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Microcontroller Programming » Tech-O-Meter

October 19, 2010
by Earnhardt
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I’m a newbie to the microcontroller and this site; I just purchased & got the NerdKits and hope you Gals & Guys could help. I am looking for a fast and easy way use my nerdKit to get RPM information from a rotating motor shaft and then display it on GLCD. I was thinking of using a Hall Effect Sensor. Since I am new at this and in the need to get it done quickly anyone have simple Source Code and parts for the circuit design I could use. Once I get this RPM information to display on the GLCD I could work on changing and trying other things later.

October 19, 2010
by Ralphxyz
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Search the Nerdkit forum for RPM, this has been discussed a lot.


October 19, 2010
by hevans
(NerdKits Staff)

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Hi Earnhardt,

One of my favourite customer projects of all time is the Hamometer described in this forum post. He did pretty much what you are describing, used a hall effect sensor to measure RPMs. Depending on how fast your motor is spinning this might not work though.


October 21, 2010
by esoderberg
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Below are the pertinent sections of code, pulled from a larger program, that I used to determine the RPM and velocity of a wheel. The setup sounds similar to Hamometer. I have two mags 180 degrees out from each other on the wheel shaft with a nearby mounted hall switch that is triggered on or off with each change in N/S polarity i.e. each pass of a magnet. I use an interrupt in the code to note the change from on to off. Timer setup below is from Nerdkits realtime clock. The velocity equation below assumes a .6 meter circumference wheel. The basic equation is v = delta d/ delta t; in below code delta d is .3 meters for each half rev and tt is delta time. The (t - ttlast)/20 part in the denominator of the velocity equation is there because the delta t value (tt) is only recalculated with an interrupt, which won't happen if the wheel stops, so I wanted the calculated velocity to decay to zero with t continuing to grow but with no new tt calculated. The numerator is multiplied by 10000 because my clock (t) is in .1 msec increments.


// Clock set up

void time_setup() {
  // setup Timer0:
  // CTC (Clear Timer on Compare Match mode)
  // TOP set by OCR0A register
  TCCR0A |= (1<<WGM01);
  // clocked from CLK/
  // which is 16000000/64, or 250000 increments per second
  TCCR0B = 0;
  TCCR0B |= (1<<CS01) | (1<<CS00);
  TCCR0B &= ~(1<<CS02);
  // set TOP to 4
  // because it counts 0, 1, 2,..23,24, 0, 1, 2 ...
  // so 0 through 24 equals 25 events
  OCR0A = 24;
  // enable interrupt on compare event
  // (250000 / 25 = 10000 per second)
  TIMSK0 |= (1<<OCIE0A);

// the_time will store the elapsed time
// (10000 = 1 second)
// note that this will overflow eventually
// This variable is marked "volatile" because it is modified
// by an interrupt handler.  Without the "volatile" marking,
// the compiler might just assume that it doesn't change in 
// the flow of any given function (if the compiler doesn't
// see any code in that function modifying it -- sounds 
// reasonable, normally!).
// But with "volatile", it will always read it from memory 
// instead of making that assumption.

ISR(TIMER0_COMPA_vect){ // when Timer0 gets to its Output Compare value,
  // elapsed time (0.0001 seconds per count).


void init_tach(){

  // make PB0 input pin

  DDRB &= ~(1<<PB0);
  // pull up resistor on
  PORTB |=  (1<<PB0);

  //Enable Pin change interrupt
  PCICR |= (1<<PCIE0); // this enables interrupts on pins PCINT 7-0

 //set the mask on pin change interrupt 0 so that only PCINT0 (PB0) triggers interrupt

  PCMSK0 |=(1<<PCINT0);


 // This is what gets executed on interupt for measuring frequency of hall switch input, set to trip with change to PB0


//Trigger on passing of mag by hall switch.

    tt=t-ttlast;//delta time between mag hits
    ttlast = t;


// clock and intterupt variables

volatile uint32_t t=1;
uint32_t tt=0, ttlast=0;

    // in main


 // turn on interrupt handler

  //init tach to gather velocity data from hall switch


// in continuing main program loop

      v = 3000/(((t - ttlast)/20)+tt);// tt is delta time between mag hits as computed in interrupt
October 25, 2010
by Earnhardt
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Hi All,

Sorry for the delay in getting back with any of you. Thanks for all the information it's very helpful, I just finished up on the Temperature Sensor Circuit, and have taken a look at the Hamometer, this is somewhat the path I would like to go but in a bigger scale. Eric thanks for the code I’ll take a look and see if I could get it working. Take a look at the How it works is they have a hall-effect sensor that’s connected to their product called the AutoBlockRPM this device provides power to the sensor and connects to a netbook via USB which they have written a small “exe” that’s is doing the conversion that is installed on the netbook then it outputs the RPM information to a Mimo display. How I know this is by talking with them. What I would like to do is basically the same thing but not use the netbook and use the nerdkit.

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