Opto interrupt sensor: – it converts each revolution of motor into a pulse. So we shall directly get the results as number of revolutions = number of pulses.

Micro controller: – it counts number of pulses (either from sensor or a direct signal) per fixed time (1 sec or 10 sec) and calculates frequency or RPM and displays them on LCD

LCD: – it is used to display text messages as well as the read outs of either frequency or RPM

Along with all these there are two SPDT switch inputs that selects frequency or RPM counter and decides to take single measurement or continuous in loop. One LED is also provided to indicate counting process is on.

Circuit Diagram & Connections

Now let us see the complete circuit.

Connections: –

· As shown in figure from the two input connections of SPDT switch SW1 is the direct frequency input.

· Another input of SW1 is connected with output of opto-interrupt sensor MOC7811. For more details or the connections of MOC7811 refer the datasheet of it.

· The output line of SW1 is connected to the base input of NPN transistor Q1 through current limiting resistor R1.

· Q1 is configured in switch mode so its output is always 5V or 0V.

· This output is connected to port 3 pins P3.3 and P3.4 that are INT0 pin and T0 pin

· The data pins D0 – D7 (7 – 14) of LCD are connected with port P0

· Two control pins Rs(4) and En (6) are connected with P2.7 and P2.6 respectively

· Read/write pin (5) is permanently connected with ground

· Pins 1 and 2 provides biasing voltage and pins 16 and 15 provides LED backlight

· A 1K pot connected to pin 3 as shown to vary LCD brightness

· One RED LED is connected on P2.5 as shown

· SPDT switch SW2 connected to P2.0 in such a way that it can provide high and low logic input to the pin. Same for SW3 connected to P2.1

· Capacitor C3 is connected between Vcc and reset input pin (9) to provide power on reset

· A push button switch is connected in parallel with C3 to provide manual reset

· A 12 MHz crystal along with two capacitors C1 and C2 connected to XTAL1 and XTAL2 provides necessary clock to micro controller

Working

Working and operation: –

Both the input signals either direct frequency or pulse output from sensor are first fed to wave shaping circuit. As we can see it’s a simple transistor as a switch circuit its output is low when input is more than 0.7 V and it is high if input is less than 0.7 V. so when any type of signal is applied directly (sine wave or any other) it will be converted into pulses from 0 to 5 V. the output of sensor is in pulse form itself but it will be inverted by this circuit.

The pulses are fed to external interrupt 1 and timer0 input pin of micro controller. So micro-controller will start counting these pulses. The LED remains ON till counting finishes. Now here there are two cases

1. If its frequency counter, the pulses are counted for 1 sec time. We shall get direct value as number of pulses per sec.

2. For RPM we know 1 RPM = 60 RPS. So as we get the value of number of revolutions per second we shall multiply it by 60. But because as RPM value is small, I am counting revolutions for 5 sec and then multiply it with 12. So finely we shall get 12×5 = 60 sec = 1 min that means we get revolutions per minute

This value is then converted into ASCII format and then it is displayed on LCD

To change from frequency counter to RPM counter and vice versa, change SW2 and SW1 to gather and then press “RESET” button. So as the program detects change in logic level at pin P2.0, it will change the measuring parameter.

Every time when the counting finishes, the program will check the status of pin P2.1. If its logic 1 that means no further measurement now stop. For next measurement one has to press “RESET” again. If P2.1 is at logic 0 that means continue to take readings after every 5 second.

Software logic: –

The program loaded into micro controller is written in C language and compiled using KEIL (IDE). Complete program is a collection of different functions.

The display function displays value of frequency or RPM in 4 digits. It first separates all four digits convert it into ASCII and display them one by one

Interrupt function calculates external pulses on T0 input pin for 1 sec or 5 sec.

Delay function is variable delay function that generates either 1 sec or 5 sec for counting frequency or RPM

Four functions are given for LCD operation

1. writedata – it sends data to be displayed on LCD screen

2. writecmd – it sends command to operate LCD

3. writestr – it displays various text messages on LCD

4. busy – it waits for LCD to become ready for accepting next data or command byte

Working Videos

Frequency Counter

RPM Counter

Project Source Code

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#include<reg51.h>
#include<string.h>
 
sbit rs = P2^7;                                                 // rs pin of LCD
sbit en = P2^6;                                                            // en pin of LCD  
sbit led= P2^5;
sbit sw = P2^1;
sbit pin = P2^0;
unsigned int flag=0,d;                                                 // rpm flag
 
void delay(int a)                                                          // 1 sec or 5 sec delay
  {
            int k;
            TL1 = 0xBF;                                       // load value 15550=3CBF
            TH1 = 0x3C;
            TR1 = 1;
            for(k=0;k<a;k++)
              {
                        while(TF1==0);
                        TF1 = 0;
                        TL1 = 0xBF;
                        TH1 = 0x3C;
              }
            TR1 = 0;
  }
void busy()                                                     // check busy flag
{
            int x;
            for(x=0;x<1500;x++);
}
void writecmd(unsigned char a)                                 // send command byte to LCD
{
            busy();
            rs = 0;              
            P0 = a; 
            en = 1;
            en = 0;
}
void writedata(unsigned char b)                                 // send data byte to LCD
{
            busy();
            rs = 1;              
            P0 = b; 
            en = 1;
            en = 0; 
}
void writestr(unsigned char *s)                                  // write string / message to LCD
{
            unsigned char l,i;
            l = strlen(s);
            for(i=0;i<l;i++)
            {
                        writedata(*s);
                        s++;                 
            }
   }
void int1(void) interrupt 2                              // external interrupt 1 function
  {
            EA=0;                                                             // first disable interrupts          
            led=0;
            TH0=0x00;                                          // clear T0
            TL0=0x00;
            if(pin==0) d=20;                                             // for freq counter delay is 1 sec and 
            else d=100;                                          // for RPM counter delay is 5 sec
            TR0=1;                                                            // start timer 0
            delay(d);                                              // give 1 sec or 5 sec delay
            TR0=0;                                                            // stop timer     
            flag=1;                                                            // if not then set the flag
            led=1;
      }
void display()                                                  // convert hex to desimal and 
       {                                                               // decimal to ascii
            unsigned int tmp1,tmp2,tmp3,tmp4,t,t1,i,j;
            unsigned char asci[5]; 
            tmp1 = (TL0 & 0x0F);
            tmp2 = TL0 >> 4;
            tmp3 = (TH0 & 0x0F);
            tmp4 = TH0>>4;
            tmp2 = tmp2*16;
            tmp3 = tmp3*256;
            tmp4 = tmp4*4096;
            t = tmp1+tmp2+tmp3+tmp4;
            if(pin==1) t=t*12;                               // for RPM multiply the value by 12 
            i=4;
            if(t>=10000)
              {
                        while(t>=10)
                          {                    
                                    t1=t%10;
                                    asci[i]=t1+0x30;
                                    t=t/10;
                                    i--;
                          }
                        asci[0]=t+0x30;
              }
            else if (t>=1000)
              {
                        while(t>=10)
                          {                    
                                    t1=t%10;
                                    asci[i]=t1+0x30;
                                    t=t/10;
                                    i--;
                          }
                        asci[1]=t+0x30;
                        asci[0]=0x20;
              }
            else
              {
                        while(t>=10)
                          {                    
                                    t1=t%10;
                                    asci[i]=t1+0x30;
                                    t=t/10;
                                    i--;
                          }
                        asci[2]=t+0x30;
                        asci[1]=0x20;
                        asci[0]=0x20;
              }
            writecmd(0x01);         
            if(pin==0) writestr("frequency:");        // display current RPM and
            else writestr("current RPM:"); 
            writecmd(0xC0);         
            for(j=0;j<5;j++)
                        writedata(asci[j]);                     // all four digits one by one
            if(pin==0) writestr(" Hz");
         }
void main()
    {
                        TMOD=0x15;            // timer 0 in 16 bit counter and timer 1 in 16 bit counter
                        P0 = 0x00;                   // P0 as output port                  
                        rs=0;                            // clear all LCD control pins
                        en=0;               
                        sw=1;
                        led=0;
                        led=1; 
                        pin=1;  
                        writecmd(0x3C);                                  // initialize LCD
                        writecmd(0x0E);
                        writecmd(0x01);
                        if(pin==0)        
                             {
                                    writecmd(0x84);                     // initially display message
                                    writestr("Frequency");             // frequency counter
                               }
                        else
                              {    
                                    writecmd(0x87);
                                    writestr("RPM");                     // or RPM counter
                             }
                        writecmd(0xC5);
                        writestr("Counter");                 
                        TH0=0x00;                                         // clear T0
                        TL0=0x00;
                        IT1=1;
                        IE=0x84;                                            // enable external interrupt 1
            next:     while(flag==0);                                              // remain within loop till rpm flag is clear
                        display();                                            // when flag is set display value          
                        flag=0;                                                 // reset the flag to 0 again
                        EA=1;                                                 // again enable interrupts
                        If(pin==0) {d=100;delay(d);}                        // for continuous freq measurement give 
                                                                                    // delay of 5 sec
                        if(sw==0) goto next;                          // again go for next measurement
                        while(1);
            }
 
 
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