One wire RPM pickup

Hello guys,

I have built a one wire rpm pick up that uses a wire wrapped around the spark plug. The circuit consists of a zener diode, a rectifier, a couple resistors, and a cap. I hooked it up to an O scope, and it works as expected. The frequency veries with RPM very consistantly.

I hook it up to the Arduino, but the output is not consistant. the RPM goes up after the a few minutes. Its not just the engine warming up either. I believe it has to be the code. I hooked up a tiny tach to it, and it does the same thing.

My tach puts out an AC wave, and the tiny tach is digital I believe. If anyone could point me into the right direction I would really appreciate it. Here is the code.

int iTestPin=13;                                   //Pin which generates test pulses, this is not needed when actual pulse is available for measurement
int iPulsePin = 2;                                //Pin serving as external interrupt (pin 2) to count the pulses 
static float siPulseCounter=0;                       //Variable keeping track of pulse counts
static float siSecondCount=0;                        //Variable keeping track no. of seconds in the timer1 block
float rpm;
void SetupTimer1()                                //Setting up timer1 with appropriate register values

      TCCR1A = 0;                                 // No PWM is used.So set this to zero.
      TCCR1B = 1<<CS12 | 0<<CS11 | 0<<CS10;       // Input clock is set to F(cpu)/256
      TIMSK1 = 1<<TOIE1;                          // Enable timer interrupt to detect overflow
      Set the timer count to overflow to in 1 second.
      My board has a 16MHz oscillator.
      Reload Timer Value  = ((2 ^ Bits) - 1) - ((Input Frequency / Prescale) / Target Frequency)
                          = 65535 - ((16 x 10 ^6 Hz)/256)/1 Hz 
                          = 65535 - 62500
                          = 3035
      TCNT1=0x0bdb;                               //3035 in hex

ISR(TIMER1_OVF_vect)                             // Timer1 ISR when it overflows
    TCNT1=0x0bdb;                                //Reset Timer1 to count another second      
    siSecondCount++;                             //Increase second count 
    if(siSecondCount>=1)                        //If 1 Seconds are attained Print the Pulse count to serial port.
        rpm = siPulseCounter*16.54;     //Multiplier has to do with Reference Voltage this is at 5v ref.
        Serial.print("RPM is ");
        Serial.println(rpm);  	//Print as decimal value
        siPulseCounter=0;                       //Reset counter values for next minute cycle

void setup(void)
      pinMode(iPulsePin, INPUT);                  // Set Pulsepin to accept inputs
      digitalWrite(iPulsePin, LOW);  
      pinMode(iTestPin, OUTPUT);                  // Test signal generator pin as output. Can be ignored if the actual digital signal is available
      attachInterrupt(0, count, RISING);          // Caputres external interrupt 0  at iPulsepin at rising edge and calls funtion count defined below 
      Serial.begin(115200);                         // Begin Serial communication at baudrate 9600 bps
      SetupTimer1();                              // Call Setuptimer1 function
      Serial.println(" Initialising, Please wait...");

void loop(void) 
  /*** Following 4 lines just generate pulses for testing purposes.
    All the 4 lines Can be ignored if the actual digital signal is available ***/
      digitalWrite(iTestPin, HIGH);             // sets the iTestPin ON
      delay(6.89);                              // waits for a second
      digitalWrite(iTestPin, LOW);              // sets the iTestPin off
      delay(2);                              // waits for a second

void count()                                    // Function called by AttachInterrupt at interrupt at int 0
      siPulseCounter++;                        //increment the pulse count

Post a schematic and the scope trace. What output do you get from your code and from the mini-tach.

I hooked up the scope today. My circuit looks like it is picking up emf. I dont have it potted or anything so this is a huge problem.

The tiny tach is a perfect digital signal 0-5v. I didnt have a chance to take a pic, but like i said its a perfect square wave. Any thoughts on why the code doesn't work? I'll just use the tiny tach for now.

Does anyone have an idea to protect it from the emf?