PulseIn tachometer

Question

I tried to create a tachometer for my motorcycle using an Arduino Uno.
I am trying to measure the voltage of the ignition coil (stepped down to not destroy the Arduino).

I am trying to determine the time between two explosions (one ignition coil high voltage).

I tried to use the pulseIn command in the following way:

unsigned pin=8;
float t;
unsigned long rpm;
void setup() {
 Serial.begin(9600);
 pinMode(pin,INPUT);
}
void loop() {
 rpm=pulseIn(pin,LOW);
 Serial.println(rpm/60);
}

The problem with it is that it won't display stable values. It will go all crazy. With an open circuit it still displays random values.

• Divisadero: Are you sure that pulse is not shorter than 10ms?
• Andrei Grigore: i tested both pulseIn(pin,LOW) and pulseIn(pin,high) and neither worked. At most, my bike does 14.000 rpm, meaning 14.000/60 (233) rps. 233 rps ..... 1000 ms x rps............1 ms x=233/1000. I guess it is less than 10 ms. Is that the problem?
• Divisadero: arduino.cc/en/Reference/pulseIn - under 10ms it wont work right
• Divisadero: but if you have tested it with smaller speed, the problem will be somewhere else
• Majenko: I suspect that it is already too late for that Arduino. How are you safely stepping the voltage down in such a way that it doesn't a) kill the Arduino, and b) kill you?

Answer 1

No one mentioned that there is back EMF from primary coil when switch (or points) becomes open. This means that there is 200-300V applied backwards to your voltage divider while you think there is only 0-12V.

Answer 2

I can't tell you why that isn't working, but I can give you a working alternative.

unsigned int pin=8;
void setup() {
 Serial.begin(9600);
 pinMode(pin,INPUT);
}
void loop() {
 unsigned float rpm = 0;
 while (digitalRead(pin) == FALSE) {
 rpm++;
 delayMicroseconds(1);
 }
 Serial.println(rpm/60);
}

I might've forgotten something, but that should work.

Edit: the comments may be right, are you stepping down your input voltage?