Tag: Engine Coil

You can measure engine RPM using the negative terminal of the coil, however you need to isolate the 12v circuit of the coil, from the 5v levels used in the Arduino. This can be done using an optical isolator circuit as detailed below.

The following schematic shows how to connect the circuit:

Assembly List

The following components, or their equivalent are required to build the RPM module.

Label	Part Type	Properties
4n25	DIP – 6 pins	package DIP (Dual Inline) [THT]; hole size 1.0mm,0.508mm; true; chip label IC; pins 6; pin spacing 300mil
Arduino	Arduino Uno (Rev3)	type Arduino UNO (Rev3)
C1	Ceramic Capacitor	package 100 mil [THT, multilayer]; capacitance 100nF; voltage 6.3V
C2	Ceramic Capacitor	package 100 mil [THT, multilayer]; capacitance 100nF; voltage 6.3V
D1	Rectifier Diode	package 300 mil [THT]; type Rectifier; part # 1N4001
D2	Zener Diode	package Melf DO-213 AB [SMD]; breakdown voltage 5.1V; type Zener; power dissipation 0.5W; part # 1N4732A
R1	390 Ω Resistor	package THT; tolerance ±5%; bands 4; resistance 390Ω; pin spacing 400 mil
R2	4.7k Ω Resistor	package THT; tolerance ±5%; bands 4; resistance 4.7kΩ; pin spacing 400 mil

Example Code

volatile unsigned long timePointsOpen, timePointsClosed, lastChange;
volatile unsigned int numBangs;

void pointsOpening(){
    unsigned long t;
    t = millis();
    if (lastChange > 0){
        timePointsClosed += (t - lastChange);
    }
    lastChange = t;
    ++numBangs;
}

void setup(){
  pinMode(3, INPUT);
  digitalWrite(3, HIGH);
}

void loop(){
  // Number of coil pulses
  numBangs = 0;
  attachInterrupt(0, pointsOpening, RISING);
  delay(50);
  detachInterrupt(0);
  numBangs = ((60000/50)*numBangs)/2;
  Serial.print("RPM: ");
  Serial.println(numBangs);
}