Counting RPM with an arduino

For the latest IgniteGuelph contest, we needed a way to measure the speed of homebuilt electric motors. Simple RPM, not torque was the goal, so we opted to go with a light sensor on an arduino to count the light/dark cycles.

First, we need to figure out the threshold value of the light sensor.  We started with a CdS photocell, but the response time was too slow to count anything faster than 150 rpm.  Working with what we had on hand, we built a photo sensor with an LED. This tested out fine up to 600 RPM, or so.  Ideal would be a proper photodiode or phototransistor, but we were limited to what we had on hand.

#define LED_N_SIDE 3
#define LED_P_SIDE 2

void setup() {
Serial.begin (115200);
Serial.println(“Start”);
}

void loop() {
Serial.print(“Sensor : “);
Serial.println(readLight());
delay(100);
}

int readLight() {
unsigned int j;

// Apply reverse voltage, charge up the pin and led capacitance
pinMode(LED_N_SIDE,OUTPUT);
pinMode(LED_P_SIDE,OUTPUT);
digitalWrite(LED_N_SIDE,HIGH);
digitalWrite(LED_P_SIDE,LOW);

// Isolate the pin 2 end of the diode
pinMode(LED_N_SIDE,INPUT);
digitalWrite(LED_N_SIDE,LOW); // turn off internal pull-up resistor

// Count how long it takes the diode to bleed back down to a logic zero
for ( j = 10000; j > 1; j–) {
if ( digitalRead(LED_N_SIDE)==0) break;
}
return j;
}

Once we have a threshold value for the light sensor, we use the code below to spit out the RPM to the Serial

#define PHOTOCELL 5
#define LED_N_SIDE 2
#define LED_P_SIDE 3

int photocellThreshold = 2000;
int currentState = 0;
int leeway = 4;
unsigned long lastOnTime = 0;
unsigned long times[5] = {0,0,0,0,0};
int timesPos = 0;

void setup() {
Serial.begin (115200);
Serial.println(“Start”);
}

void loop() {
//Serial.print(“Photocell Value: “);
int currentVal = readLight();
if (currentState == 1 && currentVal < photocellThreshold – leeway) {
lightOff();
} else if (currentState == 0 && currentVal > photocellThreshold + leeway) {
lightOn();
}
//Serial.println(analogRead(PHOTOCELL));
delay(5);
}

void lightOff() {
currentState = 0;
}

void lightOn() {
currentState = 1;
unsigned long now = millis();
if (lastOnTime) {
unsigned long thisTime = now – lastOnTime;
times[timesPos] = thisTime;
averageTimes();
timesPos = (timesPos + 1) % 5;
}
lastOnTime = now;
}

void averageTimes() {
unsigned long sum = times[0] + times[1] + times[2] + times[3] + times[4];
float avg = 300000 / float(sum);
Serial.print(“RPM : “);
Serial.println(avg);
}

int readLight() {
unsigned int j;

// Apply reverse voltage, charge up the pin and led capacitance
pinMode(LED_N_SIDE,OUTPUT);
pinMode(LED_P_SIDE,OUTPUT);
digitalWrite(LED_N_SIDE,HIGH);
digitalWrite(LED_P_SIDE,LOW);

// Isolate the pin 2 end of the diode
pinMode(LED_N_SIDE,INPUT);
digitalWrite(LED_N_SIDE,LOW); // turn off internal pull-up resistor

// Count how long it takes the diode to bleed back down to a logic zero
for ( j = 10000; j > 1; j–) {
if ( digitalRead(LED_N_SIDE)==0) break;
}
return j;
}