by 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);
}
Hi,
Great article – but one question, there is “Coil – 5V Terminal”. But car ignition coil gives max ~500v from output?
The high tension side of the coil does indeed generate a high voltage, however that is triggered by a nominal 12v circuit, which is what is used in this. The low tension side still has a bunch of interference, and the voltage can vary quite a bit hence the zeners, and optical isolator circuit.
Hi David,
¿Do you have the Fritzing proyect? I need the protoboard view and looking to your “schematic.png” I assume you done with Fritzing.
Thanks!
Sir !!
I just made up ur circuit design and i tested it ,.,its working fine,,but my problem is when i get result from the serial monitor its just only
RPM 220 ,
RPM 280 ,
RPM 320 ,
RPM 330 ,
RPM 480 , Like that ..
But i need it like counting numbers like
RMP 221
RMP 237
.
.
…
.
RPM 1223
RpM 1224
like array
please help me with this sir
thank you